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+##AbstractQueuedSynchronizer--AQS
+ 1.继承了AbstractOwnableSynchronizer
+ Thread exclusiveOwnerThread;
+ AbstractOwnableSynchronizer();
+ //设置线程独占
+ setExclusiveOwnerThread(Thread thread);
+ //获取独占线程
+ getExclusiveOwnerThread()
+
+##源码分析
+
+ public abstract class AbstractQueuedSynchronizer
+ extends AbstractOwnableSynchronizer
+ implements java.io.Serializable {
+
+ private static final long serialVersionUID = 7373984972572414691L;
+
+ //无参构造器--初始状态为0
+ protected AbstractQueuedSynchronizer() { }
+ //一个节点-----在后面对获取锁顺序方面有用
+ static final class Node {
+
+
+
+ /** Marker to indicate a node is waiting in shared mode */
+ static final Node SHARED = new Node();
+ /** Marker to indicate a node is waiting in exclusive mode */
+ static final Node EXCLUSIVE = null;
+ /** waitStatus value to indicate thread has cancelled */
+ static final int CANCELLED = 1;
+ /** waitStatus value to indicate successor's thread needs unparking */
+ static final int SIGNAL = -1;
+ /** waitStatus value to indicate thread is waiting on condition */
+ static final int CONDITION = -2;
+ /**
+ * waitStatus value to indicate the next acquireShared should
+ * unconditionally propagate
+ */
+ static final int PROPAGATE = -3;
+
+
+
+ //有五种标记
+ INITIAL = 0---初始状态
+ CANCELLED = 1---取消状态(不排队了)
+ SIGNAL = -1---释放锁了/或者取消了，(将会通知下一个节点)
+ CONDITION = -2---条件锁标记(当满足我需要的条件，我就可以去排队获取锁了)
+ PROPAGATE = -3
+ 以下是源码的解释
+
+ /**
+ * Status field, taking on only the values:
+ * SIGNAL: The successor of this node is (or will soon be)
+ * blocked (via park), so the current node must
+ * unpark its successor when it releases or
+ * cancels. To avoid races, acquire methods must
+ * first indicate they need a signal,
+ * then retry the atomic acquire, and then,
+ * on failure, block.
+ * CANCELLED: This node is cancelled due to timeout or interrupt.
+ * Nodes never leave this state. In particular,
+ * a thread with cancelled node never again blocks.
+ * CONDITION: This node is currently on a condition queue.
+ * It will not be used as a sync queue node
+ * until transferred, at which time the status
+ * will be set to 0. (Use of this value here has
+ * nothing to do with the other uses of the
+ * field, but simplifies mechanics.)
+ * PROPAGATE: A releaseShared should be propagated to other
+ * nodes. This is set (for head node only) in
+ * doReleaseShared to ensure propagation
+ * continues, even if other operations have
+ * since intervened.
+ * 0: None of the above
+ *
+ * The values are arranged numerically to simplify use.
+ * Non-negative values mean that a node doesn't need to
+ * signal. So, most code doesn't need to check for particular
+ * values, just for sign.
+ *
+ * The field is initialized to 0 for normal sync nodes, and
+ * CONDITION for condition nodes. It is modified using CAS
+ * (or when possible, unconditional volatile writes).
+ */
+ volatile int waitStatus;
+ //前驱节点(可以理解为前面的那个人-----因为排队机制)
+ volatile Node prev;
+
+ //后面的节点
+ volatile Node next;
+
+ //拿到这个锁的线程
+ volatile Thread thread;
+
+ //条件锁的--条件节点
+ Node nextWaiter;
+
+ //共享模式
+ final boolean isShared() {
+ return nextWaiter == SHARED;
+ }
+
+ //获取前一个节点
+ final Node predecessor() throws NullPointerException {
+ //前一个节点
+ Node p = prev;
+ if (p == null)
+ throw new NullPointerException();
+ else
+ return p;
+ }
+
+ Node() { // Used to establish initial head or SHARED marker
+ }
+ //用于同步队列添加节点
+ Node(Thread thread, Node mode) { // Used by addWaiter
+ this.nextWaiter = mode;
+ this.thread = thread;
+ }
+ //用于条件中
+ Node(Thread thread, int waitStatus) { // Used by Condition
+ this.waitStatus = waitStatus;
+ this.thread = thread;
+ }
+ }
+
+ //等待队列头
+ private transient volatile Node head;
+
+ //等待队列尾
+ private transient volatile Node tail;
+
+ //同步状态
+ private volatile int state;
+
+ //获取同步状态---共享变量状态
+ protected final int getState() {
+ return state;
+ }
+
+ //设置同步状态
+ protected final void setState(int newState) {
+ state = newState;
+ }
+
+ //修改最底层上的数据
+ protected final boolean compareAndSetState(int expect, int update) {
+ // See below for intrinsics setup to support this
+ return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
+ }
+
+ // Queuing utilities
+
+ /**
+ * The number of nanoseconds for which it is faster to spin
+ * rather than to use timed park. A rough estimate suffices
+ * to improve responsiveness with very short timeouts.
+ */
+ static final long spinForTimeoutThreshold = 1000L;
+
+ //插入一个节点
+ private Node enq(final Node node) {
+ for (;;) {
+ //旧队列中最后的节点
+ Node t = tail;
+ //如果没有节点
+ if (t == null) { // Must initialize
+ //初始化一个，并且该节点就是头节点
+ if (compareAndSetHead(new Node()))
+ tail = head;
+ } else {
+ //将旧队列最后的节点设置为当前节点的前一个节点
+ node.prev = t;
+ if (compareAndSetTail(t, node)) {
+ //前一个节点的后节点设置为现在这个节点
+ t.next = node;
+ //返回之前的节点
+ return t;
+ }
+ }
+ }
+ }
+
+ //把节点添加到队列中
+ private Node addWaiter(Node mode) {
+ Node node = new Node(Thread.currentThread(), mode);
+ // Try the fast path of enq; backup to full enq on failure
+ Node pred = tail;
+ if (pred != null) {
+ node.prev = pred;
+ if (compareAndSetTail(pred, node)) {
+ pred.next = node;
+ return node;
+ }
+ }
+ enq(node);
+ return node;
+ }
+
+ //设置头
+ private void setHead(Node node) {
+ head = node;
+ node.thread = null;
+ node.prev = null;
+ }
+
+ //如果节点后面还有节点，则唤醒操作
+ private void unparkSuccessor(Node node) {
+ /*
+ * If status is negative (i.e., possibly needing signal) try
+ * to clear in anticipation of signalling. It is OK if this
+ * fails or if status is changed by waiting thread.
+ */
+ //获取节点的状态
+ int ws = node.waitStatus;
+ //上面有5种状态-----
+ if (ws < 0)
+ compareAndSetWaitStatus(node, ws, 0);
+
+ /*
+ * Thread to unpark is held in successor, which is normally
+ * just the next node. But if cancelled or apparently null,
+ * traverse backwards from tail to find the actual
+ * non-cancelled successor.
+ */
+ // 当前下一个节点
+ Node s = node.next;
+ //没有下一个节点，或者下个节点已经取消了
+ if (s == null || s.waitStatus > 0) {
+ s = null;
+ //尾节点; 该节点不为空并且不是头节点；该节点的前置节点
+ for (Node t = tail; t != null && t != node; t = t.prev)
+ //从队列最后向前查找，最靠前的的状态不为取消的
+ if (t.waitStatus <= 0)
+ s = t;
+ }
+ if (s != null)
+ //取消线程挂起-----唤醒操作
+ LockSupport.unpark(s.thread);
+ }
+
+ //释放共享
+ private void doReleaseShared() {
+ /*
+ * Ensure that a release propagates, even if there are other
+ * in-progress acquires/releases. This proceeds in the usual
+ * way of trying to unparkSuccessor of head if it needs
+ * signal. But if it does not, status is set to PROPAGATE to
+ * ensure that upon release, propagation continues.
+ * Additionally, we must loop in case a new node is added
+ * while we are doing this. Also, unlike other uses of
+ * unparkSuccessor, we need to know if CAS to reset status
+ * fails, if so rechecking.
+ */
+ for (;;) {
+ //获得头节点
+ Node h = head;
+ //有队列，并且不止一个节点
+ if (h != null && h != tail) {
+ //当前节点的状态
+ int ws = h.waitStatus;
+ //是等待唤醒状态
+ if (ws == Node.SIGNAL) {
+ //把共享变量变成无人占用的状态
+ if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
+ continue; // loop to recheck cases
+ //唤醒队列中 最靠前不是取消的节点--去获取共享变量
+ unparkSuccessor(h);
+ }
+ else if (ws == 0 &&
+ !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
+ continue; // loop on failed CAS
+ }
+ //只有一个节点 跳出释放
+ if (h == head) // loop if head changed
+ break;
+ }
+ }
+
+ //设置队列头并传播
+ private void setHeadAndPropagate(Node node, int propagate) {
+ // 记录之前的队列头
+ Node h = head; // Record old head for check below
+ //把新的节点设置为队列头
+ setHead(node);
+ /*
+ * Try to signal next queued node if:
+ * Propagation was indicated by caller,
+ * or was recorded (as h.waitStatus either before
+ * or after setHead) by a previous operation
+ * (note: this uses sign-check of waitStatus because
+ * PROPAGATE status may transition to SIGNAL.)
