过滤器的API

1、API

a、生命周期（和servletcontext相似）：

（1）创建：服务器启动的时候创建（执行init方法）。

（2）销毁：服务器关闭的时候销毁（执行destory方法）。

b、filterConfig.getFilterName()：读取配置文件中的名字。

getInitParameter():读取配置文件中的初始化配置参数。

public class FilterDemo implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        String s1=filterConfig.getFilterName();
        String s2=filterConfig.getInitParameter("zhai");
        System.out.println(s1);
        System.out.println(s2);
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

    }

    @Override
    public void destroy() {

    }

配置文件：

<filter>
    <filter-name>FilterDemo</filter-name>
    <filter-class>pers.zhb.filter.FilterDemo</filter-class>
    <init-param>
        <param-name>zhai</param-name>
        <param-value></param-value>
    </init-param>
</filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>/filter</url-pattern>
    </filter-mapping>

2、配置文件：

（1）全名匹配：

<filter>
    <filter-name>FilterDemo</filter-name>
    <filter-class>pers.zhb.filter.FilterDemo</filter-class>
</filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>/servlet1</url-pattern>
    </filter-mapping>

即访问的虚拟路径名必须为servlet1，匹配的是全部的名字。

（2）目录匹配：

<filter>
    <filter-name>FilterDemo</filter-name>
    <filter-class>pers.zhb.filter.FilterDemo</filter-class>
</filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>/abc/*</url-pattern>
    </filter-mapping>

即必须为abc目录下的资源。

（3）后缀名匹配：

<filter>
    <filter-name>FilterDemo</filter-name>
    <filter-class>pers.zhb.filter.FilterDemo</filter-class>
</filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>*.jsp</url-pattern>
    </filter-mapping>

3、dispatcher：访问的方式。

（1）REQUEST：默认值，代表直接访问某个资源时执行此filter。

在部队REQUEST参数进行配置的时候：

创建两个Servlet，由servlet1请求转发到servlet2：

Servlet1：

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Servlet1运行了");
        request.getRequestDispatcher("servlet2").forward(request,response);
    }

Servlet2：

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    System.out.println("Servlet2运行了");
    }

过滤器：

 public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        filterChain.doFilter(servletRequest,servletResponse);
        System.out.println("过滤器运行了");
    }

当在配置文件中添加了dispatcher属性后：

    <filter>
        <filter-name>FilterDemo</filter-name>
        <filter-class>pers.zhb.filter.FilterDemo</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>REQUEST</dispatcher>
    </filter-mapping>

只有访问Servlet1的时候有过滤器，因为请求转发的时候，只有第一次的时候是直接访问，请求转发到Servlet2的时候不是直接访问。

当把Servlet1改为重定向到Servlet2的时候，Servlet1和Servlet2触发过滤器：

   protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Servlet1运行了");
        response.sendRedirect(request.getContextPath()+"/servlet2");
    }

因为重定向是对服务器的重新访问，而请求转发发生在服务器内部。

（2）FORWARD：

 <filter>
        <filter-name>FilterDemo</filter-name>
        <filter-class>pers.zhb.filter.FilterDemo</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>FilterDemo</filter-name>
        <url-pattern>/*</url-pattern>
        <dispatcher>FORWARD</dispatcher>
    </filter-mapping>

只有转发的时候才能触发过滤器：

  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Servlet1运行了");
        response.sendRedirect(request.getContextPath()+"/servlet2");
    }

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    System.out.println("Servlet2运行了");
    }　　

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Servlet1运行了");
       request.getRequestDispatcher("/servlet2").forward(request,response);
    }

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("Servlet2运行了");
    }

（3）INCLUDE：包含其他资源的时候触发过滤器。

（4）ERROR：发生错误的时候触发过滤器。