javaweb的三大组件之一。

Filter过滤器（/拦截器）是在URL对一组资源（servlet,html,jsp,css,js,jpg等）的请求之前，当满足条件后发行。

1. default void init(FilterConfig filterConfig)
2. void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
3. default void destroy()

1. public FilterConfig getFilterConfig()
2. public ServletContext getServletContext()
3. public void init(FilterConfig filterConfig) throws ServletException {
   this.filterConfig = filterConfig;
   this.init();
   }
4. public void init() throws ServletException

1. public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain){ this.doFilter((HttpServletRequest)request, (HttpServletResponse)response, chain);}
2. protected void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException, ServletException {
   chain.doFilter(request, response); //放行
   }

因此我们可以参照Servlet方法，继承HttpFilter即可，重写其中的doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)方法。

Filter的执行过程：

Filter的执行顺序：

注解方式：

@WebFilter(filterName = "RequestFilter",urlPatterns = "/*",dispatcherTypes = {DispatcherType.REQUEST})

拦截方式

在web.xml中设置错误页

<error-page>    
  <error-code>404</error-code> 
	<location>/404.html</location>
</error-page>

测试代码：

1.MyServlet AbsServlet见上篇

@WebServlet(name = "MyServlet",urlPatterns = {"/error","/add","/delete"})
public class MyServlet extends AbsServlet {  
  public String add(HttpServletRequest req,HttpServletResponse res){    
    System.out.println("this is add");      
    //转发     
    return "forward:/delete";   
  }    
  public String delete(HttpServletRequest req,HttpServletResponse res){      
    System.out.println("this is delete");  
    return "这是this is delete";  
  }   
  public void error (HttpServletRequest req,HttpServletResponse res)throws Exception { 
    //错误重定向到错误页     
    System.out.println("this is error");    
    res.sendRedirect(req.getContextPath()+"/404.html");  
  } 
}

2.ForwardFilter

//转发至/delete的请求进行拦截
@WebFilter(filterName = "ForwardFilter",urlPatterns = "/delete",    dispatcherTypes = {DispatcherType.FORWARD})
public class ForwardFilter extends HttpFilter {    
  @Override   
  public void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException, ServletException {      
    System.out.println("FORWARD Filter");      
    chain.doFilter(request,response); 
  }
}

3.ErrorFilter

//错误拦截，错误页在web.xml中设置
@WebFilter(filterName = "ErrorFilter",urlPatterns = "/*",    dispatcherTypes = {DispatcherType.ERROR})
public class ErrorFilter extends HttpFilter {    
  @Override 
  public void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws IOException, ServletException {     
    System.out.println("Error Filter");    
    chain.doFilter(request,response); 
  }
}

4.EncodingFilter

//拦截所有请求进行编码设计，为保证为第一个拦截器，在filterName前加了AAA
//DispatcherType.REQUEST为默认方式，可以不设置
@WebFilter(filterName = "AAAEncodingFilter",urlPatterns = "/*",  dispatcherTypes={DispatcherType.REQUEST})
public class EncodingFilter extends HttpFilter {   
  @Override    public void doFilter(HttpServletRequest req, HttpServletResponse resp, FilterChain chain) throws ServletException, IOException {        
    System.out.println("EncodingFilter");    
    req.setCharacterEncoding("utf-8");    
    resp.setContentType("text/html;charset=utf-8");  
    chain.doFilter(req, resp); 
  }
}

5.web.xml

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"     
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"      
xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"   
version="4.0">
  <error-page>    
  <!--错误编码404,错误页404.html--> 
  <error-code>404</error-code> 
	<location>/404.html</location>
  </error-page>
</web-app>

当浏览器进行：http: //localhost:8088/javaweb/add 时，会转发请求/delete，同时执行拦截器ForwardFilter

http: //localhost:8088/javaweb/add1 /add1不存在，执行错误拦截器ErrorFilter,转向错误页面404.html

关于编码的设置

//自定义类将HttpRequest进行包装，更新方法getParameter
//方法getParameter重写req.getMethod为get方式时的转换
public class EncodingRequest extends HttpServletRequestWrapper{   
  private HttpServletRequest req = null;  
  public EncodingRequest(HttpServletRequest req) {      
    super(req);     
    this.req = req; 
  }   
  @Override   
  public String getParameter(String s)  {       
    String result = req.getParameter(s);   
    try {     
      result = new String(result.getBytes("ISO-8859-1"),"utf-8");    
    } catch (UnsupportedEncodingException e) {      
      e.printStackTrace();     
    }      
    return result;  
  }
}

@WebFilter(filterName = "AAAEncodingFilter",urlPatterns = "/*", dispatcherTypes={DispatcherType.REQUEST})
public class EncodingFilter extends HttpFilter {  
  @Override    
  public void doFilter(HttpServletRequest req, HttpServletResponse resp, FilterChain chain) throws ServletException, IOException {        
    //请求为post和请求为get时的编码设置        
    if(req.getMethod().equals("post")){         
      req.setCharacterEncoding("utf-8");    
    } else if (req.getMethod().equals("get")) {     
      //req转为EncodingRequest类型，执行的是自定义类中的方法   
      req = (EncodingRequest)req;    
    }        
    System.out.println("EncodingFilter");     
    //响应编码      
    resp.setContentType("text/html;charset=utf-8");      
    chain.doFilter(req, resp);  
  }
}