Servlet学习笔记

内容整理自《head first servlet & jsp》一书，以此为蓝本进行servlet相关知识体系的搭建

[Servlet知识地图](https://naotu.baidu.com/file/ef490ad04fd6677b2fe0bd6f74c4c245)

client 请求 -> Web 服务器应用(如nginx) -> Web 容器应用(如tomcat) -> Servlets 代码

servlets 运行在 Web 容器中，容器的功能包括：
- 通信支持：容器负责与Web服务器通信
- 生命周期管理：容器管理 servlet 的声明周期方法调用
- 多线程支持：每个 servlet 请求由容器负责分配线程
- 声明方式实现安全：使用部署描述文件配置安全性
- JSP支持：容器负责解释执JSP 代码

容器如何处理请求
- 接收请求
- 创建 HttpServletRequest 和 HttpServletResponse 对象
- 分配线程
- 调用 servlet 的 services() 方法，并传入上面两个对象，services() 方法内部再调用 doGet 或 doPost 方法
- doGet 或 doPost 方法内部填充 HttpServletResponse 对象
- 容器将 HttpServletResponse 对象内容转换成 HTTP 响应返回

## 容器如何找到Servlet
Servlet 有 3 个名字：客户端使用的 URL 路径、容器内部使用的名称、Servlet 实际类名(及包名）

[![image](https://user-images.githubusercontent.com/5096493/103539872-c11e8180-4ed3-11eb-834b-85965c5994ec.png)](https://excalidraw.com/#json=5938088027095040,BrUgbz9HOaz-hUjjLkGjEA)

## Servelt 生命周期
[![](https://mermaid.ink/img/eyJjb2RlIjoiJSUgU2VydmVsdCDnlJ_lkb3lkajmnJ9cbnN0YXRlRGlhZ3JhbS12MlxuICAgIEE6IOS4jeWtmOWcqFxuICAgIEI6IOWIneWni-WMllxuICAgIEEgLS0-IEIgOiBpbml0KClcbiAgICBCIC0tPiBBOiBkZXN0b3J5KClcbiAgICBCIC0tPkI6IHNlcnZpY2UoKVxuIiwibWVybWFpZCI6e30sInVwZGF0ZUVkaXRvciI6ZmFsc2V9)](https://mermaid-js.github.io/mermaid-live-editor/#/edit/eyJjb2RlIjoiJSUgU2VydmVsdCDnlJ_lkb3lkajmnJ9cbnN0YXRlRGlhZ3JhbS12MlxuICAgIEE6IOS4jeWtmOWcqFxuICAgIEI6IOWIneWni-WMllxuICAgIEEgLS0-IEIgOiBpbml0KClcbiAgICBCIC0tPiBBOiBkZXN0b3J5KClcbiAgICBCIC0tPkI6IHNlcnZpY2UoKVxuIiwibWVybWFpZCI6e30sInVwZGF0ZUVkaXRvciI6ZmFsc2V9)