+ * and
+ * The next node is waiting in shared mode,
+ * or we don't know, because it appears null
+ *
+ * The conservatism in both of these checks may cause
+ * unnecessary wake-ups, but only when there are multiple
+ * racing acquires/releases, so most need signals now or soon
+ * anyway.
+ */
+ if (propagate > 0 || h == null || h.waitStatus < 0 ||
+ (h = head) == null || h.waitStatus < 0) {
+ Node s = node.next;
+ if (s == null || s.isShared())
+ //释放共享
+ doReleaseShared();
+ }
+ }
+
+ // Utilities for various versions of acquire
+
+
+ //不排队了--出列
+ private void cancelAcquire(Node node) {
+ // Ignore if node doesn't exist
+ if (node == null)
+ return;
+ //不和线程挂钩了
+ node.thread = null;
+
+ // Skip cancelled predecessors
+ Node pred = node.prev;
+ //一直找一个前面没有被取消的节点
+ 这个时候pred一定不是被取消的
+ 可以理解为，出列，但是要补齐位置，让前后连接起来
+ while (pred.waitStatus > 0)
+ node.prev = pred = pred.prev;
+
+ // predNext is the apparent node to unsplice. CASes below will
+ // fail if not, in which case, we lost race vs another cancel
+ // or signal, so no further action is necessary.
+
+
+ //记录一下以及新占位节点的下一个节点
+ Node predNext = pred.next;
+
+ // Can use unconditional write instead of CAS here.
+ // After this atomic step, other Nodes can skip past us.
+ // Before, we are free of interference from other threads.
+
+ //将将要出列的节点设置为取消
+ node.waitStatus = Node.CANCELLED;
+
+ // If we are the tail, remove ourselves.
+
+ //如果要出列的是最后一个节点，并且把新结尾的节点已经替换了
+ if (node == tail && compareAndSetTail(node, pred)) {
+ //把最新结尾节点的的后一个节点设置为null---表示我就是最后一个节点了
+ compareAndSetNext(pred, predNext, null);
+ } else {
+ // If successor needs signal, try to set pred's next-link
+ // so it will get one. Otherwise wake it up to propagate.
+ int ws;
+ //如果node既不是tail，又不是head的后继节点
+ //则将node的前继节点的waitStatus置为SIGNAL
+ //并使node的前继节点指向node的后继节点
+ if (pred != head &&
+ //状态是要唤醒的下一个节点的
+ ((ws = pred.waitStatus) == Node.SIGNAL ||
+ (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
+ pred.thread != null) {
+ Node next = node.next;
+ if (next != null && next.waitStatus <= 0)
+ //将当前节点的前一个节点指向当前节点的后一个节点
+ compareAndSetNext(pred, predNext, next);
+ } else {
+ //如果node是head的后继节点，则直接唤醒node的后继节点
+ unparkSuccessor(node);
+ }
+
+ node.next = node; // help GC
+ }
+ }
+
+ //线程是否应该阻塞
+ private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
+ //上一个节点的等待状态
+ int ws = pred.waitStatus;
+ //如果前面那个是等待唤醒状态
+ if (ws == Node.SIGNAL)
+ /*
+ * This node has already set status asking a release
+ * to signal it, so it can safely park.
+ */
+ //当前节点需要阻塞---因为前面的还没有执行完
+ return true;
+ //前面的节点已经取消了
+ if (ws > 0) {
+ /*
+ * Predecessor was cancelled. Skip over predecessors and
+ * indicate retry.
+ */
+ //把当前节点前面的已经取消的节点 全部剔除
+ do {
+ node.prev = pred = pred.prev;
+ } while (pred.waitStatus > 0);
+ pred.next = node;
+ } else {
+ /*
+ * waitStatus must be 0 or PROPAGATE. Indicate that we
+ * need a signal, but don't park yet. Caller will need to
+ * retry to make sure it cannot acquire before parking.
+ */
+ //设置为等待唤醒
+ compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
+ }
+ return false;
+ }
+
+ //中断当前线程
+ static void selfInterrupt() {
+ Thread.currentThread().interrupt();
+ }
+
+ //线程挂起，并且检查是否已经中断
+ private final boolean parkAndCheckInterrupt() {
+ LockSupport.park(this);
+ return Thread.interrupted();
+ }
+
+ /*
+ * Various flavors of acquire, varying in exclusive/shared and
+ * control modes. Each is mostly the same, but annoyingly
+ * different. Only a little bit of factoring is possible due to
+ * interactions of exception mechanics (including ensuring that we
+ * cancel if tryAcquire throws exception) and other control, at
+ * least not without hurting performance too much.
+ */
+
+ //节点获取锁
+ final boolean acquireQueued(final Node node, int arg) {
+ //标识
+ boolean failed = true;
+ try {
+ //中断标识
+ boolean interrupted = false;
+ //无线循环
+ for (;;) {
+ //当前节点的前一个节点
+ final Node p = node.predecessor();
+ //是头节点(该它去获取锁了),并且获取锁成功(具体是看那个模板用的这个方法)
+ if (p == head && tryAcquire(arg)) {
+ //当前节点作为头节点
+ setHead(node);
+ p.next = null; // help GC
+ failed = false;
+ return interrupted;
+ }
+ //不是头节点 就看看是不是需要阻塞，并且线程挂起和中断
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ interrupted = true;
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ //一直获取
+ private void doAcquireInterruptibly(int arg)
+ throws InterruptedException {
+ //获取添加到队列中的
+ //Node.EXCLUSIVE是null，即使插入了一个null在尾节点
+ final Node node = addWaiter(Node.EXCLUSIVE);
+ boolean failed = true;
+ try {
+ for (;;) {
+ //获取当前null前面一个节点
+ final Node p = node.predecessor();
+ //一直加锁直到上锁成功
+ if (p == head && tryAcquire(arg)) {
+ setHead(node);
+ p.next = null; // help GC
+ failed = false;
+ return;
+ }
+ //不是头节点的 就判断一下要不要线程挂起
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ //排它定时获取
+ private boolean doAcquireNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ if (nanosTimeout <= 0L)
+ return false;
+ //一个时间范围
+ final long deadline = System.nanoTime() + nanosTimeout;
+ final Node node = addWaiter(Node.EXCLUSIVE);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ //头节点获取成功
+ if (p == head && tryAcquire(arg)) {
+ setHead(node);
+ p.next = null; // help GC
+ failed = false;
+ return true;
+ }
+ nanosTimeout = deadline - System.nanoTime();
+ if (nanosTimeout <= 0L)
+ return false;
+ //线程挂起
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ nanosTimeout > spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ }
+ } finally {
+ //取消这个节点
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+ //获取锁，头节点就直到获取成功为止，其他节点，判断是否等待，等待就挂起
+ private void doAcquireShared(int arg) {
+ final Node node = addWaiter(Node.SHARED);
+ boolean failed = true;
+ try {
+ boolean interrupted = false;
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head) {
+ int r = tryAcquireShared(arg);
+ if (r >= 0) {
+ setHeadAndPropagate(node, r);
+ p.next = null; // help GC
+ if (interrupted)
+ selfInterrupt();
+ failed = false;
+ return;
+ }
+ }
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ interrupted = true;
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ //不断获取锁，抛出中断异常，有些业务是需要这个异常
+ private void doAcquireSharedInterruptibly(int arg)
+ throws InterruptedException {
+ final Node node = addWaiter(Node.SHARED);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head) {
+ int r = tryAcquireShared(arg);
+ if (r >= 0) {
+ setHeadAndPropagate(node, r);
+ p.next = null; // help GC
+ failed = false;
+ return;
+ }
+ }
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ parkAndCheckInterrupt())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ //一段时间内不断获取
+ private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ if (nanosTimeout <= 0L)
+ return false;
+ final long deadline = System.nanoTime() + nanosTimeout;
+ final Node node = addWaiter(Node.SHARED);
+ boolean failed = true;
+ try {
+ for (;;) {
+ final Node p = node.predecessor();
+ if (p == head) {
+ int r = tryAcquireShared(arg);
+ if (r >= 0) {
+ setHeadAndPropagate(node, r);
+ p.next = null; // help GC
+ failed = false;
+ return true;
+ }
+ }
+ nanosTimeout = deadline - System.nanoTime();
+ if (nanosTimeout <= 0L)
+ return false;
+ if (shouldParkAfterFailedAcquire(p, node) &&
+ nanosTimeout > spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ }
+ } finally {
+ if (failed)
+ cancelAcquire(node);
+ }
+ }
+
+ // Main exported methods
+
+ //尝试获取锁，具体实现有子类去实现
+ protected boolean tryAcquire(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+ //尝试释放
+ protected boolean tryRelease(int arg) {
+ throw new UnsupportedOperationException();
+ }
+ //共享模式获取
+ protected int tryAcquireShared(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+
+ protected boolean tryReleaseShared(int arg) {
+ throw new UnsupportedOperationException();
+ }
+
+
+ protected boolean isHeldExclusively() {
+ throw new UnsupportedOperationException();
+ }
+
+ //获取失败就排队
+ public final void acquire(int arg) {
