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理解Servlet

本文依然使用00-03、从JSP开始中创建的项目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

Hello Servlet

项目结构如下:

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。

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。

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);
    }

}

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>

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");

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:

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");

Servlet如何返回JSON数据

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。

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。