《JSP2.0技术手册》读书笔记三Filter

Filter，即过滤器了，很形象地说明了它的作用-在结果发生前获得控制做出相应操作，主要是改变一个请求或修改一个响应。具体运用情况如下：

1. 统一的认证处理。 2. 对用户的请求做检查、做更精确的记录。

3. 监视或对用户所传递的参数做前置处理。 4. 改变图像文件的格式

5. 对响应做编码的动作 6. 对响应做压缩处理

7. 对xml的输出使用xslt来转换

想要清楚地明白它在web应用中的位置和作用，就需要了解它的运作方式，下图较为形象：

由图可知：

1。一个响应或请求是经由过滤链层层转发的。一个Filter做完处理后，调用FilterChain.doFilter()来转送给其他Filter做处理。

2。Filter可以在request到达servlet或Jsp之前预处理请求，也可以在响应到达客户端前处理响应。

3。当客户端发出web资源请求时，web服务器根据web.xml设置的过滤规则进行检查，若客户请求满足过滤规则，则对客户请求/响应进行拦截，对请求头和请求数据进行检查或改动，并依次通过过滤器链，最后把请求/响应交给请求的web资源处理。

补：Filter做完处理后一般使用FilterChain.doFilter()来让其他Filter继续过滤。但也可以调用RequestDispatcher的forward()、include()方法转发请求，RequestDispatcher对象可由HttpServletRequest.getRequestDispatcher(uri)得到。还可以使用HttpServletResponse的sendRedirect(uri)、sendError(int statusCode)或sendError(int statusCode,String msg)来做出相应。

Filter可以处理不同的请求，这需要在web.xml中设置，如下：

<filter> <filter-name>filterSamp</filter-name> <filter-class>test.FilterSamp</filter-class> </filter> <filter-mapping> <filter-name>filterSamp</filter-name> <url-pattern>/*</url-pattern> <dispatcher>REQUEST</dispatcher> <dispatcher>FORWARD</dispatcher> <dispatcher>INCLUDE</dispatcher> <dispatcher>ERROR</dispatcher> </filter-mapping> 

REQUEST:只有当用户直接对网页做出请求的动作时，通过此Filter。一般默认情况为REQUEST。

FORWARD:当用户的请求是通过RequestDispatcher.forward()方法时，通过此Filter。

INCLUDE:当用户的请求是通过RequestDispatcher.include方法时，通过此Filter。还必须把<url-pattern>改成<servlet-name>，意思是当请求从servlet-name指定的servlet发出并且通过requestdispatcher.include()时，才通过此Filter。

ERROR:当用户的请求是通过错误机制处理的时候，通过此Filter。

我们来了解下RequestDispatcher接口。通常可用获得一个requestDispatcher对象，封装由一个特定的URL定义的服务器资源。从客户端接收请求，然后发给服务器端的可用资源(servlet/JSP等)。通常是通过HttpServletRequest.getRequestDispatcher(uri)来获得具体对象。forward(request,response)方法用来从生成它的servlet向其他资源传递请求;include(request,response)方法用来包括发送给其他服务器端资源的响应的内容。

Filter的使用：

1. 所有过滤器都必须实现javax.servlet.Filter接口。此接口中方法有：

1.1 public void init(FilterConfig config):当Filter被加载时执行。FilterConfig可用获得web.xml定义的该过滤器的名字-config.getFilterName()，可以获得ServletContext对象、web.xml设定的过滤器初始参数(param-name,param-value)。

1.2 public void doFilter(ServletRequest,ServletResponse,FilterChain)：可以对请求和响应做处理，然后调用filterChain.doFilter()将控制权传给下一个Filter。若此Filter已为过滤链中最后一个过滤器，则运行最初请求的servlet或jsp。

1.3 public void destroy()：当doFilter执行完毕就会调用此方法，释放资源，一般不需要使用。

2. 在web.xml中配置此过滤器的过滤规则。

<web-app> <filter> <filter-name></filter-name> <filter-class></filter-class> <init-param> <param-name>encoding</param-name> <param-value>UTF-8</param-value> </init-param> </filter> <filter-mapping> <filter-name></filter-name> <servlet-name></servlet-name> <url-pattern></url-pattern> <dispatcher></dispatcher> </web-app> 

