1、实例化
通过web.xml配置，web容器启动时就会加载过滤器，实例化只会实例化一次。Servlet3.0规范中新增了@WebFilter的方式，这种方式用于讲一个类声明为过滤器，该注解将会在部署时被web容器处理，容器将根据具体的属性配置将相应的类部署为过滤器。
2、初始化
调用过滤器的init()，这是过滤器的初始化方法，web容器创建过滤器实例后将调用这个方法。这个方法中可以读取web.xml文件中过滤器的参数。
3、过滤方法
每次请求都会调用过滤器的doFilter()，这个方法完成实际的过滤操作。这个是过滤器的核心方法，当用户请求访问与过滤器关联的URL时，web容器将先调用过滤器的doFilter()。FilterChain参数可以调用chain.doFilter()，将请求传递给下一个过滤器（或目标资源），或利用转发，重定向将请求转发到其他资源。
4、销毁
当web容器在销毁过滤器实例前调用destroy()，在这个方法中可以释放过滤器占用的资源。
过滤器的分类：
指定过滤器的转发模式，对应filter中的dispatchTypes属性，具体属性值包括以下5个，其中Servlet2.5支持REQUEST、FORWARD、INCLUDE、ERROR，Servlet3.0新增ASYNC
1、REQUEST
用户直接访问页面时，web容器将会调用过滤器，过滤器的默认种类。
2、FORWARD
目标资源是通过RequestDispatch的forward访问时，该过滤器将被调用。
3、INCLUDE
目标资源是通过RequestDispatch的include访问时，该过滤器将被调用。
4、ERROR
目标资源是通过声明式异常处理机制调用时，过滤器将被调用。
5、ASYNC
支持异步处理
过滤器示例：
web.xml
loginFilter com.pumpkin.filter.LoginFilter unfilteredPaths login.jsp;failure.jsp;login EncodingFilter com.pumpkin.filter.EncodingFilter charset UTF-8 loginFilter /* EncodingFilter /* LoginServlet com.pumpkin.servlet.LoginServlet LoginServlet /login login.jsp
login.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
Filter实例
<%
System.out.println("===login.jsp===");
%>
success.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
成功页面
<%
System.out.println("===success.jsp===");
%>
${username}，登陆成功！
failure.jsp
<%--
Created by IntelliJ IDEA.
User: Administrator
Date: 2017/6/17
Time: 22:47
To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
失败页面
<%
System.out.println("===failure.jsp===");
%>
登录失败！
LoginServlet.java
package com.pumpkin.servlet;
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
public class LoginServlet extends HttpServlet {
public LoginServlet() {
super();
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("===LoginServlet===doGet===");
doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("===LoginServlet===doPost===start===");
String username = req.getParameter("username");
String password = req.getParameter("password");
System.out.println("===username===" + username + "===");
if ("admin".equals(username) && "admin".equals(password)) {
HttpSession session = req.getSession();
session.setAttribute("username",username);
resp.sendRedirect(req.getContextPath() + "/success.jsp");
}else {
resp.sendRedirect(req.getContextPath() + "/failure.jsp");
}
System.out.println("===LoginServlet===doPost===end===");
}
}
LoginFilter.java
package com.pumpkin.filter;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.util.logging.Filter;
public class LoginFilter implements javax.servlet.Filter {
private FilterConfig config;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("===LoginFilter===init===");
config = filterConfig;
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
System.out.println("===LoginFilter===doFilter===start===");
HttpServletRequest request = (HttpServletRequest) servletRequest;
HttpServletResponse response = (HttpServletResponse) servletResponse;
HttpSession session = request.getSession();
String unfilteredPaths = config.getInitParameter("unfilteredPaths");
if (unfilteredPaths != null) {
String[] strArray = unfilteredPaths.split(";");
for (int i = 0; i < strArray.length; i++) {
if (strArray[i] == null || "".equals(strArray[i])) {
continue;
}
if (request.getRequestURI().indexOf(strArray[i]) != -1) {
filterChain.doFilter(request, response);
return;
}
}
}
if (session.getAttribute("username") != null) {
filterChain.doFilter(request, response);
} else {
response.sendRedirect("login.jsp");
}
System.out.println("===LoginFilter===doFilter===end===");
}
@Override
public void destroy() {
System.out.println("===LoginFilter===destroy===");
}
}
EncodingFilter.java
package com.pumpkin.filter;
import com.sun.net.httpserver.Filter;
import com.sun.net.httpserver.HttpExchange;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import java.io.IOException;
public class EncodingFilter implements javax.servlet.Filter{
private FilterConfig config;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("===EncodingFilter===init===");
config = filterConfig;
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
System.out.println("===EncodingFilter===doFilter===start===");
String charset = config.getInitParameter("charset");
if (charset == null) {
charset = "UTF-8";
}
servletRequest.setCharacterEncoding(charset);
filterChain.doFilter(servletRequest,servletResponse);
System.out.println("===EncodingFilter===doFilter===end===");
}
@Override
public void destroy() {
System.out.println("===EncodingFilter===destroy===");
}
}
Servlet3.0特性：
Servlet3.0支持注解的方式注册servlet和filter，我们不需要在web.xml中配置servlet和filter，只需要在servlet和filter类中用@方式注入即可，代码如下：