+ if (!tryAcquire(arg) &&
+ acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
+ selfInterrupt();
+ }
+
+ //一直获取
+ public final void acquireInterruptibly(int arg)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if (!tryAcquire(arg))
+ doAcquireInterruptibly(arg);
+ }
+
+ //一段时间里不管获取
+ public final boolean tryAcquireNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ return tryAcquire(arg) ||
+ doAcquireNanos(arg, nanosTimeout);
+ }
+
+ //释放
+ public final boolean release(int arg) {
+ if (tryRelease(arg)) {
+ Node h = head;
+ if (h != null && h.waitStatus != 0)
+ unparkSuccessor(h);
+ return true;
+ }
+ return false;
+ }
+
+
+ public final void acquireShared(int arg) {
+ if (tryAcquireShared(arg) < 0)
+ doAcquireShared(arg);
+ }
+
+ public final void acquireSharedInterruptibly(int arg)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if (tryAcquireShared(arg) < 0)
+ doAcquireSharedInterruptibly(arg);
+ }
+
+
+ public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ return tryAcquireShared(arg) >= 0 ||
+ doAcquireSharedNanos(arg, nanosTimeout);
+ }
+
+
+ public final boolean releaseShared(int arg) {
+ if (tryReleaseShared(arg)) {
+ doReleaseShared();
+ return true;
+ }
+ return false;
+ }
+
+ // Queue inspection methods
+
+
+ public final boolean hasQueuedThreads() {
+ return head != tail;
+ }
+
+
+ public final boolean hasContended() {
+ return head != null;
+ }
+
+ public final Thread getFirstQueuedThread() {
+ // handle only fast path, else relay
+ return (head == tail) ? null : fullGetFirstQueuedThread();
+ }
+
+ /**
+ * Version of getFirstQueuedThread called when fastpath fails
+ */
+ private Thread fullGetFirstQueuedThread() {
+ /*
+ * The first node is normally head.next. Try to get its
+ * thread field, ensuring consistent reads: If thread
+ * field is nulled out or s.prev is no longer head, then
+ * some other thread(s) concurrently performed setHead in
+ * between some of our reads. We try this twice before
+ * resorting to traversal.
+ */
+ Node h, s;
+ Thread st;
+ if (((h = head) != null && (s = h.next) != null &&
+ s.prev == head && (st = s.thread) != null) ||
+ ((h = head) != null && (s = h.next) != null &&
+ s.prev == head && (st = s.thread) != null))
+ return st;
+
+ /*
+ * Head's next field might not have been set yet, or may have
+ * been unset after setHead. So we must check to see if tail
+ * is actually first node. If not, we continue on, safely
+ * traversing from tail back to head to find first,
+ * guaranteeing termination.
+ */
+
+ Node t = tail;
+ Thread firstThread = null;
+ while (t != null && t != head) {
+ Thread tt = t.thread;
+ if (tt != null)
+ firstThread = tt;
+ t = t.prev;
+ }
+ return firstThread;
+ }
+
+ /**
+ * Returns true if the given thread is currently queued.
+ *
+ * This implementation traverses the queue to determine
+ * presence of the given thread.
+ *
+ * @param thread the thread
+ * @return {@code true} if the given thread is on the queue
+ * @throws NullPointerException if the thread is null
+ */
+ public final boolean isQueued(Thread thread) {
+ if (thread == null)
+ throw new NullPointerException();
+ for (Node p = tail; p != null; p = p.prev)
+ if (p.thread == thread)
+ return true;
+ return false;
+ }
+
+ /**
+ * Returns {@code true} if the apparent first queued thread, if one
+ * exists, is waiting in exclusive mode. If this method returns
+ * {@code true}, and the current thread is attempting to acquire in
+ * shared mode (that is, this method is invoked from {@link
+ * #tryAcquireShared}) then it is guaranteed that the current thread
+ * is not the first queued thread. Used only as a heuristic in
+ * ReentrantReadWriteLock.
+ */
+ final boolean apparentlyFirstQueuedIsExclusive() {
+ Node h, s;
+ return (h = head) != null &&
+ (s = h.next) != null &&
+ !s.isShared() &&
+ s.thread != null;
+ }
+
+ /**
+ * Queries whether any threads have been waiting to acquire longer
+ * than the current thread.
+ *
+ *
An invocation of this method is equivalent to (but may be
+ * more efficient than):
+ *
{@code
+ * getFirstQueuedThread() != Thread.currentThread() &&
+ * hasQueuedThreads()}
+ *
+ * Note that because cancellations due to interrupts and
+ * timeouts may occur at any time, a {@code true} return does not
+ * guarantee that some other thread will acquire before the current
+ * thread. Likewise, it is possible for another thread to win a
+ * race to enqueue after this method has returned {@code false},
+ * due to the queue being empty.
+ *
+ *
This method is designed to be used by a fair synchronizer to
+ * avoid barging.
+ * Such a synchronizer's {@link #tryAcquire} method should return
+ * {@code false}, and its {@link #tryAcquireShared} method should
+ * return a negative value, if this method returns {@code true}
+ * (unless this is a reentrant acquire). For example, the {@code
+ * tryAcquire} method for a fair, reentrant, exclusive mode
+ * synchronizer might look like this:
+ *
+ *
{@code
+ * protected boolean tryAcquire(int arg) {
+ * if (isHeldExclusively()) {
+ * // A reentrant acquire; increment hold count
+ * return true;
+ * } else if (hasQueuedPredecessors()) {
+ * return false;
+ * } else {
+ * // try to acquire normally
+ * }
+ * }}
+ *
+ * @return {@code true} if there is a queued thread preceding the
+ * current thread, and {@code false} if the current thread
+ * is at the head of the queue or the queue is empty
+ * @since 1.7
+ */
+ public final boolean hasQueuedPredecessors() {
+ // The correctness of this depends on head being initialized
+ // before tail and on head.next being accurate if the current
+ // thread is first in queue.
+ Node t = tail; // Read fields in reverse initialization order
+ Node h = head;
+ Node s;
+ return h != t &&
+ ((s = h.next) == null || s.thread != Thread.currentThread());
+ }
+
+
+ // Instrumentation and monitoring methods
+
+ /**
+ * Returns an estimate of the number of threads waiting to
+ * acquire. The value is only an estimate because the number of
+ * threads may change dynamically while this method traverses
+ * internal data structures. This method is designed for use in
+ * monitoring system state, not for synchronization
+ * control.
+ *
+ * @return the estimated number of threads waiting to acquire
+ */
+ public final int getQueueLength() {
+ int n = 0;
+ for (Node p = tail; p != null; p = p.prev) {
+ if (p.thread != null)
+ ++n;
+ }
+ return n;
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to
+ * acquire. Because the actual set of threads may change
+ * dynamically while constructing this result, the returned
+ * collection is only a best-effort estimate. The elements of the
+ * returned collection are in no particular order. This method is
+ * designed to facilitate construction of subclasses that provide
+ * more extensive monitoring facilities.
+ *
+ * @return the collection of threads
+ */
+ public final Collection getQueuedThreads() {
+ ArrayList list = new ArrayList();
+ for (Node p = tail; p != null; p = p.prev) {
+ Thread t = p.thread;
+ if (t != null)
+ list.add(t);
+ }
+ return list;
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to
+ * acquire in exclusive mode. This has the same properties
+ * as {@link #getQueuedThreads} except that it only returns
+ * those threads waiting due to an exclusive acquire.
+ *
+ * @return the collection of threads
+ */
+ public final Collection getExclusiveQueuedThreads() {
+ ArrayList list = new ArrayList();
+ for (Node p = tail; p != null; p = p.prev) {
+ if (!p.isShared()) {
+ Thread t = p.thread;
+ if (t != null)
+ list.add(t);
+ }
+ }
+ return list;
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to
+ * acquire in shared mode. This has the same properties
+ * as {@link #getQueuedThreads} except that it only returns
+ * those threads waiting due to a shared acquire.
