过滤器（Filter)是 J2EE Servlet模块性的组件。Filter的作用是对URL进行统一的拦截处理。Filter通常用于应用程序层面进行全局处理。

1. 实现Filter

开发过滤器三要素

任何过滤器都要实现javax.servlet.Filter接口
在 Filter接口的 doFilter()方法中编写过滤器的功能代码。
在web.xml中对过滤器进行配置，说明拦截URL的范围

接下来具体实现：

xml配置方式：

public class FirstFilter implements Filter {
    public void destroy() {
//...
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
       System.out.println("过滤器FirstFilter已生效");
       chain.doFilter(req,resp);
    }

    public void init(FilterConfig config) throws ServletException {
     //...
    }

}

<filter>
       <filter-name>First</filter-name>
       <filter-class>Filter.FirstFilter</filter-class>
   </filter>
    <filter-mapping>
        <filter-name>First</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

注解@WebFilter配置方式

属性名	类型	描述
filterName	String	指定过滤器的 name 属性，等价于 <filter-name>
value	String[]	该属性等价于 urlPatterns 属性。但是两者不应该同时使用。
urlPatterns	String[]	指定一组过滤器的 URL 匹配模式。等价于 <url-pattern> 标签。
servletNames	String[]	指定过滤器将应用于哪些 Servlet。取值是 @WebServlet 中的 name 属性的取值，或者是 web.xml 中 <servlet-name> 的取值。
dispatcherTypes	DispatcherType	指定过滤器的转发模式。具体取值包括：ASYNC、ERROR、FORWARD、INCLUDE、REQUEST。
initParams	WebInitParam[]	指定一组过滤器初始化参数，等价于 <init-param> 标签。
asyncSupported	boolean	声明过滤器是否支持异步操作模式，等价于 <async-supported> 标签。
description	String	该过滤器的描述信息，等价于 <description> 标签。
displayName	String	该过滤器的显示名，通常配合工具使用，等价于 <display-name> 标签。

@WebFilter(filterName = "FirstFilter", urlPatterns = "/*")
public class FirstFilter implements Filter { //...}

2. 过滤器生命周期

初始化：Filter.init()服务器启动后
提供服务：Filter.doFilter()
销毁：Filter.destroy()服务器关闭前
过滤器对象在Web应用启动时被创建且全局唯一。唯一的过滤器对象在并发环境中采用”多线程“提供服务。

3. 过滤器参数化

过滤器为了增强灵活性，允许配置信息放在web.xml。在web.xml中配置<init-param>设置过滤器参数

xml方式

<filter>
       <filter-name>First</filter-name>
       <filter-class>Filter.FirstFilter</filter-class>
        <init-param>
              <param-name>encoding</param-name>
              <param-value>UTF-8
        </init-param>
   </filter>
    <filter-mapping>
        <filter-name>First</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

public class FirstFilter implements Filter {
       String encoding;
         public void init(FilterConfig config) throws ServletException {
                encoding =  config.getInitParameter("encoding");
          }
......
    }

注解方式

@WebFilter(filterName = "FirstFilter", urlPatterns = "/*",
        initParams = {
                @WebInitParam(name = "encoding",value = "UTF-8"),
                @WebInitParam(name = "P2",value = "V2")
        })
public class FirstFilter implements Filter {
       String encoding;
         public void init(FilterConfig config) throws ServletException {
                encoding =  config.getInitParameter("encoding");
          }
    }

4. 过滤链

过滤链

过滤器的执行顺序以<filter-mapping>的顺序为准。调用chain.doFilter()将请求向后转递。

xml方式

<filter>
       <filter-name>FilterA</filter-name>
       <filter-class>Filter.FilterA</filter-class>
   </filter>
    <filter-mapping>
        <filter-name>FilterA</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

<filter>
       <filter-name>FilterB</filter-name>
       <filter-class>Filter.FilterB</filter-class>
   </filter>
    <filter-mapping>
        <filter-name>FilterB</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

<filter>
       <filter-name>FilterC</filter-name>
       <filter-class>Filter.FilterC</filter-class>
   </filter>
    <filter-mapping>
        <filter-name>FilterC</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

public class FilterA implements Filter {
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
          System.out.println("A is printed")
           chain.doFilter(req,resp);
    }
}
public class FilterB implements Filter {
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
          System.out.println("B is printed")
           chain.doFilter(req,resp);
    }
}
public class FilterC implements Filter {
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
          System.out.println("C is printed")
           chain.doFilter(req,resp);
    }
}
public class FirstServlet extends HttpServlet {
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
       System.out.println("Servlet is printed")
    }
}

输出：

A is printed
B is printed
C  is printed
Servlet is printed

如果将输出语句位置调换

public class FilterA implements Filter {
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
           chain.doFilter(req,resp);
           System.out.println("A is printed")
    }
}

输出：

Servlet is printed
C  is printed
B is printed
A is printed

注解方式
使用注解方式时，过滤链按照名称的字典顺序，很不合理。

eg:字符集过滤器

Web中文乱码的解决

GET请求：server.xml增加URIEncoding="UTF-8"
POST请求：使用request.setCharacterEncoding("UTF-8")
响应 response.setContentType("text/html;charset=UTF-8")

@WebFilter(filterName = "CharacterEncodingFilter")
public class CharacterEncodingFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
       //因为Tomcat中request的实现类继承自HttpServletRequest
        //所以可以转化为HttpServletRequest，以防ServletRequest不含有某些方法
        //详情见前面的 Java Web(3）HttpServletRequest和HttpServletResponse
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response =   (HttpServletResponse) resp;
        request.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
        chain.doFilter(request, response);

    }

    public void init(FilterConfig config) throws ServletException {
    }

}