摘要：攔截所有的請(qǐng)求，放行登錄頁面登錄操作請(qǐng)求，其余請(qǐng)求需要在登錄后才可訪問。同時(shí)配置參數(shù)，指定要放行的路徑和請(qǐng)求的字符集。

作用: 用于將一個(gè)類聲明為 Servlet

描述: 該注解將會(huì)在部署時(shí)被容器處理, 容器將根據(jù)具體的屬性配置將相應(yīng)的類部署為 Servlet.

屬性詳解: 該注解具有下表給出的一些常用屬性(以下所有屬性均為可選屬性, 但是 vlaue 或者 urlPatterns 通常是必需的, 且二者不能共存, 如果同時(shí)指定, 通常是忽略 value 的取值)

import java.io.IOException;
 
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
//注解配置  
@WebServlet(displayName = "UserServlet", // 描述
        name = "UserServlet", // servlet名稱
        urlPatterns = { "/user" }, // url
        loadOnStartup = 1, // 啟動(dòng)項(xiàng)
        initParams = { @WebInitParam(name = "username", value = "張三") }) // 初始化參數(shù)
 
public class UserServlet extends HttpServlet {
 
    private String username;
 
    public UserServlet() {
        System.out.println("UserServlet...");
    }
 
    @Override
    public void init(ServletConfig config) throws ServletException {
        // 獲取初始化信息：張三
        username = config.getInitParameter("username");
    }
 
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 主要內(nèi)容操作
    }
 
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 主要內(nèi)容操作
        System.out.println(username);
    }
}

Filter（過濾器）主要對(duì)請(qǐng)求到達(dá)前進(jìn)行處理，也可以在請(qǐng)求結(jié)束后進(jìn)行處理，類似于鏈?zhǔn)健?/p>

一個(gè)請(qǐng)求可以被多個(gè)過濾器攔截到，會(huì)依次進(jìn)入各個(gè)Filter中，放行后直至進(jìn)入Servlet，Servlet處理請(qǐng)求結(jié)束后，回到各個(gè)Filter繼續(xù)執(zhí)行后面的代碼，先執(zhí)行的Filter后執(zhí)行完。

常用到的地方：

用戶權(quán)限過濾

記錄日志

字符編碼處理

@WebFilter 常用屬性

用戶權(quán)限過濾示例：

自定義過濾器，實(shí)現(xiàn) javax.servlet.Filter 接口，通過注解方式配置。攔截所有的請(qǐng)求，放行登錄頁面、登錄操作請(qǐng)求，其余請(qǐng)求需要在登錄后才可訪問。同時(shí)配置參數(shù)，指定要放行的路徑和請(qǐng)求的字符集。

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.annotation.WebFilter;
import javax.servlet.annotation.WebInitParam;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
@WebFilter(filterName = "loginFilter", urlPatterns = "/*", initParams = {
        @WebInitParam(name = "loginUI", value = "/home/loginUI"),
        @WebInitParam(name = "loginProcess", value = "home/login"),
        @WebInitParam(name = "encoding", value = "utf-8") })
public class LoginFilter implements Filter {
    private FilterConfig config;
 
    @Override
    public void init(FilterConfig config) throws ServletException {
        this.config = config;
    }
 
    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
            throws IOException, ServletException {
        // 獲取配置參數(shù)
        String loginUI = config.getInitParameter("loginUI");
        String loginProcess = config.getInitParameter("loginProcess");
        String encoding = config.getInitParameter("encoding");
 
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
 
        // 設(shè)置請(qǐng)求的字符集（post請(qǐng)求方式有效）
        request.setCharacterEncoding(encoding);
 
        // 不帶http://域名:端口的地址
        String uri = request.getRequestURI();
        if (uri.contains(loginUI) || uri.contains(loginProcess)) {
            // 請(qǐng)求的登錄，放行
            chain.doFilter(request, response);
        } else {
            if (request.getSession().getAttribute("user") == null) {
                // 重定向到登錄頁面
                response.sendRedirect(request.getContextPath() + loginUI);
            } else {
                // 已經(jīng)登錄，放行
                chain.doFilter(request, response);
            }
        }
    }
 
    @Override
    public void destroy() {
        this.config = null;
    }
}

該注解用于將類聲明為監(jiān)聽器，被 @WebListener 標(biāo)注的類必須實(shí)現(xiàn)以下至少一個(gè)接口：

ServletContextListener

ServletContextAttributeListener

ServletRequestListener

ServletRequestAttributeListener

HttpSessionListener

HttpSessionAttributeListener

@WebListener("This is only a demo listener") 
public class SimpleListener implements ServletContextListener{...}

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
 
@WebListener
public class Test implements ServletRequestListener {
 
    public Test() {
    }
 
    
    public void requestDestroyed(ServletRequestEvent arg0)  { 
    }
 
    public void requestInitialized(ServletRequestEvent arg0)  { 
    }
    
}