+ *
+ * @return the collection of threads
+ */
+ public final Collection getSharedQueuedThreads() {
+ ArrayList list = new ArrayList();
+ for (Node p = tail; p != null; p = p.prev) {
+ if (p.isShared()) {
+ Thread t = p.thread;
+ if (t != null)
+ list.add(t);
+ }
+ }
+ return list;
+ }
+
+ /**
+ * Returns a string identifying this synchronizer, as well as its state.
+ * The state, in brackets, includes the String {@code "State ="}
+ * followed by the current value of {@link #getState}, and either
+ * {@code "nonempty"} or {@code "empty"} depending on whether the
+ * queue is empty.
+ *
+ * @return a string identifying this synchronizer, as well as its state
+ */
+ public String toString() {
+ int s = getState();
+ String q = hasQueuedThreads() ? "non" : "";
+ return super.toString() +
+ "[State = " + s + ", " + q + "empty queue]";
+ }
+
+
+ // Internal support methods for Conditions
+
+ /**
+ * Returns true if a node, always one that was initially placed on
+ * a condition queue, is now waiting to reacquire on sync queue.
+ * @param node the node
+ * @return true if is reacquiring
+ */
+ final boolean isOnSyncQueue(Node node) {
+ if (node.waitStatus == Node.CONDITION || node.prev == null)
+ return false;
+ if (node.next != null) // If has successor, it must be on queue
+ return true;
+ /*
+ * node.prev can be non-null, but not yet on queue because
+ * the CAS to place it on queue can fail. So we have to
+ * traverse from tail to make sure it actually made it. It
+ * will always be near the tail in calls to this method, and
+ * unless the CAS failed (which is unlikely), it will be
+ * there, so we hardly ever traverse much.
+ */
+ return findNodeFromTail(node);
+ }
+
+ /**
+ * Returns true if node is on sync queue by searching backwards from tail.
+ * Called only when needed by isOnSyncQueue.
+ * @return true if present
+ */
+ private boolean findNodeFromTail(Node node) {
+ Node t = tail;
+ for (;;) {
+ if (t == node)
+ return true;
+ if (t == null)
+ return false;
+ t = t.prev;
+ }
+ }
+
+ /**
+ * Transfers a node from a condition queue onto sync queue.
+ * Returns true if successful.
+ * @param node the node
+ * @return true if successfully transferred (else the node was
+ * cancelled before signal)
+ */
+ final boolean transferForSignal(Node node) {
+ /*
+ * If cannot change waitStatus, the node has been cancelled.
+ */
+ if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
+ return false;
+
+ /*
+ * Splice onto queue and try to set waitStatus of predecessor to
+ * indicate that thread is (probably) waiting. If cancelled or
+ * attempt to set waitStatus fails, wake up to resync (in which
+ * case the waitStatus can be transiently and harmlessly wrong).
+ */
+ Node p = enq(node);
+ int ws = p.waitStatus;
+ if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
+ LockSupport.unpark(node.thread);
+ return true;
+ }
+
+ /**
+ * Transfers node, if necessary, to sync queue after a cancelled wait.
+ * Returns true if thread was cancelled before being signalled.
+ *
+ * @param node the node
+ * @return true if cancelled before the node was signalled
+ */
+ final boolean transferAfterCancelledWait(Node node) {
+ if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
+ enq(node);
+ return true;
+ }
+ /*
+ * If we lost out to a signal(), then we can't proceed
+ * until it finishes its enq(). Cancelling during an
+ * incomplete transfer is both rare and transient, so just
+ * spin.
+ */
+ while (!isOnSyncQueue(node))
+ Thread.yield();
+ return false;
+ }
+
+ /**
+ * Invokes release with current state value; returns saved state.
+ * Cancels node and throws exception on failure.
+ * @param node the condition node for this wait
+ * @return previous sync state
+ */
+ final int fullyRelease(Node node) {
+ boolean failed = true;
+ try {
+ int savedState = getState();
+ if (release(savedState)) {
+ failed = false;
+ return savedState;
+ } else {
+ throw new IllegalMonitorStateException();
+ }
+ } finally {
+ if (failed)
+ node.waitStatus = Node.CANCELLED;
+ }
+ }
+
+ // Instrumentation methods for conditions
+
+ /**
+ * Queries whether the given ConditionObject
+ * uses this synchronizer as its lock.
+ *
+ * @param condition the condition
+ * @return {@code true} if owned
+ * @throws NullPointerException if the condition is null
+ */
+ public final boolean owns(ConditionObject condition) {
+ return condition.isOwnedBy(this);
+ }
+
+ /**
+ * Queries whether any threads are waiting on the given condition
+ * associated with this synchronizer. Note that because timeouts
+ * and interrupts may occur at any time, a {@code true} return
+ * does not guarantee that a future {@code signal} will awaken
+ * any threads. This method is designed primarily for use in
+ * monitoring of the system state.
+ *
+ * @param condition the condition
+ * @return {@code true} if there are any waiting threads
+ * @throws IllegalMonitorStateException if exclusive synchronization
+ * is not held
+ * @throws IllegalArgumentException if the given condition is
+ * not associated with this synchronizer
+ * @throws NullPointerException if the condition is null
+ */
+ public final boolean hasWaiters(ConditionObject condition) {
+ if (!owns(condition))
+ throw new IllegalArgumentException("Not owner");
+ return condition.hasWaiters();
+ }
+
+ /**
+ * Returns an estimate of the number of threads waiting on the
+ * given condition associated with this synchronizer. Note that
+ * because timeouts and interrupts may occur at any time, the
+ * estimate serves only as an upper bound on the actual number of
+ * waiters. This method is designed for use in monitoring of the
+ * system state, not for synchronization control.
+ *
+ * @param condition the condition
+ * @return the estimated number of waiting threads
+ * @throws IllegalMonitorStateException if exclusive synchronization
+ * is not held
+ * @throws IllegalArgumentException if the given condition is
+ * not associated with this synchronizer
+ * @throws NullPointerException if the condition is null
+ */
+ public final int getWaitQueueLength(ConditionObject condition) {
+ if (!owns(condition))
+ throw new IllegalArgumentException("Not owner");
+ return condition.getWaitQueueLength();
+ }
+
+ /**
+ * Returns a collection containing those threads that may be
+ * waiting on the given condition associated with this
+ * synchronizer. Because the actual set of threads may change
+ * dynamically while constructing this result, the returned
+ * collection is only a best-effort estimate. The elements of the
+ * returned collection are in no particular order.
+ *
+ * @param condition the condition
+ * @return the collection of threads
+ * @throws IllegalMonitorStateException if exclusive synchronization
+ * is not held
+ * @throws IllegalArgumentException if the given condition is
+ * not associated with this synchronizer
+ * @throws NullPointerException if the condition is null
+ */
+ public final Collection getWaitingThreads(ConditionObject condition) {
+ if (!owns(condition))
+ throw new IllegalArgumentException("Not owner");
+ return condition.getWaitingThreads();
+ }
+
+ /**
+ * Condition implementation for a {@link
+ * AbstractQueuedSynchronizer} serving as the basis of a {@link
+ * Lock} implementation.
+ *
+ * Method documentation for this class describes mechanics,
+ * not behavioral specifications from the point of view of Lock
+ * and Condition users. Exported versions of this class will in
+ * general need to be accompanied by documentation describing
+ * condition semantics that rely on those of the associated
+ * {@code AbstractQueuedSynchronizer}.
+ *
+ *
This class is Serializable, but all fields are transient,
+ * so deserialized conditions have no waiters.
+ */
+ public class ConditionObject implements Condition, java.io.Serializable {
+ private static final long serialVersionUID = 1173984872572414699L;
+ /** First node of condition queue. */
+ private transient Node firstWaiter;
+ /** Last node of condition queue. */
+ private transient Node lastWaiter;
+
+ /**
+ * Creates a new {@code ConditionObject} instance.
+ */
+ public ConditionObject() { }
+
+ // Internal methods
+
+ /**
+ * Adds a new waiter to wait queue.
+ * @return its new wait node
+ */
+ private Node addConditionWaiter() {
+ Node t = lastWaiter;
+ // If lastWaiter is cancelled, clean out.
+ if (t != null && t.waitStatus != Node.CONDITION) {
+ unlinkCancelledWaiters();
+ t = lastWaiter;
+ }
+ Node node = new Node(Thread.currentThread(), Node.CONDITION);
+ if (t == null)
+ firstWaiter = node;
+ else
+ t.nextWaiter = node;
+ lastWaiter = node;
+ return node;
+ }
+
+ /**
+ * Removes and transfers nodes until hit non-cancelled one or
+ * null. Split out from signal in part to encourage compilers
+ * to inline the case of no waiters.
