JSP中的Servlet及Filter

菩提树下的杨过

发布于 2018-01-24 11:46:15

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发布于 2018-01-24 11:46:15

asp.net中，如果开发人员想自己处理http请求响应，可以利用HttpHandler来满足这一要求；类似的，如果要拦截所有http请求，可以使用HttpMoudle。java的web开发中，也有类似的处理机制，与HttpHandler应对的是HttpServlet，与HttpModule对应的则是Filter。

一、HttpServlet

先看一个简单的示例：

 1 package com.cnblogs.yjmyzz.servlet;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.ServletException;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 
10 public class SampleServlet extends HttpServlet {
11 
12     private static final long serialVersionUID = 7065409287377444221L;
13 
14     public SampleServlet(){
15         System.out.println("SampleServlet is initialized!");
16     }
17     
18     protected void doGet(HttpServletRequest request,
19             HttpServletResponse response) throws ServletException, IOException {
20 
21         response.getWriter().append("<h1>SampleServlet.doGet() is called!</h1>");
22 
23     }
24 
25     protected void doPost(HttpServletRequest request,
26             HttpServletResponse response) throws ServletException, IOException {
27 
28         response.getWriter()
29                 .append("<h1>SampleServlet.doPost() is called!</h1>");
30 
31     }
32 
33 }

在HttpServlet中，程序员得自己控制所有要在页面上输出的内容，类似ASP.NET HttpHandler中Response.Write(...)一样。

自定义的Servlet必须在web.xml中注册才能使用，参考下面的配置片段：

1     <servlet>
2         <servlet-name>Sample</servlet-name>
3         <servlet-class>com.cnblogs.yjmyzz.servlet.SampleServlet</servlet-class>
4         <load-on-startup>1</load-on-startup>
5     </servlet>
6     <servlet-mapping>
7         <servlet-name>Sample</servlet-name>
8         <url-pattern>/A/*</url-pattern>
9     </servlet-mapping>

第2行与第7行的servlet-name要一致；url-pattern表示该Servlet要拦截的url，如果写成"/*"，则表示拦截所有url请求；load-on-startup是可选节点，如果该节点值>0时，webapp一启动就会自动实例化该Servlet，否则将延时到第一次访问被拦截的url时，才会被实例化。

如果web.xml中同时注册了多个Servlet，且都指定了load-on-startup，将按照load-on-startup节点值从小到大的优先级顺序，依次实例化所有注册的Servlet。

如果多个Servlet同时拦截了相同的url，则根据它们出现在web.xml中的顺序，仅最后出现的Servlet具有拦截处理权。

二、Filter

还是先来一个最基本的示例

 1 package com.cnblogs.yjmyzz.filter;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.Filter;
 6 import javax.servlet.FilterChain;
 7 import javax.servlet.FilterConfig;
 8 import javax.servlet.ServletException;
 9 import javax.servlet.ServletRequest;
10 import javax.servlet.ServletResponse;
11 
12 public class AnotherFilter implements Filter {
13 
14     @Override
15     public void destroy() {
16         // TODO Auto-generated method stub
17 
18     }
19 
20     @Override
21     public void doFilter(ServletRequest reqeust, ServletResponse response,
22             FilterChain chain) throws IOException, ServletException {
23         response.getWriter().append("<h1>AnotherFilter.doFilter is called!</h1>");
24         chain.doFilter(reqeust, response);
25     }
26 
27     @Override
28     public void init(FilterConfig arg0) throws ServletException {
29         // TODO Auto-generated method stub
30 
31     }
32 
33 }

注意下24行，开发人员自定义的处理完成后，最后记得调用chain.doFilter(reqeust, response)，因为每一次http请求的完整处理通常会有很多个Filter按顺序协作完成，这些Filter形成一个”链式结构“，这一行的作用，就是当自己的处理完成后，继续交给Filter链中的下一个Filter去处理。

同样，Filter也必须在web.xml中注册方能使用：

1     <filter>
2         <filter-name>Filter2</filter-name>
3         <filter-class>com.cnblogs.yjmyzz.filter.AnotherFilter</filter-class>
4     </filter>
5     <filter-mapping>
6         <filter-name>Filter2</filter-name>
7         <url-pattern>/*</url-pattern>
8     </filter-mapping>

第2行与第6行的filter-name要保持一致；url-pattern为要拦截的url；如果一个web.xml中同时注册多个Filter，所有这些Filter都将起作用，处理的顺序按照在web.xml中出现的顺序，先出现的Filter先处理。

