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 }
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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>
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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 }
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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>
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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 }
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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>
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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 }
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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 }
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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>
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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>
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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 }
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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 }
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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 }
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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>
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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>
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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。
时间： 2024-10-01 10:20:41