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：

package com.cnblogs.yjmyzz.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class SampleServlet extends HttpServlet {

    private static final long serialVersionUID = 7065409287377444221L;

    public SampleServlet(){
        System.out.println("SampleServlet is initialized!");
    }
    
    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        response.getWriter().append("<h1>SampleServlet.doGet() is called!</h1>");

    }

    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        response.getWriter()
                .append("<h1>SampleServlet.doPost() is called!</h1>");

    }

}

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：

    <servlet>
        <servlet-name>Sample</servlet-name>
        <servlet-class>com.cnblogs.yjmyzz.servlet.SampleServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>Sample</servlet-name>
        <url-pattern>/A/*</url-pattern>
    </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

package com.cnblogs.yjmyzz.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class AnotherFilter implements Filter {

    @Override
    public void destroy() {
        // TODO Auto-generated method stub

    }

    @Override
    public void doFilter(ServletRequest reqeust, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        response.getWriter().append("<h1>AnotherFilter.doFilter is called!</h1>");
        chain.doFilter(reqeust, response);
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub

    }

}

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：

    <filter>
        <filter-name>Filter2</filter-name>
        <filter-class>com.cnblogs.yjmyzz.filter.AnotherFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>Filter2</filter-name>
        <url-pattern>/*</url-pattern>
    </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

package com.cnblogs.yjmyzz.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class AnotherFilter implements Filter {
    // 定义参数变量
    private String someParamter;

    @Override
    public void destroy() {

    }

    @Override
    public void doFilter(ServletRequest reqeust, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        response.getWriter().append(
                "<h1>AnotherFilter.doFilter is called!" + someParamter
                        + "</h1>");
        chain.doFilter(reqeust, response);
    }

    @Override
    public void init(FilterConfig cfg) throws ServletException {
        // 取得传入的参数
        someParamter = cfg.getInitParameter("someParameter");

    }

}

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

    <filter>
        <filter-name>Filter2</filter-name>
        <filter-class>com.cnblogs.yjmyzz.filter.AnotherFilter</filter-class>
        <init-param>
            <param-name>someParameter</param-name>
            <param-value>HelloWorld</param-value>
        </init-param>
    </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：

package com.cnblogs.yjmyzz.filter;

public class SampleData {
    
    private String someField;

    public String getSomeField() {
        return someField;
    }

    public void setSomeField(String someField) {
        this.someField = someField;
    }

}

为了对比，再来一个Filter

package com.cnblogs.yjmyzz.filter;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import org.springframework.beans.factory.annotation.Autowired;

public class SampleFilter implements Filter {

    @Autowired
    SampleData someData;

    @Override
    public void destroy() {

    }

    @Override
    public void doFilter(ServletRequest reqeust, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {
        response.getWriter().append(
                "<h1>SampleFilter.doFilter is called!"
                        + someData.getSomeField() + "</h1>");
        chain.doFilter(reqeust, response);
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    public SampleData getSomeData() {
        return someData;
    }

    public void setSomeData(SampleData someData) {
        this.someData = someData;
    }

}

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：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="someData" class="com.cnblogs.yjmyzz.filter.SampleData">
        <property name="someField" value="abc"></property>
    </bean>

    <bean id="sampleFilter" class="com.cnblogs.yjmyzz.filter.SampleFilter">
        <property name="someData" ref="someData"></property>
    </bean>

</beans>

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

    <filter>
        <description>Filter1</description>
        <display-name>Filter1</display-name>
        <filter-name>Filter1</filter-name>
        <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
        <init-param>
            <param-name>targetBeanName</param-name>
            <param-value>sampleFilter</param-value>
        </init-param>
    </filter>

    <filter-mapping>
        <filter-name>Filter1</filter-name>
        <url-pattern>/*</url-pattern>
    </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

package com.cnblogs.yjmyzz.servlet;

import java.io.IOException;

import javax.servlet.*;
import javax.servlet.http.*;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;

import com.cnblogs.yjmyzz.filter.SampleData;

public class SampleServlet extends HttpServlet {

    private static final long serialVersionUID = 7065409287377444221L;

    SampleData someData;

    public SampleServlet() {
        System.out.println("SampleServlet is initialized!");
    }

    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        response.getWriter().append(
                "<h1>SampleServlet.doGet() is called!"
                        + someData.getSomeField() + "</h1>");

    }

    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        response.getWriter().append(
                "<h1>SampleServlet.doPost() is called!</h1>");

    }

    public void init() throws ServletException {
        super.init();
        ServletContext servletContext = this.getServletContext();
        WebApplicationContext ctx = WebApplicationContextUtils
                .getWebApplicationContext(servletContext);
        someData = ctx.getBean("someData", SampleData.class);
    }
}

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：

package com.cnblogs.yjmyzz.servlet;

import java.io.IOException;

import javax.servlet.*;
import javax.servlet.http.HttpServlet;

import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;

public class HttpServletProxy extends HttpServlet {

    private static final long serialVersionUID = 4358391761577767574L;

    private String targetBean;
    private HttpServlet proxy;

    public void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException {
        proxy.service(req, res);
    }

    public void init() throws ServletException {
        this.targetBean = getServletName();
        getServletBean();
        proxy.init(getServletConfig());
    }

    private void getServletBean() {
        WebApplicationContext wac = WebApplicationContextUtils
                .getRequiredWebApplicationContext(getServletContext());
        this.proxy = (HttpServlet) wac.getBean(targetBean);
    }

}

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

package com.cnblogs.yjmyzz.servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.cnblogs.yjmyzz.filter.SampleData;

public class AnotherServlet extends HttpServlet {

    private static final long serialVersionUID = -3797187540470927379L;

    // 需要注入的Bean
    SampleData someData;

    public AnotherServlet() {
        System.out.println("AnotherServlet is initialized!");
    }

    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        response.getWriter().append(
                "<h1>AnotherServlet.doGet() is called!"
                        + someData.getSomeField() + "</h1>");

    }

    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        response.getWriter().append(
                "<h1>AnotherServlet.doPost() is called!</h1>");

    }

    public void setSomeData(SampleData someData) {
        this.someData = someData;
    }

}

在spring的beans配置文件中，配置该Servlet Bean

    <bean id="someData" class="com.cnblogs.yjmyzz.filter.SampleData">
        <property name="someField" value="abc"></property>
    </bean>

    <bean id="anotherServlet" class="com.cnblogs.yjmyzz.servlet.AnotherServlet">
        <property name="someData" ref="someData"></property>
    </bean>

最后是web.xml配置

    <servlet>
        <servlet-name>anotherServlet</servlet-name>
        <servlet-class>com.cnblogs.yjmyzz.servlet.HttpServletProxy</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>anotherServlet</servlet-name>
        <url-pattern>/A/*</url-pattern>
    </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。