00-04、理解Servlet · 浅入浅出Java Web

本文依然使用[00-03、从JSP开始](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-03.md)中创建的项目HelloJSP。本文主要有以下内容： * 如何使用Servlet编写Hello Servlet * 如何将Servlet与URL对应起来 * Servlet如何调用JSP * Servlet如何返回JSON数据 * 如何编写一个Dispatcher ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#hello-servlet)Hello Servlet 项目结构如下： [![](https://github.com/someus/another-tutorial-about-java-web/raw/master/img/00-04/01.png)](https://github.com/someus/another-tutorial-about-java-web/blob/master/img/00-04/01.png) `HelloServlet.java`内容如下： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class HelloServlet extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); try (PrintWriter out = response.getWriter()) { out.println("<!DOCTYPE html>"); out.println("<html>"); out.println("<head>"); out.println("<title>Servlet HelloServlet</title>"); out.println("</head>"); out.println("<body>"); out.println("<h1>Servlet HelloServlet at " + request.getContextPath() + "</h1>"); out.println("</body>"); out.println("</html>"); } } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } } ~~~ HTTP最常见的方法是GET和POST，在一个Servlet中对应的处理方法分别是doGet()和doPost()。`response.setContentType("text/html;charset=UTF-8");` 用来设置HTTP响应头中的Content-Type。 PrintWriter对象out的输出内容则是响应正文。 `web.xml`内容如下： ~~~ <?xml version="1.0" encoding="UTF-8"?> <web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"> <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>me.letiantian.servlet.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/HelloServlet</url-pattern> </servlet-mapping> <session-config> <session-timeout> 30 </session-timeout> </session-config> </web-app> ~~~ 在这个配置中，`me.letiantian.servlet.HelloServlet`与URL`/HelloServlet`对应。 `session-timeout`设置了session的有效时间，单位是分钟（不过目前的程序里还没用过session）。浏览器访问`http://127.0.0.1:8084/HelloJSP`会显示404；访问`http://127.0.0.1:8084/HelloJSP/HelloServlet`会显示`Servlet HelloServlet at /HelloJSP`，这也正是`me.letiantian.servlet.HelloServlet`输出的HTML的渲染结果。 ### [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#也可以使用注解将servlet和url对应起来)也可以使用注解将Servlet和URL对应起来首先清空web.xml中关于URL的配置，web.xml最终内容如下： ~~~ <?xml version="1.0" encoding="UTF-8"?> <web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"> <session-config> <session-timeout> 30 </session-timeout> </session-config> </web-app> ~~~ 然后对`me.letiantian.servlet.HelloServlet`类略做修改： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/HelloServlet") public class HelloServlet extends HttpServlet { // ...... } ~~~ 重新启动项目，浏览器访问效果和之前是相同的。 ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#servlet调用jsp)Servlet调用JSP 改写me.letiantian.servlet.HelloServlet类，内容如下： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.RequestDispatcher; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/HelloServlet") public class HelloServlet extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); request.setAttribute("title", "Hello Servlet"); request.setAttribute("content", "你好"); RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/jsp/hello.jsp"); rd.forward(request, response); } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } } ~~~ 在`WEB-INF/`下创建目录`jsp`，然后在`jsp`目录下新建`hello.jsp`，内容如下： ~~~ <!DOCTYPE html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>${title}</title> </head> <body> <h1>${content}</h1> </body> </html> ~~~ 重启该项目，访问`http://127.0.0.1:8084/HelloJSP/HelloServlet`： ~~~ $ curl -i http://127.0.0.1:8084/HelloJSP/HelloServlet HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Set-Cookie: JSESSIONID=7CCCFD5467F8330066F827623802FB23; Path=/HelloJSP/; HttpOnly Content-Type: text/html;charset=UTF-8 Content-Length: 215 Date: Fri, 18 Sep 2015 08:09:58 GMT <!DOCTYPE html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Hello Servlet</title> </head> <body> <h1>你好</h1> </body> </html> ~~~ ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#css等静态文件放在什么地方)CSS等静态文件放在什么地方在项目下建立static目录，再这个目录下添加`test.js`，内容如下： ~~~ console.log("hello world"); ~~~ 在`web.xml`添加以下内容： ~~~ <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.jpg</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.png</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.js</url-pattern> </servlet-mapping> <servlet-mapping> <servlet-name>default</servlet-name> <url-pattern>*.css</url-pattern> </servlet-mapping> ~~~ 此时，项目结构如下： [![](https://github.com/someus/another-tutorial-about-java-web/raw/master/img/00-04/02.png)](https://github.com/someus/another-tutorial-about-java-web/blob/master/img/00-04/02.png) 启动项目，访问 ~~~ $ curl -i http://localhost:8084/HelloJSP/static/test.js HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytes ETag: W/"27-1442566151000" Last-Modified: Fri, 18 Sep 2015 08:49:11 GMT Content-Type: application/javascript Content-Length: 27 Date: Fri, 18 Sep 2015 08:58:00 GMT console.log("hello world"); ~~~ ~~~ $ curl -i http://localhost:8084/HelloJSP/static/test.js?time=123 HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytes ETag: W/"27-1442566151000" Last-Modified: Fri, 18 Sep 2015 08:49:11 GMT Content-Type: application/javascript Content-Length: 27 Date: Fri, 18 Sep 2015 08:58:09 GMT console.log("hello world"); ~~~ ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#servlet如何返回json数据)Servlet如何返回JSON数据将 ~~~ response.setContentType("text/html;charset=UTF-8"); ~~~ 修改为 ~~~ response.setContentType("application/json;charset=UTF-8"); ~~~ 。 `out.println`输出JSON格式的字符串即可。 ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#编写dispatcher)编写Dispatcher 基于以上的学习，已经可以编写一个分发器了。将HelloServlet.java修改为DispatcherServlet.java，内容修改为： ~~~ package me.letiantian.servlet; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/") public class HelloServlet extends HttpServlet { protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/plain;charset=UTF-8"); try (PrintWriter out = response.getWriter()) { out.println("context: " + request.getContextPath()); out.println("request uri: " + request.getRequestURI()); out.println("params: " + request.getParameterMap()); } } @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } } ~~~ 运行项目，访问结果如下： ~~~ $ curl http://localhost:8084/HelloJSP/user context: /HelloJSP request uri: /HelloJSP/user params: {} ~~~ ~~~ $ curl http://localhost:8084/HelloJSP/user?name=letian context: /HelloJSP request uri: /HelloJSP/user params: {name=[Ljava.lang.String;@49ea47b4} ~~~ ~~~ $ curl http://localhost:8084/HelloJSP/static/test.js console.log("hello world"); ~~~ （这个代码并没什么用～）从这段代码中可以看到，我们可以通过request对象得到HTTP请求信息，特别是request URI。在这个程序的基础上，我们可以继续扩充它，使得其遇到某个URI，就调用指定的处理函数。慢慢地补充，一个框架就出来了。 ## [](https://github.com/someus/another-tutorial-about-java-web/blob/master/00-04.md#资料)资料本节中，JSP使用了表达式语言，可以参考： [JSTL 入门: 表达式语言](http://www.ibm.com/developerworks/cn/java/j-jstl0211/index.html) [JSP 表达式语言](http://www.runoob.com/jsp/jsp-expression-language.html)