Java Web 基础 - 过滤器

• 过滤器是一种服务端组件，可以截取客户端请求和响应信息，对信息进行过滤，包括过滤源 -> 过滤规则 -> 过滤结果
• 过滤器可以改变用户请求的 Web 资源（用户请求路径），但不能直接返回数据、处理用户请求（不是标准的 Servlet）
• 常见使用场景：对请求进行统一认证、编码转换、对发送的数据进行过滤替换、转换图形格式、对响应内容压缩等

工作原理与生命周期

Web 容器启动使过滤器即启动，接收到用户请求后判断是否符合规则，过滤器把符合规则的请求发送至 Web 资源，处理完成后再经过滤器返回响应给用户。
生命周期：

1. 实例化（Web 容器启动时加载 web.xml，只实例化一次 ）
2. 初始化（init()方法）
3. 对每个请求过滤方法（doFilter()方法）
4. 销毁（Web 容器关闭时执行destroy() 方法销毁）

实现一个过滤器

web.xml

<filter>        
    <filter-name>FirstFilter</filter-name>          <!-- 过滤器类完整名字和包名 -->
    <filter-class>com.ywh.filter.FirstFilter</filter-class>
    <init-param>
        <param-name>name</param-name>               <!-- 初始化参数 -->
        <param-value>ywh</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>FirstFilter</filter-name>          <!-- 用户请求的 URL 匹配时触发过滤器工作 -->
    <url-pattern>/index.jsp</url-pattern>
    <dispatcher>FORWARD</dispatcher>                <!-- 执行操作，可以是 REQUEST、INCLUDE、FORWARD、ERROR，默认为 REQUEST -->
</filter-mapping>

FirstFilter.java

public class FirstFilter implements Filter {

    @Override
    public void destroy() {
        System.out.println("destroy---FirstFilter");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        System.out.println("start----doFilter--FirstFilter");
//      chain.doFilter(request, response);
        HttpServletRequest req =(HttpServletRequest) request;
        HttpServletResponse response2 =(HttpServletResponse) response;
        response2.sendRedirect(req.getContextPath() + "/main.jsp");
//      req.getRequestDispatcher("main.jsp").forward(request, response);
//      req.getRequestDispatcher("main.jsp").include(request, response);
        System.out.println("end------doFilter--FirstFilter");
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("init----FirstFilter");
    }
}

过滤器链

对于同一个 URL 可以设置多个过滤器按指定顺序执行

过滤器链的执行逻辑

<filter>
    <filter-name>FirstFilter</filter-name>          <!-- 过滤器类完整名字和包名 -->
    <filter-class>com.ywh.filter.FirstFilter</filter-class>
    <init-param>
        <param-name>name</param-name>               <!-- 初始化参数 -->
        <param-value>zhangsan</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>FirstFilter</filter-name>          <!-- 用户请求的 URL 匹配时触发过滤器工作 -->
    <url-pattern>/index.jsp</url-pattern>
</filter-mapping>
<filter>
    <filter-name>SecondFilter</filter-name>
    <filter-class>com.ywh.filter.SecondFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SecondFilter</filter-name>
    <url-pattern>/index.jsp</url-pattern>
</filter-mapping>

过滤器分类

在 Servlet 2.5 中过滤器有四种：

• REQUEST：用户直接访问页面时调用（重定向、转发）；
• FORWARD：目标资源是通过 RequestDispatcher 的 forward 访问时调用；
• INCLUDE：目标资源是通过 RequestDispatcher 的 incluer 方法调用时调用；
• ERROR：目标资源是通过声明式异常处理机制调用时调用。

在 Servlet 3.0 中加入了 ASYNC 支持异步处理。

过滤器注解

使用 @WebFilter 注解将一个类生命为过滤器（在部署时被容器处理，容器将根据具体的属性配置将类部署为过滤器），代替 web.xml 配置过滤器类

@WebFilter(servletNames = {"SimpleServlet"}, filterName="SimpleFilter", value={"/index.jsp"}, dispatcherTypes={DispatcherType.Async})
public class TestFilter implements Filter {
    // ...
}

@WebFilter 常用属性
实例：测试异步过滤器
web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
         xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <display-name></display-name>
    <servlet>
        <description>This is the description of my J2EE component</description>
        <display-name>This is the display name of my J2EE component</display-name>
        <servlet-name>AsynServlet</servlet-name>
        <servlet-class>com.ywh.servlet.AsynServlet</servlet-class>
        <async-supported>true</async-supported>
    </servlet>