如果web.xml中同时注册了Servlet、Filter，且拦截的url相同时，Filter先处理，之后才轮到Servlet处理。

三、参数注入

通常在写Servlet、Filter时，有时候需要从外界获取一些参数，先来看下Filter的参数处理：

a) Filter基本String参数注入

 1 package com.cnblogs.yjmyzz.filter;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.Filter;
 6 import javax.servlet.FilterChain;
 7 import javax.servlet.FilterConfig;
 8 import javax.servlet.ServletException;
 9 import javax.servlet.ServletRequest;
10 import javax.servlet.ServletResponse;
11 
12 public class AnotherFilter implements Filter {
13     // 定义参数变量
14     private String someParamter;
15 
16     @Override
17     public void destroy() {
18 
19     }
20 
21     @Override
22     public void doFilter(ServletRequest reqeust, ServletResponse response,
23             FilterChain chain) throws IOException, ServletException {
24         response.getWriter().append(
25                 "<h1>AnotherFilter.doFilter is called!" + someParamter
26                         + "</h1>");
27         chain.doFilter(reqeust, response);
28     }
29 
30     @Override
31     public void init(FilterConfig cfg) throws ServletException {
32         // 取得传入的参数
33         someParamter = cfg.getInitParameter("someParameter");
34 
35     }
36 
37 }

代码很简单，在init方法中接收参数即可，这个参数是从哪里传进来的呢？看下面的web.xml配置

1     <filter>
2         <filter-name>Filter2</filter-name>
3         <filter-class>com.cnblogs.yjmyzz.filter.AnotherFilter</filter-class>
4         <init-param>
5             <param-name>someParameter</param-name>
6             <param-value>HelloWorld</param-value>
7         </init-param>
8     </filter>

init-param节点就是答案

b) Filter复杂对象的参数注入

如果要传的参数是一个复杂对象，上面的方法就不太适合（当然：你可以把对象序列化成json字符串，然后到init中接收，再反序列，理论上也可行，但是比较感觉比较怪。）

先定义一个参数对象：

 1 package com.cnblogs.yjmyzz.filter;
 2 
 3 public class SampleData {
 4     
 5     private String someField;
 6 
 7     public String getSomeField() {
 8         return someField;
 9     }
10 
11     public void setSomeField(String someField) {
12         this.someField = someField;
13     }
14 
15 }

为了对比，再来一个Filter

 1 package com.cnblogs.yjmyzz.filter;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.Filter;
 6 import javax.servlet.FilterChain;
 7 import javax.servlet.FilterConfig;
 8 import javax.servlet.ServletException;
 9 import javax.servlet.ServletRequest;
10 import javax.servlet.ServletResponse;
11 
12 import org.springframework.beans.factory.annotation.Autowired;
13 
14 public class SampleFilter implements Filter {
15 
16     @Autowired
17     SampleData someData;
18 
19     @Override
20     public void destroy() {
21 
22     }
23 
24     @Override
25     public void doFilter(ServletRequest reqeust, ServletResponse response,
26             FilterChain chain) throws IOException, ServletException {
27         response.getWriter().append(
28                 "<h1>SampleFilter.doFilter is called!"
29                         + someData.getSomeField() + "</h1>");
30         chain.doFilter(reqeust, response);
31     }
32 
33     @Override
34     public void init(FilterConfig filterConfig) throws ServletException {
35 
36     }
37 
38     public SampleData getSomeData() {
39         return someData;
40     }
41 
42     public void setSomeData(SampleData someData) {
43         this.someData = someData;
44     }
45 
46 }

这里，我们希望SomeFilter在运行时，能动态注入一个SomeData实例。下面是配置部分：

 1 <?xml version="1.0" encoding="UTF-8"?>
 2 <beans xmlns="http://www.springframework.org/schema/beans"
 3     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 4     xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
 5 
 6     <bean id="someData" class="com.cnblogs.yjmyzz.filter.SampleData">
 7         <property name="someField" value="abc"></property>
 8     </bean>
 9 
10     <bean id="sampleFilter" class="com.cnblogs.yjmyzz.filter.SampleFilter">
11         <property name="someData" ref="someData"></property>
12     </bean>
13 
14 </beans>

spring的xml配置中，先定义好SomeFilter的bean，然后是web.xml的Filter配置：

 1     <filter>
 2         <description>Filter1</description>
 3         <display-name>Filter1</display-name>
 4         <filter-name>Filter1</filter-name>
 5         <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
 6         <init-param>
 7             <param-name>targetBeanName</param-name>
 8             <param-value>sampleFilter</param-value>
 9         </init-param>
10     </filter>
11 
12     <filter-mapping>
13         <filter-name>Filter1</filter-name>
14         <url-pattern>/*</url-pattern>
15     </filter-mapping>

