过滤器

JavaWeb三大组件：Servlet、Listener(2个感知监听器不需要配置)、Filter，都需要在web.xml中进行配置

过滤器的作用是拦截请求，它会在一组资源（jsp、servlet、.css、.html等等）的前面执行！

过滤器如何编写

1. 写一个类实现Filter接口

2. 在web.xml中进行配置

package 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 AFilter implements Filter {

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

@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
// TODO Auto-generated method stub
//chain.doFilter(request, response);  //放行
}

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

}

<filter>
<filter-name>Afilter</filter-name>
<filter-class>filter.AFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>Afilter</filter-name>
<url-pattern>/AServlet</url-pattern>
</filter-mapping>

Filter接口

void init(FilterConfig)

  * 创建之后，马上执行；Filter会在服务器启动时就创建！

  *FilterConfig有三个方法，作用是获取参数，获取过滤器名，获取appliction

        **getInitParameter()

        ** getFilterName()

       **getServletContext()

void destory()

  * 销毁之前执行！在服务器关闭时销毁

void doFilter(ServletRequest,ServletResponse,FilterChain)

 * 每次过滤时都会执行

*  FilterChain有一个方法，doFilter(ServletRequest, ServletResponse)： 放行，就相当于调用了目标Servlet的service()方法！

Filter是单例的！

过滤器的四种拦截方式

  <dispatcher>REQUEST</dispatcher>默认的！

  <dispatcher>FORWARD</dispatcher>

  <dispatcher>INCLUDE</dispatcher>

  <dispatcher>ERROR</dispatcher>

在<filter-mapping>中进行配置!

举个栗子：

 <filter>
<filter-name>Afilter</filter-name>
<filter-class>filter.AFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>Afilter</filter-name>
<url-pattern>/AServlet</url-pattern>
</filter-mapping>

<filter>
<filter-name>Bfilter</filter-name>
<filter-class>filter.BFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>Bfilter</filter-name>
<url-pattern>/AServlet</url-pattern>
<dispatcher>FORWARD</dispatcher>
</filter-mapping>

public class AServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
System.out.println("AServlet...");
}

}

public class BServlet extends HttpServlet {

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

request.getRequestDispatcher("/AServlet").forward(request, response);
}

}

public class AFilter implements Filter {

@Override
public void destroy() {
// TODO Auto-generated method stub
System.out.println("过滤器死亡了");
}

@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
// TODO Auto-generated method stub
System.out.println("AServlet statrt！");
chain.doFilter(request, response);
System.out.println("AServlet end");
}

@Override
public void init(FilterConfig filterConfig) throws ServletException {
// TODO Auto-generated method stub
System.out.println("过滤器出生了");
}

}

public class BFilter implements Filter {

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

}

@Override
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
// TODO Auto-generated method stub
System.out.println("BServlet statrt！");
chain.doFilter(request, response);
System.out.println("BServlet end");
}

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

}

}

在地址栏输入http://localhost:8080/项目名/BServlet

运行结果：

BServlet statrt！

AServlet...

BServlet end

程序首先查看web.xml中的配置，AFilter拦截了AServlet，BFilter拦截了AServlet以及转发功能，BFilter拦截BServlet进行转发，然后放行，执行了AServlet

<filter-mapping>的配置顺序决定了过滤器的执行顺序！

过滤器的应用场景：

*执行目标资源之前做预处理工作，例如设置编码，这种试通常都会放行

*通过条件判断是否放行，例如校验当前用户是否已经登录，或者用户IP是否已经被禁用

*在目标资源执行后，做一些后续的特殊处理工作，例如把目标资源输出的数据进行处理