LoginServlet.java
package com.pumpkin.servlet;
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
@WebServlet(name = "LoginServlet",urlPatterns = "/login")
public class LoginServlet extends HttpServlet {
public LoginServlet() {
super();
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("===LoginServlet===doGet===");
doPost(req, resp);
}
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
System.out.println("===LoginServlet===doPost===start===");
String username = req.getParameter("username");
String password = req.getParameter("password");
System.out.println("===username===" + username + "===");
if ("admin".equals(username) && "admin".equals(password)) {
HttpSession session = req.getSession();
session.setAttribute("username",username);
resp.sendRedirect(req.getContextPath() + "/success.jsp");
}else {
resp.sendRedirect(req.getContextPath() + "/failure.jsp");
}
System.out.println("===LoginServlet===doPost===end===");
}
}
LoginFilter.java
package com.pumpkin.filter;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.util.logging.Filter;
@WebFilter(filterName = "Filter1_LoginFilter", value = {"/*"}, initParams = {@WebInitParam(name = "unfilteredPaths", value = "login.jsp;failure.jsp;login")})
public class LoginFilter implements javax.servlet.Filter {
private FilterConfig config;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("===LoginFilter===init===");
config = filterConfig;
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
System.out.println("===LoginFilter===doFilter===start===");
HttpServletRequest request = (HttpServletRequest) servletRequest;
HttpServletResponse response = (HttpServletResponse) servletResponse;
HttpSession session = request.getSession();
String unfilteredPaths = config.getInitParameter("unfilteredPaths");
if (unfilteredPaths != null) {
String[] strArray = unfilteredPaths.split(";");
for (int i = 0; i < strArray.length; i++) {
if (strArray[i] == null || "".equals(strArray[i])) {
continue;
}
if (request.getRequestURI().indexOf(strArray[i]) != -1) {
filterChain.doFilter(request, response);
return;
}
}
}
if (session.getAttribute("username") != null) {
filterChain.doFilter(request, response);
} else {
response.sendRedirect("login.jsp");
}
System.out.println("===LoginFilter===doFilter===end===");
}
@Override
public void destroy() {
System.out.println("===LoginFilter===destroy===");
}
}
EncodingFilter.java
package com.pumpkin.filter;
import com.sun.net.httpserver.Filter;
import com.sun.net.httpserver.HttpExchange;
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import java.io.IOException;
@WebFilter(filterName = "Filter0_EncodingFilter", value = {"/*"}, initParams = {@WebInitParam(name = "charset", value = "UTF-8")})
public class EncodingFilter implements javax.servlet.Filter{
private FilterConfig config;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
System.out.println("===EncodingFilter===init===");
config = filterConfig;
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
System.out.println("===EncodingFilter===doFilter===start===");
String charset = config.getInitParameter("charset");
if (charset == null) {
charset = "UTF-8";
}
servletRequest.setCharacterEncoding(charset);
filterChain.doFilter(servletRequest,servletResponse);
System.out.println("===EncodingFilter===doFilter===end===");
}
@Override
public void destroy() {
System.out.println("===EncodingFilter===destroy===");
}
}
LoginServlet.java
@WebServlet(name = "LoginServlet",urlPatterns = "/login")
这样的配置等同web.xml中的配置
LoginServlet com.pumpkin.servlet.LoginServlet LoginServlet /login
LoginFilter.java
@WebFilter(filterName = "Filter1_LoginFilter", value = {"/*"}, initParams = {@WebInitParam(name = "unfilteredPaths", value = "login.jsp;failure.jsp;login")})
这样的配置等同web.xml中的配置
loginFilter com.pumpkin.filter.LoginFilter unfilteredPaths login.jsp;failure.jsp;login loginFilter /*
EncodingFilter.java
@WebFilter(filterName = "Filter0_EncodingFilter", value = {"/*"}, initParams = {@WebInitParam(name = "charset", value = "UTF-8")})
这样的配置等同web.xml中的配置
EncodingFilter com.pumpkin.filter.EncodingFilter charset UTF-8 EncodingFilter /*
Servlet3.0过滤器执行顺序：
Servlet3.0之前Filter过滤的顺序是由用户在web.xml中配置的顺序决定的，如我们的程序会先执行LoginFilter,再执行encodingFilter。
在3.0之后新增@WebFilter注解，当使用注解配置多个Filter时，用户无法控制其执行顺序，此时Filter过滤的顺序是按照Filter的类名来控制的，按自然排序的规则。LoginFilter.java 和 EncodingFilter.java 这两个文件里面分别是“用户登录过滤器”和“字符集过滤器”，因为这两个文件的首字母E排L之前，导致每次执行的时候都是先执行“字符集过滤器”再执行“用户登录过滤器”，所以我们现在修改两个文件的名称分别为
Filter0_EncodingFilter.java
Filter1_LoginFilter.java
这样就能先执行“用户登录过滤器”再执行“字符集过滤器”。