+ * @param first (non-null) the first node on condition queue
+ */
+ private void doSignal(Node first) {
+ do {
+ if ( (firstWaiter = first.nextWaiter) == null)
+ lastWaiter = null;
+ first.nextWaiter = null;
+ } while (!transferForSignal(first) &&
+ (first = firstWaiter) != null);
+ }
+
+ /**
+ * Removes and transfers all nodes.
+ * @param first (non-null) the first node on condition queue
+ */
+ private void doSignalAll(Node first) {
+ lastWaiter = firstWaiter = null;
+ do {
+ Node next = first.nextWaiter;
+ first.nextWaiter = null;
+ transferForSignal(first);
+ first = next;
+ } while (first != null);
+ }
+
+ /**
+ * Unlinks cancelled waiter nodes from condition queue.
+ * Called only while holding lock. This is called when
+ * cancellation occurred during condition wait, and upon
+ * insertion of a new waiter when lastWaiter is seen to have
+ * been cancelled. This method is needed to avoid garbage
+ * retention in the absence of signals. So even though it may
+ * require a full traversal, it comes into play only when
+ * timeouts or cancellations occur in the absence of
+ * signals. It traverses all nodes rather than stopping at a
+ * particular target to unlink all pointers to garbage nodes
+ * without requiring many re-traversals during cancellation
+ * storms.
+ */
+ private void unlinkCancelledWaiters() {
+ Node t = firstWaiter;
+ Node trail = null;
+ while (t != null) {
+ Node next = t.nextWaiter;
+ if (t.waitStatus != Node.CONDITION) {
+ t.nextWaiter = null;
+ if (trail == null)
+ firstWaiter = next;
+ else
+ trail.nextWaiter = next;
+ if (next == null)
+ lastWaiter = trail;
+ }
+ else
+ trail = t;
+ t = next;
+ }
+ }
+
+ // public methods
+
+ /**
+ * Moves the longest-waiting thread, if one exists, from the
+ * wait queue for this condition to the wait queue for the
+ * owning lock.
+ *
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ public final void signal() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ Node first = firstWaiter;
+ if (first != null)
+ doSignal(first);
+ }
+
+ /**
+ * Moves all threads from the wait queue for this condition to
+ * the wait queue for the owning lock.
+ *
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ public final void signalAll() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ Node first = firstWaiter;
+ if (first != null)
+ doSignalAll(first);
+ }
+
+ /**
+ * Implements uninterruptible condition wait.
+ *
+ * - Save lock state returned by {@link #getState}.
+ *
- Invoke {@link #release} with saved state as argument,
+ * throwing IllegalMonitorStateException if it fails.
+ *
- Block until signalled.
+ *
- Reacquire by invoking specialized version of
+ * {@link #acquire} with saved state as argument.
+ *
+ */
+ public final void awaitUninterruptibly() {
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ boolean interrupted = false;
+ while (!isOnSyncQueue(node)) {
+ LockSupport.park(this);
+ if (Thread.interrupted())
+ interrupted = true;
+ }
+ if (acquireQueued(node, savedState) || interrupted)
+ selfInterrupt();
+ }
+
+ /*
+ * For interruptible waits, we need to track whether to throw
+ * InterruptedException, if interrupted while blocked on
+ * condition, versus reinterrupt current thread, if
+ * interrupted while blocked waiting to re-acquire.
+ */
+
+ /** Mode meaning to reinterrupt on exit from wait */
+ private static final int REINTERRUPT = 1;
+ /** Mode meaning to throw InterruptedException on exit from wait */
+ private static final int THROW_IE = -1;
+
+ /**
+ * Checks for interrupt, returning THROW_IE if interrupted
+ * before signalled, REINTERRUPT if after signalled, or
+ * 0 if not interrupted.
+ */
+ private int checkInterruptWhileWaiting(Node node) {
+ return Thread.interrupted() ?
+ (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
+ 0;
+ }
+
+ /**
+ * Throws InterruptedException, reinterrupts current thread, or
+ * does nothing, depending on mode.
+ */
+ private void reportInterruptAfterWait(int interruptMode)
+ throws InterruptedException {
+ if (interruptMode == THROW_IE)
+ throw new InterruptedException();
+ else if (interruptMode == REINTERRUPT)
+ selfInterrupt();
+ }
+
+ /**
+ * Implements interruptible condition wait.
+ *
+ * - If current thread is interrupted, throw InterruptedException.
+ *
- Save lock state returned by {@link #getState}.
+ *
- Invoke {@link #release} with saved state as argument,
+ * throwing IllegalMonitorStateException if it fails.
+ *
- Block until signalled or interrupted.
+ *
- Reacquire by invoking specialized version of
+ * {@link #acquire} with saved state as argument.
+ *
- If interrupted while blocked in step 4, throw InterruptedException.
+ *
+ */
+ public final void await() throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ LockSupport.park(this);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null) // clean up if cancelled
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ }
+
+ /**
+ * Implements timed condition wait.
+ *
+ * - If current thread is interrupted, throw InterruptedException.
+ *
- Save lock state returned by {@link #getState}.
+ *
- Invoke {@link #release} with saved state as argument,
+ * throwing IllegalMonitorStateException if it fails.
+ *
- Block until signalled, interrupted, or timed out.
+ *
- Reacquire by invoking specialized version of
+ * {@link #acquire} with saved state as argument.
+ *
- If interrupted while blocked in step 4, throw InterruptedException.
+ *
+ */
+ public final long awaitNanos(long nanosTimeout)
+ throws InterruptedException {
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ final long deadline = System.nanoTime() + nanosTimeout;
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ if (nanosTimeout <= 0L) {
+ transferAfterCancelledWait(node);
+ break;
+ }
+ if (nanosTimeout >= spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ nanosTimeout = deadline - System.nanoTime();
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null)
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ return deadline - System.nanoTime();
+ }
+
+ /**
+ * Implements absolute timed condition wait.
+ *
+ * - If current thread is interrupted, throw InterruptedException.
+ *
- Save lock state returned by {@link #getState}.
+ *
- Invoke {@link #release} with saved state as argument,
+ * throwing IllegalMonitorStateException if it fails.
+ *
- Block until signalled, interrupted, or timed out.
+ *
- Reacquire by invoking specialized version of
+ * {@link #acquire} with saved state as argument.
+ *
- If interrupted while blocked in step 4, throw InterruptedException.
+ *
- If timed out while blocked in step 4, return false, else true.
+ *
+ */
+ public final boolean awaitUntil(Date deadline)
+ throws InterruptedException {
+ long abstime = deadline.getTime();
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ boolean timedout = false;
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ if (System.currentTimeMillis() > abstime) {
+ timedout = transferAfterCancelledWait(node);
+ break;
+ }
+ LockSupport.parkUntil(this, abstime);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null)
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ return !timedout;
+ }
+
+ /**
+ * Implements timed condition wait.
+ *
+ * - If current thread is interrupted, throw InterruptedException.
+ *
- Save lock state returned by {@link #getState}.
+ *
- Invoke {@link #release} with saved state as argument,
+ * throwing IllegalMonitorStateException if it fails.
+ *
- Block until signalled, interrupted, or timed out.
+ *
- Reacquire by invoking specialized version of
+ * {@link #acquire} with saved state as argument.
+ *
- If interrupted while blocked in step 4, throw InterruptedException.
+ *
- If timed out while blocked in step 4, return false, else true.
+ *
+ */
+ public final boolean await(long time, TimeUnit unit)
+ throws InterruptedException {
+ long nanosTimeout = unit.toNanos(time);
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ Node node = addConditionWaiter();
+ int savedState = fullyRelease(node);
+ final long deadline = System.nanoTime() + nanosTimeout;
+ boolean timedout = false;
+ int interruptMode = 0;
+ while (!isOnSyncQueue(node)) {
+ if (nanosTimeout <= 0L) {
+ timedout = transferAfterCancelledWait(node);
+ break;
+ }
+ if (nanosTimeout >= spinForTimeoutThreshold)
+ LockSupport.parkNanos(this, nanosTimeout);
+ if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
+ break;
+ nanosTimeout = deadline - System.nanoTime();
+ }
+ if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
+ interruptMode = REINTERRUPT;
+ if (node.nextWaiter != null)
+ unlinkCancelledWaiters();
+ if (interruptMode != 0)
+ reportInterruptAfterWait(interruptMode);
+ return !timedout;
+ }
+
+ // support for instrumentation
+
+ /**
+ * Returns true if this condition was created by the given
+ * synchronization object.
+ *
+ * @return {@code true} if owned
+ */
+ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
+ return sync == AbstractQueuedSynchronizer.this;
+ }
+
+ /**
+ * Queries whether any threads are waiting on this condition.
+ * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
+ *
+ * @return {@code true} if there are any waiting threads
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ protected final boolean hasWaiters() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+ if (w.waitStatus == Node.CONDITION)
+ return true;
+ }
+ return false;
+ }
+
+ /**
+ * Returns an estimate of the number of threads waiting on
+ * this condition.
+ * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
+ *
+ * @return the estimated number of waiting threads
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ protected final int getWaitQueueLength() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ int n = 0;
+ for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+ if (w.waitStatus == Node.CONDITION)
+ ++n;
+ }
+ return n;
+ }
+
+ /**
+ * Returns a collection containing those threads that may be
+ * waiting on this Condition.
+ * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
+ *
+ * @return the collection of threads
+ * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
+ * returns {@code false}
+ */
+ protected final Collection getWaitingThreads() {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ ArrayList list = new ArrayList();
+ for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
+ if (w.waitStatus == Node.CONDITION) {
+ Thread t = w.thread;
+ if (t != null)
+ list.add(t);
+ }
+ }
+ return list;
+ }
+ }
+
+ /**
+ * Setup to support compareAndSet. We need to natively implement
+ * this here: For the sake of permitting future enhancements, we
+ * cannot explicitly subclass AtomicInteger, which would be
+ * efficient and useful otherwise. So, as the lesser of evils, we
+ * natively implement using hotspot intrinsics API. And while we
+ * are at it, we do the same for other CASable fields (which could
+ * otherwise be done with atomic field updaters).
+ */
+ private static final Unsafe unsafe = Unsafe.getUnsafe();
+ private static final long stateOffset;
+ private static final long headOffset;
+ private static final long tailOffset;
+ private static final long waitStatusOffset;
+ private static final long nextOffset;
+
+ static {
+ try {
+ stateOffset = unsafe.objectFieldOffset
+ (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
+ headOffset = unsafe.objectFieldOffset
+ (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
+ tailOffset = unsafe.objectFieldOffset
+ (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
+ waitStatusOffset = unsafe.objectFieldOffset
+ (Node.class.getDeclaredField("waitStatus"));
+ nextOffset = unsafe.objectFieldOffset
+ (Node.class.getDeclaredField("next"));
+
+ } catch (Exception ex) { throw new Error(ex); }
+ }
+
+ /**
+ * CAS head field. Used only by enq.
+ */
+ private final boolean compareAndSetHead(Node update) {
+ return unsafe.compareAndSwapObject(this, headOffset, null, update);
+ }
+
+ /**
+ * CAS tail field. Used only by enq.
+ */
+ private final boolean compareAndSetTail(Node expect, Node update) {
+ return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
+ }
+
+ /**
+ * CAS waitStatus field of a node.
+ */
+ private static final boolean compareAndSetWaitStatus(Node node,
+ int expect,
+ int update) {
+ return unsafe.compareAndSwapInt(node, waitStatusOffset,
+ expect, update);
+ }
+
+ /**
+ * CAS next field of a node.
+ */
+ private static final boolean compareAndSetNext(Node node,
+ Node expect,
+ Node update) {
+ return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
+ }
+}
+
+
diff --git a/week_03/17/ReentrantLock b/week_03/17/ReentrantLock
new file mode 100644
index 0000000000000000000000000000000000000000..d3fc968c2a94b70cfdc22302458d1c16cccf3324
--- /dev/null
+++ b/week_03/17/ReentrantLock
@@ -0,0 +1,293 @@
+##ReentrantLock---重入锁(公平和非公平)
+ 1.实现Lock
+ //锁
+ void lock();
+ //可中断锁
+ void lockInterruptibly() throws InterruptedException;
+ //尝试加锁
+ boolean tryLock();
+ //一个时间段内尝试加锁
+ boolean tryLock(long time, TimeUnit unit) throws InterruptedException;
+ //释放锁
+ void unlock();
+ //条件锁
+ Condition newCondition();
+##理解
+ 1.公平锁，先看下有人在用锁么,如果没有去去检查一下有没有等待加锁的节点
+ 没有节点，就拿走锁，有节点就去后面排队
+ 如果排队获取锁成功，就把自己的这个节点变成老大，之前的那个老大删掉，返回。
+ 如果排队获取锁失败，就阻塞在这里,
+ 如果当前节点前面一位老哥已经取消了就把前面所有取消的，剔除队伍。
+ 如果前面都在等待唤醒排队中，那自己也就老老实实在等待唤醒的队伍中。
+ 2.非公平锁，直接上锁，如果上面已经有锁了,就在旁边等(可以理解为插队到等待加锁的队伍的第一个)，这样他一来就2次机会。
+##源码分析
+
+ public class ReentrantLock implements Lock, java.io.Serializable {
+ private static final long serialVersionUID = 7373984872572414699L;
+
+ /** Synchronizer providing all implementation mechanics */
+ private final Sync sync;
+
+ //模板模式
+ abstract static class Sync extends AbstractQueuedSynchronizer {
+ private static final long serialVersionUID = -5179523762034025860L;
+
+
+ abstract void lock();
+
+ //非公平锁
+ final boolean nonfairTryAcquire(int acquires) {
+ final Thread current = Thread.currentThread();
+ int c = getState();
+ if (c == 0) {
+ if (compareAndSetState(0, acquires)) {
+ setExclusiveOwnerThread(current);
+ return true;
+ }
+ }
+ else if (current == getExclusiveOwnerThread()) {
+ int nextc = c + acquires;
+ if (nextc < 0) // overflow
+ throw new Error("Maximum lock count exceeded");
+ setState(nextc);
+ return true;
+ }
+ return false;
+ }
+ //尝试释放
+ protected final boolean tryRelease(int releases) {
+ int c = getState() - releases;
+ if (Thread.currentThread() != getExclusiveOwnerThread())
+ throw new IllegalMonitorStateException();
+ boolean free = false;
+ if (c == 0) {
+ free = true;
+ setExclusiveOwnerThread(null);
+ }
+ setState(c);
+ return free;
+ }
+ //当前线程是不是独占了
+ protected final boolean isHeldExclusively() {
+ // While we must in general read state before owner,
+ // we don't need to do so to check if current thread is owner
+ return getExclusiveOwnerThread() == Thread.currentThread();
+ }
+
+ final ConditionObject newCondition() {
+ return new ConditionObject();
+ }
+
+ // Methods relayed from outer class
+ //获取状态
+ final Thread getOwner() {
+ return getState() == 0 ? null : getExclusiveOwnerThread();
+ }
+
+ final int getHoldCount() {
+ return isHeldExclusively() ? getState() : 0;
+ }
+
+ final boolean isLocked() {
+ return getState() != 0;
+ }
+
+ /**
+ * Reconstitutes the instance from a stream (that is, deserializes it).
+ */
+ private void readObject(java.io.ObjectInputStream s)
+ throws java.io.IOException, ClassNotFoundException {
+ s.defaultReadObject();
+ setState(0); // reset to unlocked state
+ }
+ }
+
+ //非公平锁
+ static final class NonfairSync extends Sync {
+ private static final long serialVersionUID = 7316153563782823691L;
+
+ /**
+ * Performs lock. Try immediate barge, backing up to normal
+ * acquire on failure.
+ */
+ final void lock() {
+ //直接上底层改
+ if (compareAndSetState(0, 1))
+ //设置为当前线程独占
+ setExclusiveOwnerThread(Thread.currentThread());
+ else
+ //如果直接修改失败的话，去排队
+
+ acquire(1);
+ }
+
+ protected final boolean tryAcquire(int acquires) {
+ return nonfairTryAcquire(acquires);
+ }
+ }
+
+ //公平锁
+ static final class FairSync extends Sync {
+ private static final long serialVersionUID = -3000897897090466540L;
+
+ final void lock() {
+ acquire(1);
+ }
+
+ /**
+ * Fair version of tryAcquire. Don't grant access unless
+ * recursive call or no waiters or is first.
+ */
+ protected final boolean tryAcquire(int acquires) {
+ final Thread current = Thread.currentThread();
+ //获取共享变量的状态
+ int c = getState();
+ //没有被使用
+ if (c == 0) {
+ //检查队列
+ if (!hasQueuedPredecessors() &&
+ //尝试修改
+ compareAndSetState(0, acquires)) {
+ //设置独占
+ setExclusiveOwnerThread(current);
+ return true;
+ }
+ }
+ //检查是不是当前线程独占
+ else if (current == getExclusiveOwnerThread()) {
+ int nextc = c + acquires;
+ if (nextc < 0)
+ throw new Error("Maximum lock count exceeded");
+ setState(nextc);
+ return true;
+ }
+ return false;
+ }
+ }
+
+ //初始化
+ public ReentrantLock() {
+ sync = new NonfairSync();
+ }
+
+ //选择初始化
+ public ReentrantLock(boolean fair) {
+ sync = fair ? new FairSync() : new NonfairSync();
+ }
+
+
+ public void lock() {
+ sync.lock();
+ }
+
+ //可中断加锁
+ public void lockInterruptibly() throws InterruptedException {
+ sync.acquireInterruptibly(1);
+ }
+
+ //尝试加锁(非公平加锁)
+ public boolean tryLock() {
+ return sync.nonfairTryAcquire(1);
+ }
+
+ //释放锁
+ public void unlock() {
+ sync.release(1);
+ }
+
+ //条件锁
+ public Condition newCondition() {
+ return sync.newCondition();
+ }
+
+ //占用次数
+ public int getHoldCount() {
+ return sync.getHoldCount();
+ }
+
+
+ public boolean isHeldByCurrentThread() {
+ return sync.isHeldExclusively();
+ }
+
+ //查询锁是否被上锁了
+ public boolean isLocked() {
+ return sync.isLocked();
+ }
+
+ /**
+ * Returns {@code true} if this lock has fairness set true.