[![](https://mermaid.ink/img/eyJjb2RlIjoic2VxdWVuY2VEaWFncmFtXG4gIGF1dG9udW1iZXJcbiAgcGFydGljaXBhbnQgQSBhcyBXZWLlrrnlmahcbiAgcGFydGljaXBhbnQgQiBhcyBTZXJ2bGV057G7XG4gIHBhcnRpY2lwYW50IEMgYXMgU2VydmlsZXTlrp7kvotcblxuICBBLT4-Qjog5Yqg6L2957G7XG4gIEEtPj5DOiDliJvlu7pTZXJ2ZWx05a6e5L6L77yI5p6E6YCg5Ye95pWw77yJXG4gIEEtPj5DOiBpbml0KClcbiAgbG9vcCDmjqXmlLbliLDor7fmsYJcbiAgICBBLT4-Qzogc2VydmljZSgpXG4gICAgYWx0XG4gICAgICBDLT4-QzogZG9HZXQoKVxuICAgIGVsc2UgXG4gICAgICBDLT4-QzogZG9Qb3N0KClcbiAgICBlbmRcbiAgZW5kXG4gIEEtPj5DOiBkZXN0b3J5KCkiLCJtZXJtYWlkIjp7fSwidXBkYXRlRWRpdG9yIjpmYWxzZX0)](https://mermaid-js.github.io/mermaid-live-editor/#/edit/eyJjb2RlIjoic2VxdWVuY2VEaWFncmFtXG4gIGF1dG9udW1iZXJcbiAgcGFydGljaXBhbnQgQSBhcyBXZWLlrrnlmahcbiAgcGFydGljaXBhbnQgQiBhcyBTZXJ2bGV057G7XG4gIHBhcnRpY2lwYW50IEMgYXMgU2VydmlsZXTlrp7kvotcblxuICBBLT4-Qjog5Yqg6L2957G7XG4gIEEtPj5DOiDliJvlu7pTZXJ2ZWx05a6e5L6L77yI5p6E6YCg5Ye95pWw77yJXG4gIEEtPj5DOiBpbml0KClcbiAgbG9vcCDmjqXmlLbliLDor7fmsYJcbiAgICBBLT4-Qzogc2VydmljZSgpXG4gICAgYWx0XG4gICAgICBDLT4-QzogZG9HZXQoKVxuICAgIGVsc2UgXG4gICAgICBDLT4-QzogZG9Qb3N0KClcbiAgICBlbmRcbiAgZW5kXG4gIEEtPj5DOiBkZXN0b3J5KCkiLCJtZXJtYWlkIjp7fSwidXBkYXRlRWRpdG9yIjpmYWxzZX0)

## Servlet 相关类

[![](https://mermaid.ink/img/eyJjb2RlIjoiY2xhc3NEaWFncmFtXG5TZXJ2bGV0IDx8Li4gR2VuZXJpY1NlcnZsZXRcblNlcnZsZXRDb25maWcgPC4uIFNlcnZsZXRcblNlcnZsZXRDb250ZXh0IDwuLiBTZXJ2bGV0Q29uZmlnXG5TZXJ2bGV0Q29uZmlnIDx8Li4gR2VuZXJpY1NlcnZsZXRcbkdlbmVyaWNTZXJ2bGV0IDx8LS0gSHR0cFNlcnZsZXRcbmNsYXNzIFNlcnZsZXQge1xuICA8PGludGVyZmFjZT4-XG4gIGluaXQoKSB2b2lkXG4gIHNlcnZpY2UoU2VydmxldFJlcXVlc3QsIFNlcnZsZXRSZXNwb25zZSkgdm9pZFxuICBkZXN0cm95KCkgdm9pZFxuICBnZXRTZXJ2bGV0Q29uZmlnKCkgU2VydmxldENvbmZpZ1xuICBnZXRTZXJ2bGV0SW5mbygpIFN0cmluZ1xufVxuY2xhc3MgU2VydmxldENvbmZpZyB7XG4gIDw8aW50ZXJmYWNlPj5cbiAgZ2V0U2VydmxldE5hbWUoKSBTdHJpbmdcbiAgZ2V0U2VydmxldENvbnRleHQoKSBTZXJ2bGV0Q29udGV4dFxuICBnZXRJbml0UGFyYW1ldGVyKCkgU3RyaW5nXG4gIGdldEluaXRQYXJhbWV0ZXJOYW1lcygpIEVudW1lcmF0aW9uflN0cmluZ35cbn1cbmNsYXNzIFNlcnZsZXRDb250ZXh0IHtcbiAgPDxpbnRlcmZhY2U-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)](https://mermaid-js.github.io/mermaid-live-editor/#/edit/eyJjb2RlIjoiY2xhc3NEaWFncmFtXG5TZXJ2bGV0IDx8Li4gR2VuZXJpY1NlcnZsZXRcblNlcnZsZXRDb25maWcgPC4uIFNlcnZsZXRcblNlcnZsZXRDb250ZXh0IDwuLiBTZXJ2bGV0Q29uZmlnXG5TZXJ2bGV0Q29uZmlnIDx8Li4gR2VuZXJpY1NlcnZsZXRcbkdlbmVyaWNTZXJ2bGV0IDx8LS0gSHR0cFNlcnZsZXRcbmNsYXNzIFNlcnZsZXQge1xuICA8PGludGVyZmFjZT4-XG4gIGluaXQoKSB2b2lkXG4gIHNlcnZpY2UoU2VydmxldFJlcXVlc3QsIFNlcnZsZXRSZXNwb25zZSkgdm9pZFxuICBkZXN0cm95KCkgdm9pZFxuICBnZXRTZXJ2bGV0Q29uZmlnKCkgU2VydmxldENvbmZpZ1xuICBnZXRTZXJ2bGV0SW5mbygpIFN0cmluZ1xufVxuY2xhc3MgU2VydmxldENvbmZpZyB7XG4gIDw8aW50ZXJmYWNlPj5cbiAgZ2V0U2VydmxldE5hbWUoKSBTdHJpbmdcbiAgZ2V0U2VydmxldENvbnRleHQoKSBTZXJ2bGV0Q29udGV4dFxuICBnZXRJbml0UGFyYW1ldGVyKCkgU3RyaW5nXG4gIGdldEluaXRQYXJhbWV0ZXJOYW1lcygpIEVudW1lcmF0aW9uflN0cmluZ35cbn1cbmNsYXNzIFNlcnZsZXRDb250ZXh0IHtcbiAgPDxpbnRlcmZhY2U-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)

