Filter内容
1.利用Filter来过滤的时候大都是Http请求和Http响应,在doFilter()方法中,参数类是ServletRequest和ServletResponse ,使用的时候一般需要强制转换为HttpServletRequest和HttpServletResponse 。针对这个问题,可以仿照GenericServlet 的做法,来定义一个HttpFilter,用作所有Filter的父类。
HttpFilter.java
package com.javaweb.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 javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse; public abstract class HttpFilter implements Filter {
@Override
public void destroy() { }
/*
* 原生的doFilter方法,在方法内部把ServletRequest和ServletResponse转为了HttpServletRequest和HttpServletResponse,
* 并调用了doFilter(HttpServletRequest request,HttpServletResponse response,FilterChain filterchain)方法。
* 若编写Filter的过滤方法不建议直接继承该方法,而建议继承doFilter(HttpServletRequest request,HttpServletResponse response,
* FilterChain filterchain)方法。
* */
@Override
public void doFilter(ServletRequest req, ServletResponse resp, FilterChain filterchain)
throws IOException, ServletException {
HttpServletRequest request=(HttpServletRequest) req;
HttpServletResponse response=(HttpServletResponse) resp;
doFilter(request,response,filterchain);
}
//抽象方法,为Http请求定制必须实现的方法。
public abstract void doFilter(HttpServletRequest request,HttpServletResponse response,FilterChain filterchain) throws IOException, ServletException;
private FilterConfig filterconfig;
//不建议子类直接覆盖,若直接覆盖,将可能会导致filterConfig成员变量初始化失败。
@Override
public void init(FilterConfig filterconfig) throws ServletException {
this.filterconfig =filterconfig;
init();
}
//供子类继承的初始化方法,可以通过getFilterConfig()获取FilterConfig对象。
public void init(){}
//直接返回init(ServletConfig)的ServletConfig对象。
public FilterConfig getFilterConfig(){
return filterconfig;
}
}
2.理解多个Filter代码的执行顺序
示例代码:
index.html
<body>
<a href="http://localhost:8080/MyWebProject/jspTest/test.jsp">to test jsp page</a>
</body>
test.jsp
<body>
<%
System.out.println("5. Test page");
%>
</body>
过滤器拦截:
filterTest.java
package com.javaweb.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 filterTest implements Filter {
@Override
public void destroy() {
System.out.println("destroy...");
} @Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
System.out.println("1. before filterTest's chain.doFilter...");
chain.doFilter(request,response);
System.out.println("2. after filterTest's chain.doFilter...");
} @Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("init...");
} }
secondFilter.java
package com.javaweb.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 secondFilter implements Filter {
@Override
public void destroy() {
System.out.println("secondFilter's destroy..."); } @Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
System.out.println("3. before secondFilter's chain doFilter...");
chain.doFilter(request,response);
System.out.println("4. after secondFilter's chain doFilter...");
} @Override
public void init(FilterConfig arg0) throws ServletException {
System.out.println("secondFilter's init..."); } }
在web.xml中进行配置:
<filter>
<filter-name>filterTest</filter-name>
<filter-class>com.javaweb.Filter.filterTest</filter-class>
</filter>
<filter-mapping>
<filter-name>filterTest</filter-name>
<url-pattern>/jspTest/test.jsp</url-pattern>
</filter-mapping>
<filter>
<filter-name>secondFilter</filter-name>
<filter-class>com.javaweb.Filter.secondFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>secondFilter</filter-name>
<url-pattern>/jspTest/test.jsp</url-pattern>
</filter-mapping>
执行顺序:
index.html页面点击链接时去test.jsp页面,被第一个过滤器filterTest拦截,
①首先执行其中的doFilter()方法,执行方法体中的System.out.println("1. before filterTest's chain.doFilter...");
②之后chain.doFilter(request,response);将拦截传递给下一个过滤器secondFilter,执行其中的doFilter()方法,同样执行方法体中的System.out.println("3. before secondFilter's chain doFilter...");
③接着chain.doFilter(request,response);因为没有过滤器,所以将请求转发到test.jsp,执行<% System.out.println("5. Test page"); %>;
④接下来执行方法体中的System.out.println("4. after secondFilter's chain doFilter...");
⑤最后回到filterTest的doFilter()方法中,执行System.out.println("2. after filterTest's chain.doFilter...");
3.<dispatcher>
<dispatcher></dispatcher>指定过滤器所拦截的资源被Servlet容器调用的方式,可以是REQUEST,INCLUDE,FORWARD和ERROR之一,默认为REQUEST。可以设置多个<dispatcher>子元素来指定Filter对资源的多种调用方式进行拦截。
<dispatcher>子元素可以设置的值及其意义:
REQUEST:当用户直接访问页面时,Web容器将会调用过滤器。如果目标资源是通过RequestDispatcher的include()或forward()方法访问时,那么该过滤器将不会被调用。
INCLUDE:如果目标资源是通过RequestDispatcher的include()方法访问时,那么该过滤器将会被调用,除此之外,该过滤器不会被调用。
FORWARD:如果目标资源是通过RequestDispatcher的forward()方法访问时,那么该过滤器将会被调用,除此之外,该过滤器不会被调用。
ERROR:如果目标资源是通过声明式异常处理机制调用时,那么该过滤器将会被调用,除此之外,过滤器不会被调用。
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