+ *
+ * @return {@code true} if this lock has fairness set true
+ */
+ public final boolean isFair() {
+ return sync instanceof FairSync;
+ }
+
+ //返回拥有锁的线程
+ protected Thread getOwner() {
+ return sync.getOwner();
+ }
+
+ //查询是否还有线程在等改锁
+ public final boolean hasQueuedThreads() {
+ return sync.hasQueuedThreads();
+ }
+
+ //这个线程是否在等这个锁
+ public final boolean hasQueuedThread(Thread thread) {
+ return sync.isQueued(thread);
+ }
+
+ //队列长度
+ public final int getQueueLength() {
+ return sync.getQueueLength();
+ }
+
+ //等待该锁的所有线程
+ protected Collection getQueuedThreads() {
+ return sync.getQueuedThreads();
+ }
+
+ //AQS中的方法
+ public boolean hasWaiters(Condition condition) {
+ if (condition == null)
+ throw new NullPointerException();
+ if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+ throw new IllegalArgumentException("not owner");
+ return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
+ }
+
+ //AQS中的方法---返回正在等待的线程数
+ public int getWaitQueueLength(Condition condition) {
+ if (condition == null)
+ throw new NullPointerException();
+ if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+ throw new IllegalArgumentException("not owner");
+ return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
+ }
+
+ //AQS中的方法--返回在等待条件的线程
+ protected Collection getWaitingThreads(Condition condition) {
+ if (condition == null)
+ throw new NullPointerException();
+ if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
+ throw new IllegalArgumentException("not owner");
+ return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
+ }
+
+ /**
+ * Returns a string identifying this lock, as well as its lock state.
+ * The state, in brackets, includes either the String {@code "Unlocked"}
+ * or the String {@code "Locked by"} followed by the
+ * {@linkplain Thread#getName name} of the owning thread.
+ *
+ * @return a string identifying this lock, as well as its lock state
+ */
+ public String toString() {
+ Thread o = sync.getOwner();
+ return super.toString() + ((o == null) ?
+ "[Unlocked]" :
+ "[Locked by thread " + o.getName() + "]");
+ }
+ }
diff --git a/week_03/17/Semaphore b/week_03/17/Semaphore
new file mode 100644
index 0000000000000000000000000000000000000000..3942b4dcd13db740e3ced800d2776b4efba459d1
--- /dev/null
+++ b/week_03/17/Semaphore
@@ -0,0 +1,247 @@
+##Semaphore----信号量
+ 1.可以理解为锁次数的管理员
+ 2.加一次锁 就消耗一次，释放一次就增加一次。
+ 3.通俗的来说，可以理解为，锁的限流。
+
+##代码中也存在NonfairSync，FairSync
+ 1.所有这个也是有公平和非公平的说法。
+
+##源码分析
+
+ {
+ private static final long serialVersionUID = -3222578661600680210L;
+ /** All mechanics via AbstractQueuedSynchronizer subclass */
+ private final Sync sync;
+
+ /**
+ * Synchronization implementation for semaphore. Uses AQS state
+ * to represent permits. Subclassed into fair and nonfair
+ * versions.
+ */
+ abstract static class Sync extends AbstractQueuedSynchronizer {
+ private static final long serialVersionUID = 1192457210091910933L;
+ //初始化，把许可次数放进去
+ Sync(int permits) {
+ setState(permits);
+ }
+ //获取一个许可
+ final int getPermits() {
+ return getState();
+ }
+ //默认非公平
+ final int nonfairTryAcquireShared(int acquires) {
+ for (;;) {
+ int available = getState();
+ //用剩余的许可总数减去马上用的这个
+ int remaining = available - acquires;
+ //如果小于0或者已经更改成功了就返回
+ if (remaining < 0 ||
+ compareAndSetState(available, remaining))
+ return remaining;
+ }
+ }
+ //归还许可
+ protected final boolean tryReleaseShared(int releases) {
+ for (;;) {
+ int current = getState();
+ int next = current + releases;
+ if (next < current) // overflow
+ throw new Error("Maximum permit count exceeded");
+ if (compareAndSetState(current, next))
+ return true;
+ }
+ }
+ //减少许可证
+ final void reducePermits(int reductions) {
+ for (;;) {
+ //当前剩余许可证
+ int current = getState();
+ int next = current - reductions;
+ if (next > current) // underflow
+ throw new Error("Permit count underflow");
+ //把许可次数换成减少之后的
+ if (compareAndSetState(current, next))
+ return;
+ }
+ }
+ //销毁许可
+ final int drainPermits() {
+ for (;;) {
+ int current = getState();
+ if (current == 0 || compareAndSetState(current, 0))
+ return current;
+ }
+ }
+ }
+
+ //非公平获取一个许可
+ static final class NonfairSync extends Sync {
+ private static final long serialVersionUID = -2694183684443567898L;
+
+ NonfairSync(int permits) {
+ super(permits);
+ }
+
+ protected int tryAcquireShared(int acquires) {
+ return nonfairTryAcquireShared(acquires);
+ }
+ }
+
+ //公平的获取一个许可-----检查一下同步队列
+ static final class FairSync extends Sync {
+ private static final long serialVersionUID = 2014338818796000944L;
+
+ FairSync(int permits) {
+ super(permits);
+ }
+
+ protected int tryAcquireShared(int acquires) {
+ for (;;) {
+ if (hasQueuedPredecessors())
+ return -1;
+ int available = getState();
+ int remaining = available - acquires;
+ if (remaining < 0 ||
+ compareAndSetState(available, remaining))
+ return remaining;
+ }
+ }
+ }
+
+ //指定许可次数
+ public Semaphore(int permits) {
+ sync = new NonfairSync(permits);
+ }
+
+ //指定许可次数，并且指定公平和非公平
+ public Semaphore(int permits, boolean fair) {
+ sync = fair ? new FairSync(permits) : new NonfairSync(permits);
+ }
+
+ //获取一个许可--可以中断，失败就要去排队
+ public void acquire() throws InterruptedException {
+ sync.acquireSharedInterruptibly(1);
+ }
+
+ //获取一个许可--失败就排队
+ public void acquireUninterruptibly() {
+ sync.acquireShared(1);
+ }
+
+ //可中断获取许可成功/失败，如果失败，不排队
+ public boolean tryAcquire() {
+ return sync.nonfairTryAcquireShared(1) >= 0;
+ }
+
+ //获取许可，失败就在一段时间里获取，超时false,不在队伍中
+ public boolean tryAcquire(long timeout, TimeUnit unit)
+ throws InterruptedException {
+ return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
+ }
+
+ //释放许可--
+ public void release() {
+ sync.releaseShared(1);
+ }
+
+ //获取多个许可--可中断
+ public void acquire(int permits) throws InterruptedException {
+ if (permits < 0) throw new IllegalArgumentException();
+ sync.acquireSharedInterruptibly(permits);
+ }
+ //一次获取多个许可--不断尝试
+ public void acquireUninterruptibly(int permits) {
+ if (permits < 0) throw new IllegalArgumentException();
+ sync.acquireShared(permits);
+ }
+
+ //一次获取多个-只是一次
+ public boolean tryAcquire(int permits) {
+ if (permits < 0) throw new IllegalArgumentException();
+ return sync.nonfairTryAcquireShared(permits) >= 0;
+ }
+ //尝试获取多个--一段时间内没有获取到则返回
+ public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
+ throws InterruptedException {
+ if (permits < 0) throw new IllegalArgumentException();
+ return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
+ }
+
+ //释放多个许可
+ public void release(int permits) {
+ if (permits < 0) throw new IllegalArgumentException();
+ sync.releaseShared(permits);
+ }
+
+ //获取许可次数
+ public int availablePermits() {
+ return sync.getPermits();
+ }
+
+ //销毁剩余的许可次数，
+ public int drainPermits() {
+ return sync.drainPermits();
+ }
+
+ //减少许可次数
+ protected void reducePermits(int reduction) {
+ if (reduction < 0) throw new IllegalArgumentException();
+ sync.reducePermits(reduction);
+ }
+ //判断非公平
+ public boolean isFair() {
+ return sync instanceof FairSync;
+ }
+
+ /**
+ * Queries whether any threads are waiting to acquire. Note that
+ * because cancellations may occur at any time, a {@code true}
+ * return does not guarantee that any other thread will ever
+ * acquire. This method is designed primarily for use in
+ * monitoring of the system state.
