1. 引言
Filter是拦截Request请求的对象,在用户的请求访问资源前处理ServletRequest以及ServletResponse,他可用于日志记录、加解密、Session检查、图像文件保护等。
2. Filter API
Filter相关接口包括Filter、FilterConfig、FilterChain。
2.1 Filter接口
包含三个生命周期init、doFilter、destroy,servlet容器初始化Filter时,会触发Filter的init方法,FilterConfig实例由Servlet容器传入init方法中。
void init(FilterConfig filterConfig)
Servlet容器每次处理Filter相关资源时,会调用相关Filter实例的doFilter方法
void doFilter(ServletRequest request, Servlet Response, FilterChanin filterChain)
在Filter的doFilter的实现中,最后一行需要调用FilterChain中的doChain方法
filterChain.doFilter(request, response)
一个资源可能被多个Filter关联到(Filter链条),这时Filter.doFilter()方法将触发Filter链条中下一个Filter。只有在Filter链条中的最后一个Filter里调用FilterChain.doFilter()才会触发处理资源方法,否则Request请求终止。
Filter接口最后一个方法是destroy,在servlet容器要销毁时触发
void destroy();
3. Filter配置
Filter配置需要如下步骤:
1. 确认哪些资源需要使用这个Filter拦截器
2. 配置Filter的初始化参数值
3. 给Filter取一个名称,方便识别
FilterConfig接口允许通过他的getServletContext方法访问ServletContext
ServletContext getServletContext(); //返回ServletContext
java.lang.String getFilterName(); //返回Filter名字
java.util.Enumeration<java.lang.String> getInitParameterNames() //返回初始化参数名
java.lang.String getInitParameter(java.lang.String parameterName)//返回参数
配置方式有两种,一种是@WebFilter注解方法,另一种是通过XML注册方式,以@WebFilter注解为例,下面是可选参数
属性 | 描述 |
---|---|
asyncSupported | Filter是否支持异步操作 |
description | Filter描述 |
dispatcherTypes | Filter生效范围 |
displayName | Filter显示名 |
filterName | Filter名称 |
initParams | Filter初始化参数 |
urlPatterns | Filter生效的URL路径 |
value | Filter生效的URL路径 |
@WebFilter(filterName = "Security Filter", urlPatterns = "/*",
initParams = {
@WebInitParam(name = "A", value = "1")
@WebInitParam(name = "B", value = "2")
})
4. 示例程序
package config.filter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;
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.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServletRequest;
/**
* 日志监听器,记录每天的访问情况
* @author littlemotor
* @since 18.8.22
*/
@WebFilter(filterName = "LoggingFilter",
urlPatterns = {"/*"},
initParams = {
@WebInitParam(name = "logFileName", value = "log.txt")
})
public class LoggingFilter implements Filter{
private PrintWriter viewLogger;
private PrintWriter counterLogger;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
//String logFileName = filterConfig.getInitParameter("logFileName");
//String appPath = filterConfig.getServletContext().getRealPath("/");
//System.out.println(appPath);
try {
viewLogger = new PrintWriter(new File("/Users/littlemotor/webLog/","log.txt"));
} catch (FileNotFoundException e) {
e.printStackTrace();
}
}
/**
* Filter链,注意在最后加doFilter
*/
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
HttpServletRequest httpRequest = (HttpServletRequest)request;
viewLogger.println(new Date() + " " + "URI: " + httpRequest.getRequestURI() );
viewLogger.flush();
chain.doFilter(request, response);
}
@Override
public void destroy() {
System.out.println("destroying filter");
if(viewLogger != null) {
viewLogger.close();
}
}
}