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JSP中的Servlet及Filter

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     @Override15     public void destroy() {16         // TODO Auto-generated method stub17 18     }19 20     @Override21     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     @Override28     public void init(FilterConfig arg0) throws ServletException {29         // TODO Auto-generated method stub30 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     @Override17     public void destroy() {18 19     }20 21     @Override22     public void doFilter(ServletRequest reqeust, ServletResponse response,23             FilterChain chain) throws IOException, ServletException {24         response.getWriter().append(25                 "<h1>AnotherFilter.doFilter is called!" + someParamter26                         + "</h1>");27         chain.doFilter(reqeust, response);28     }29 30     @Override31     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     @Autowired17     SampleData someData;18 19     @Override20     public void destroy() {21 22     }23 24     @Override25     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     @Override34     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 =http://www.mamicode.com/ 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

JSP中的Servlet及Filter