## 问题：tomcat 返回页面中文变成问号

原因（猜测）：从 HttpServletResponse  获取 PrintWriter 时其默认编码不是 UTF-8，导致写入到 HTML 中的中文出现乱码，设置 HTTP 头 `Content-Type` 后，获取到的 PrintWriter 会以此为字符编码，从而正确写入字符串内容

解决办法：
```java
  protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    // 设置 HTTP 响应报文的编码为 UTF-8（注意：在往响应中写入数据之前设置，否则对于已写入的内容不生效)
    resp.setContentType("text/html;charset=utf-8");
    PrintWriter out = resp.getWriter();
}
```

## tomcat 日志

tomcat 日志位于 $CATALINA_HOME/logs 目录下，包含下面几类

localhost_access_log.YYYY-MM-DD.txt 是接口访问日志
```
$ tail -f localhost_access_log.2021-01-05.txt 
10.14.87.202 - - [05/Jan/2021:21:23:13 +0800] "GET /Beer-v1/index.html HTTP/1.1" 200 717
10.14.87.202 - - [05/Jan/2021:21:23:15 +0800] "GET /Beer-v1/form.html HTTP/1.1" 200 701
```

localhost.YYYY-MM-DD.log 是 Sevlet 运行日志，包含通过 HttpServlet 的log 方法、System.out、JavaSE 提供的 logging 包输出的日志，以及异常堆栈信息等。
```
$ tail -n 30 -f localhost.2021-01-05.log
INFO: Marking servlet TestInitParameter as unavailable
Jan 05, 2021 5:51:06 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Allocate exception for servlet TestInitParameter
java.lang.ClassNotFoundException: com.example.web.TestInitParameter
        at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1892)
        at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1735)
        ...

Jan 05, 2021 9:25:14 PM org.apache.catalina.core.ApplicationContext log
INFO: TestInitParameter: getServletPath(): /TestInitParameter
```

catalina.YYYY-MM-DD.log  包含 tomcat 容器运行日志，如 App （Context）重启等。
```
$tail -f catalina.2021-01-05.log 
Jan 05, 2021 9:22:51 PM org.apache.catalina.core.StandardContext reload
INFO: Reloading Context with name [/Beer-v1] is completed
Jan 05, 2021 9:25:11 PM org.apache.catalina.startup.HostConfig reload
INFO: Reloading context [/Beer-v1]
```

还有 manager.YYYY-MM-DD.log 和 host-manager.YYYY-MM-DD.log 日志，暂时未用到，功能未知。

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