我们经常用过滤器来进行访问权限控制、编码处理等。下面就举一个最实用的例子，解决乱码！

        首先定义过滤器SetCharacterEncodingFilter，它是tomcat自带的一个类，可以在网上找到源代码，引入到项目中就可使用了，现贴出代码如下：

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; /** * <p>Example filter that sets the character encoding to be used in parsing the * incoming request, either unconditionally or only if the client did not * specify a character encoding. Configuration of this filter is based on * the following initialization parameters:</p> * <ul> * <li><strong>encoding</strong> - The character encoding to be configured * for this request, either conditionally or unconditionally based on * the <code>ignore</code> initialization parameter. This parameter * is required, so there is no default.</li> * <li><strong>ignore</strong> - If set to "true", any character encoding * specified by the client is ignored, and the value returned by the * <code>selectEncoding()</code> method is set. If set to "false, * <code>selectEncoding()</code> is called <strong>only</strong> if the * client has not already specified an encoding. By default, this * parameter is set to "true".</li> * </ul> * * <p>Although this filter can be used unchanged, it is also easy to * subclass it and make the <code>selectEncoding()</code> method more * intelligent about what encoding to choose, based on characteristics of * the incoming request (such as the values of the <code>Accept-Language</code> * and <code>User-Agent</code> headers, or a value stashed in the current * user's session.</p> * * @@author Craig McClanahan * @@version $Revision: 1.1 $ $Date: 2009/07/06 07:54:40 $ */ public class SetCharacterEncodingFilter implements Filter { // ----------------------------------------------------- Instance Variables /** * The default character encoding to set for requests that pass through * this filter. */ protected String encoding = null; /** * The filter configuration object we are associated with. If this value * is null, this filter instance is not currently configured. */ protected FilterConfig filterConfig = null; /** * Should a character encoding specified by the client be ignored? */ protected boolean ignore = true; // --------------------------------------------------------- Public Methods /** * Take this filter out of service. */ public void destroy() { this.encoding = null; this.filterConfig = null; } /** * Select and set (if specified) the character encoding to be used to * interpret request parameters for this request. * * @@param request The servlet request we are processing * @@param result The servlet response we are creating * @@param chain The filter chain we are processing * * @@exception IOException if an input/output error occurs * @@exception ServletException if a servlet error occurs */ public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { // Conditionally select and set the character encoding to be used if (ignore || (request.getCharacterEncoding() == null)) { String encoding = selectEncoding(request); if (encoding != null) request.setCharacterEncoding(encoding); } // Pass control on to the next filter chain.doFilter(request, response); } /** * Place this filter into service. * * @@param filterConfig The filter configuration object */ public void init(FilterConfig filterConfig) throws ServletException { this.filterConfig = filterConfig; this.encoding = filterConfig.getInitParameter("encoding"); String value = filterConfig.getInitParameter("ignore"); if (value == null) this.ignore = true; else if (value.equalsIgnoreCase("true")) this.ignore = true; else if (value.equalsIgnoreCase("yes")) this.ignore = true; else this.ignore = false; } // ------------------------------------------------------ Protected Methods /** * Select an appropriate character encoding to be used, based on the * characteristics of the current request and/or filter initialization * parameters. If no character encoding should be set, return * <code>null</code>. * <p> * The default implementation unconditionally returns the value configured * by the <strong>encoding</strong> initialization parameter for this * filter. * * @@param request The servlet request we are processing */ protected String selectEncoding(ServletRequest request) { return (this.encoding); } } 

其次就是配置过滤规则了：

<filter> <filter-name>Set Character Encoding</filter-name> <filter-class>other.SetCharacterEncodingFilter</filter-class> <init-param> <param-name>encoding</param-name> <param-value>UTF-8</param-value> </init-param> </filter> <filter-mapping> <filter-name>Set Character Encoding</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> 

真是太舒服了！这比任何其他解决中文乱码问题的办法都有效简洁！