对比下刚才的Filter配置，有几个变化：

filter-class 换成了 org.springframework.web.filter.DelegatingFilterProxy

init-param 节点通过targetBeanName 这个参数名，将sampleFilter bean动态注入

再来看看Servlet的参数注入，spring并没有提供类似DelegatingServletProxy的代理类，所以只能自己动手了，下面是二种常见做法：

a) 通过init方法，实现Servlet的Spring bean注入

 1 package com.cnblogs.yjmyzz.servlet;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.*;
 6 import javax.servlet.http.*;
 7 import org.springframework.web.context.WebApplicationContext;
 8 import org.springframework.web.context.support.WebApplicationContextUtils;
 9 
10 import com.cnblogs.yjmyzz.filter.SampleData;
11 
12 public class SampleServlet extends HttpServlet {
13 
14     private static final long serialVersionUID = 7065409287377444221L;
15 
16     SampleData someData;
17 
18     public SampleServlet() {
19         System.out.println("SampleServlet is initialized!");
20     }
21 
22     protected void doGet(HttpServletRequest request,
23             HttpServletResponse response) throws ServletException, IOException {
24 
25         response.getWriter().append(
26                 "<h1>SampleServlet.doGet() is called!"
27                         + someData.getSomeField() + "</h1>");
28 
29     }
30 
31     protected void doPost(HttpServletRequest request,
32             HttpServletResponse response) throws ServletException, IOException {
33 
34         response.getWriter().append(
35                 "<h1>SampleServlet.doPost() is called!</h1>");
36 
37     }
38 
39     public void init() throws ServletException {
40         super.init();
41         ServletContext servletContext = this.getServletContext();
42         WebApplicationContext ctx = WebApplicationContextUtils
43                 .getWebApplicationContext(servletContext);
44         someData = ctx.getBean("someData", SampleData.class);
45     }
46 }

关键在于init方法，通过Spring的WebApplicationContext拿到上下文，然后手动去获取bean实例

b) 自己实现ServletProxy，实现注入

先定义ServletProxy代理类：

 1 package com.cnblogs.yjmyzz.servlet;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.*;
 6 import javax.servlet.http.HttpServlet;
 7 
 8 import org.springframework.web.context.WebApplicationContext;
 9 import org.springframework.web.context.support.WebApplicationContextUtils;
10 
11 public class HttpServletProxy extends HttpServlet {
12 
13     private static final long serialVersionUID = 4358391761577767574L;
14 
15     private String targetBean;
16     private HttpServlet proxy;
17 
18     public void service(ServletRequest req, ServletResponse res)
19             throws ServletException, IOException {
20         proxy.service(req, res);
21     }
22 
23     public void init() throws ServletException {
24         this.targetBean = getServletName();
25         getServletBean();
26         proxy.init(getServletConfig());
27     }
28 
29     private void getServletBean() {
30         WebApplicationContext wac = WebApplicationContextUtils
31                 .getRequiredWebApplicationContext(getServletContext());
32         this.proxy = (HttpServlet) wac.getBean(targetBean);
33     }
34 
35 }

本质上ServletProxy也是一个Servlet，在init方法中，通过动态获取servletName，利用Spring的WebApplicationContextt得到真正需要的Servlet Bean实例并保存在proxy变量中，最终对http执行处理的（即：调用service方法的），是proxy变量所指向的Servlet Bean实例。

定义真正需要使用的Servlet

 1 package com.cnblogs.yjmyzz.servlet;
 2 
 3 import java.io.IOException;
 4 
 5 import javax.servlet.ServletException;
 6 import javax.servlet.http.HttpServlet;
 7 import javax.servlet.http.HttpServletRequest;
 8 import javax.servlet.http.HttpServletResponse;
 9 import com.cnblogs.yjmyzz.filter.SampleData;
10 
11 public class AnotherServlet extends HttpServlet {
12 
13     private static final long serialVersionUID = -3797187540470927379L;
14 
15     // 需要注入的Bean
16     SampleData someData;
17 
18     public AnotherServlet() {
19         System.out.println("AnotherServlet is initialized!");
20     }
21 
22     protected void doGet(HttpServletRequest request,
23             HttpServletResponse response) throws ServletException, IOException {
24 
25         response.getWriter().append(
26                 "<h1>AnotherServlet.doGet() is called!"
27                         + someData.getSomeField() + "</h1>");
28 
29     }
30 
31     protected void doPost(HttpServletRequest request,
32             HttpServletResponse response) throws ServletException, IOException {
33 
34         response.getWriter().append(
35                 "<h1>AnotherServlet.doPost() is called!</h1>");
36 
37     }
38 
39     public void setSomeData(SampleData someData) {
40         this.someData = someData;
41     }
42 
43 }

在spring的beans配置文件中，配置该Servlet Bean

1     <bean id="someData" class="com.cnblogs.yjmyzz.filter.SampleData">
2         <property name="someField" value="abc"></property>
3     </bean>
4 
5     <bean id="anotherServlet" class="com.cnblogs.yjmyzz.servlet.AnotherServlet">
6         <property name="someData" ref="someData"></property>
7     </bean>

最后是web.xml配置

1     <servlet>
2         <servlet-name>anotherServlet</servlet-name>
3         <servlet-class>com.cnblogs.yjmyzz.servlet.HttpServletProxy</servlet-class>
4         <load-on-startup>1</load-on-startup>
5     </servlet>
6     <servlet-mapping>
7         <servlet-name>anotherServlet</servlet-name>
8         <url-pattern>/A/*</url-pattern>
9     </servlet-mapping>

注：web.xml中的servlet-name节点值，必须于spring beans配置文件中的bean id一致，因为ServletProxy是根据ServletName来查找Bean实例的。

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原始发表：2014-08-24 ，如有侵权请联系 cloudcommunity@tencent.com 删除

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