    <servlet-mapping>
        <servlet-name>AsynServlet</servlet-name>
        <url-pattern>/servlet/AsynServlet</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
</web-app>

AsynServlet.java

public class AsynServlet extends HttpServlet {

    public AsynServlet() {super(); }

    public void destroy() { super.destroy(); }

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Servletִ 执行开始时间：" + new Date());
        AsyncContext context = request.startAsync();
        new Thread(new Executor(context)).start();
        System.out.println("Servletִ 执行结束时间：" + new Date());
    }

    public class Executor implements Runnable {
        private AsyncContext context;

        public Executor(AsyncContext context) { this.context = context; }

        @Override
        public void run() {
            try {
                Thread.sleep(1000 * 10);
                System.out.println("业务执行完成时间：" + new Date());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
        out.println("<HTML>");
        out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");
        out.println("  <BODY>");
        out.print("    This is ");
        out.print(this.getClass());
        out.println(", using the POST method");
        out.println("  </BODY>");
        out.println("</HTML>");
        out.flush();
        out.close();
    }

    public void init() throws ServletException { }
}

AsynFilter.java

@WebFilter(
    filterName = "AsynFilter", 
    asyncSupported = true, 
    value = {"/servlet/AsynServlet"}, 
    dispatcherTypes = {DispatcherType.REQUEST, DispatcherType.ASYNC}
    )
public class AsynFilter implements Filter {

    @Override
    public void destroy() {}

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException {
        System.out.println("start.....AsynFilter");
        arg2.doFilter(arg0, arg1);
        System.out.println("end.....AsynFilter");
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {}
}

实例：对用户请求进行统一认证
login.jsp

<body>
    <form action="<%=request.getContextPath() %>/servlet/LoginServlet" method="post">
        用户名:<input type="text" name="username">
        密码:<input type="password" name="password">
        <input type="submit" value="提交">
    </form>
</body>

LoginServlet.java

public class LoginServlet extends HttpServlet {

    public LoginServlet() { super(); }

    public void destroy() { super.destroy(); }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        System.out.println(username);

        if ("admin".equals(username) && "admin".equals(password)) {
            // 校验通过
            HttpSession session = request.getSession();
            session.setAttribute("username", username);
            response.sendRedirect(request.getContextPath() + "/sucess.jsp");
        } else {
            // 校验失败
            response.sendRedirect(request.getContextPath() + "/fail.jsp");
        }
    }

    public void init() throws ServletException { }
}

LoginFilter.java

public class LoginFilter implements Filter {

    private FilterConfig config;

    @Override
    public void destroy() { }

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) arg0;
        HttpServletResponse response = (HttpServletResponse) arg1;
        HttpSession session = request.getSession();

        String noLoginPaths = config.getInitParameter("noLoginPaths");

        String charset = config.getInitParameter("charset");
        if (charset == null) {
            charset = "UTF-8";
        }
        request.setCharacterEncoding(charset);

        if (noLoginPaths != null) {
            String[] strArray = noLoginPaths.split(";");
            // 获取不需要重定向的页面，符合条件则放行（如对于 login.jsp 不再执行重定向）
            for (int i = 0; i < strArray.length; i++) {
                if (strArray[i] == null || "".equals(strArray[i])) 
                    continue;
                if (request.getRequestURI().indexOf(strArray[i]) != -1) {
                    arg2.doFilter(arg0, arg1);
                    return;
                }
            }
        }
        if (session.getAttribute("username") != null) {
            arg2.doFilter(arg0, arg1);
        } else {
            response.sendRedirect("login.jsp");
        }
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException { config = arg0; }

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5"
         xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
    <display-name></display-name>
    <servlet>
        <description>This is the description of my J2EE component</description>
        <display-name>This is the display name of my J2EE component</display-name>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.ywh.serlvet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/servlet/LoginServlet</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    <filter>
        <filter-name>LoginFilter</filter-name>
        <filter-class>com.ywh.filter.LoginFilter</filter-class>
        <init-param>
            <param-name>noLoginPaths</param-name>    <!-- 不需要重定向的页面 -->
            <param-value>login.jsp;fail.jsp;LoginServlet</param-value>
        </init-param>
        <init-param>
            <param-name>charset</param-name>
            <param-value>UTF-8</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>LoginFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>