+ *
+ * @return {@code true} if there may be other threads waiting to
+ * acquire the lock
+ */
+ public final boolean hasQueuedThreads() {
+ return sync.hasQueuedThreads();
+ }
+
+ /**
+ * Returns an estimate of the number of threads waiting to acquire.
+ * The value is only an estimate because the number of threads may
+ * change dynamically while this method traverses internal data
+ * structures. This method is designed for use in monitoring of the
+ * system state, not for synchronization control.
+ *
+ * @return the estimated number of threads waiting for this lock
+ */
+ public final int getQueueLength() {
+ return sync.getQueueLength();
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to acquire.
+ * Because the actual set of threads may change dynamically while
+ * constructing this result, the returned collection is only a best-effort
+ * estimate. The elements of the returned collection are in no particular
+ * order. This method is designed to facilitate construction of
+ * subclasses that provide more extensive monitoring facilities.
+ *
+ * @return the collection of threads
+ */
+ protected Collection getQueuedThreads() {
+ return sync.getQueuedThreads();
+ }
+
+ /**
+ * Returns a string identifying this semaphore, as well as its state.
+ * The state, in brackets, includes the String {@code "Permits ="}
+ * followed by the number of permits.
+ *
+ * @return a string identifying this semaphore, as well as its state
+ */
+ public String toString() {
+ return super.toString() + "[Permits = " + sync.getPermits() + "]";
+ }
+}
\ No newline at end of file
diff --git a/week_03/17/synchronized b/week_03/17/synchronized
new file mode 100644
index 0000000000000000000000000000000000000000..b837d5db2bf5cb4be9db8d8045e61baf5a93cac8
--- /dev/null
+++ b/week_03/17/synchronized
@@ -0,0 +1,51 @@
+##synchronized----同步
+ 1.我们都知道保证同步可以用这个实现。
+ 及对变量加锁，操作，解锁
+ 加锁会保证只有一个线程在用---及中间不会有其他干扰，是原子的
+ 解锁的时候，会将新的数据放到主内存在，是可见的
+ 加锁的过程是同步队列排队进行的，所以也是有序的。
+ 2.是个可重入锁，非公平的锁
+ 3.偏向锁，轻量锁，重量锁
+ 解释：偏向锁是指一段同步代码一直被一个线程访问，那么这个线程会自动获取锁，降低获取锁的代价。
+ 轻量级锁，是指当锁是偏向锁时，被另一个线程所访问，偏向锁会升级为轻量级锁，这个线程会通过自旋的方式尝试获取锁，不会阻塞，提高性能。
+ 重量级锁，是指当锁是轻量级锁时，当自旋的线程自旋了一定的次数后，还没有获取到锁，就会进入阻塞状态，该锁升级为重量级锁，重量级锁会使其他线程阻塞，性能降低
+
+ 注意:多个synchronized只有锁的是同一个对象，它们之间的代码才是同步的，这一点在使用synchronized的时候一定要注意.
+ synchronized只是jvm级别的锁(就是都放在一台tomcat上是没有问题的)，
+ 但是，现在有很多都采用分布式的，这个就满足不了，就会用到分布式锁。
+
+
+##分布式锁
+ 1.通过之前对锁的理解，多个线程维护同一个共享变量，依据该变量则可以保证。
+
+ 那么分布式我们可以理解为几个线程(一台服务器看成一个线程),共同去维护同一个变量就可以实现了。
+
+ 可以利用：
+ 1.mysql数据库主键唯一性。
+ // 加锁、释放锁必须使用同一个session。
+ // 会单独开一个session来维护这个，所有增加开销以及数据库的压力。
+ 2.redis的key唯一性，涉及方法setex方法，以及它的lau脚本
+ 推荐使用redisson,能解决，加锁和释放之间的原子性，锁的唯一性，以及锁续命的效果
+ 缺点：集群模式下会在所有master节点执行加锁命令，大部分（2N+1）成功了则获得锁，节点越多，加锁的过程越慢；
+ 高并发情况下，未获得锁的线程会睡眠重试，如果同一把锁竞争非常激烈，会占用非常多的系统资源
+
+
+ 3.ZK临时有序节点
+ 因为ZK相同的节点只能创建一个。
+ 有序节点可以解决，惊群效应；
+ 大家根据自己创建的顺序去依次获取，最后一个用完之后，检查后面没有了，就把锁删掉
+ 和公平锁的一样的道理，排队获取。
+ 有现有的轮子curator可以使用,又互斥锁，读写锁、多重锁、信号量
+
+ 缺点： 加锁会频繁地“写”zookeeper，增加zookeeper的压力
+ 写zookeeper的时候会在集群进行同步，节点数越多，同步越慢，获取锁的过程越慢；
+ 依赖zookeeper，如果本系统没有依赖的话，相对增加了复杂度
+
+ 惊群效应：都监听了加锁那个老哥有没有释放锁，如果释放了，一窝蜂的就去抢锁,但是又只有一个能创建成功
+ 就会产生不必要的开销。
+
+
+
+
+
+
\ No newline at end of file
diff --git a/week_03/17/volatile b/week_03/17/volatile
new file mode 100644
index 0000000000000000000000000000000000000000..8d614e2852fe5730cf494799555f20e2d92f7c6a
--- /dev/null
+++ b/week_03/17/volatile
@@ -0,0 +1,21 @@
+##volatile---可见性，有序性
+ 1.保证了可见性。
+ 被volatile修饰的变量，只要一被改动，其他的线程就都知道了。
+ 2.禁止重排序
+ 在JVM中,有时候会提速，你所有的变量，可能不会按照你的代码那样顺序的做
+ 例如：int a=1; int b=2;
+ 我们会觉得 应该是先初始a=1,再b=2。但是JVM的时候，有可能是先b,再a的。
+ 这种被成为代码重新排序。
+ 实现方法：内存屏障
+ 内存屏障的作用：
+ （1）阻止屏障两侧的指令重排序；
+ （2）强制把写缓冲区/高速缓存中的数据回写到主内存，让缓存中相应的数据失效
+ (个人理解可见性)在我们主内存外面还有一层消息总线触发机制，类似与关卡。
+ 其他线程和这个消息总线之间，有类似与发布订阅的关系。所以当我们其中一个线程对这个变量做了修改，并且
+ 写回主内存的时候，就回触发该机制，其他线程就会再去主内存中获取这个新的值。
+##缺点---不能保证原子性
+ 因为我们程序中的lang和double是高32+低32组成的，所以建议将共享的这两个类型用volatile修饰(因为它能保证单次读写的原子性)
+ 这两个类是由2次操作完成的
+
+
+
\ No newline at end of file
diff --git "a/week_03/17/\345\206\205\345\255\230\346\250\241\345\236\213" "b/week_03/17/\345\206\205\345\255\230\346\250\241\345\236\213"
new file mode 100644
index 0000000000000000000000000000000000000000..e9498deee55e1085af18fd3f1b28e6e4792bb06f
--- /dev/null
+++ "b/week_03/17/\345\206\205\345\255\230\346\250\241\345\236\213"
@@ -0,0 +1,32 @@
+##java内存模型
+ 1.组成
+ 1.主内存
+ 2.多个工作线程---多个工作内存
+ 类似：
+ 主 | 工作内存--------------java线程
+ | 工作内存--------------java线程
+ 内 | 工作内存--------------java线程
+ | 工作内存--------------java线程
+ 存 | 工作内存--------------java线程
+ 3.存的东西
+ 主内存是被用来java线程之间的数据交互，因为java线程之间是不能访问的，数据都是通过主内存交互。
+ 工作内存,会用来存一些局部变量，和方法的。
+
+##内存区域
+ 1.虚拟机栈，本地方法栈，PC寄存器，方法区，堆
+
+ 2.还有一个元空间(1.8之前用的java内存，1.8开始是直接在主内存上申请了一块)
+
+##GC
+ 1.过程
+ 年轻代 老年代 本地内存
+ Eden------S0----S1,oldMe, 元空间
+ 1.当Eden园满了之后会触发minorGC,这个时候会将Eden中还存在的对象 移动S区域。
+ 2.这个对象会在S0-S1周而复始(minorGC一次，对象年龄+1),当这个对象的年龄在15岁的时候，会放到老年代去。
+ 3.如果老年代也满了，就会触发FullGC(如果有比较大的对象，会直接放到老年代中)
+
+ 对象的年龄是放在对象头中的
+
+
+
+