Filter

过滤器 (Filter)
	1、概述：过滤器概述过滤器就像一个保安。能够对请求和响应进行拦截。
	2、编写过滤的步骤：
		1）编写一个类。实现javax.servlet.Filter接口，这种类一般称之为过滤器类
			public class FilterDemo1 implements Filter {

				public void init(FilterConfig filterConfig) throws ServletException {
					System.out.println("FilerDemo1初始化了");
				}
			
				//对于每一次的用户訪问。都会调用该方法。
				public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
					throws IOException, ServletException {

					System.out.println("FilterDemo1拦截前");
					chain.doFilter(request, response);//放行，让目标资源运行
					System.out.println("FilterDemo1拦截后");
				}

				public void destroy() {
					// TODO Auto-generated method stub
				}
			}
		2）在web.xml中进行配置，要拦截哪些资源。
			<filter>
				<filter-name>FilterDemo1</filter-name>
				<filter-class>cn.jxn.filter.FilterDemo1</filter-class>
			</filter>
			<filter-mapping>
				<filter-name>FilterDemo1</filter-name>
				<url-pattern>/*</url-pattern>
			</filter-mapping>
	3、过滤器的运行过程：
		1）过滤器仅仅会被初始化一次。应用被载入时就完毕了初始化。
		2）多个过滤器的拦截顺序是依照web.xml中filter-mapping元素的出现顺序进行拦截的。
	
	4、过滤器简单案例：
		1）界面输出中文及中文请求參数（POST方式有效）编码过滤器：SetCharacterEncodingFilter.java
			web.xml中的配置：
			<filter>
				<filter-name>SetCharacterEncodingFilter</filter-name>
				<filter-class>cn.jxn.filter.example.SetCharacterEncodingFilter</filter-class>
				<init-param>
					<param-name>encoding</param-name>
					<param-value>UTF-8</param-value>
				</init-param>
			  </filter>
			  <filter-mapping>
				<filter-name>SetCharacterEncodingFilter</filter-name>
				<url-pattern>/*</url-pattern>
			  </filter-mapping>
			
			//界面输出中文及中文请求參数（POST方式有效）编码过滤器
			public class SetCharacterEncodingFilter implements Filter {
			
				private FilterConfig filterConfig;
				
				public void destroy() {
					// TODO Auto-generated method stub
				}

				public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) 
					throws IOException, ServletException {

					String encodingValue = filterConfig.getInitParameter("encoding");//获取过滤器配置中的參数
					if(encodingValue==null)//假设没有配置该參数
						encodingValue="UTF-8";//给定默认值UTF-8
					request.setCharacterEncoding(encodingValue);//仅仅对POST请求方式实用
					response.setContentType("text/html;charset="+encodingValue);
					chain.doFilter(request, response);
				}

				public void init(FilterConfig filterConfig) throws ServletException {
					this.filterConfig = filterConfig;
				}
			}
		2）控制动态资源(Servlet JSP)不要缓存的过滤器
			web.xml中的配置
			<filter>
				<filter-name>NoCacheFilter</filter-name>
				<filter-class>cn.jxn.filter.example.NoCacheFilter</filter-class>
			</filter>
			<filter-mapping>
				<filter-name>NoCacheFilter</filter-name>
				<url-pattern>*.jsp</url-pattern>
			</filter-mapping>
			<filter-mapping>
				<filter-name>NoCacheFilter</filter-name>
				<url-pattern>/servlet/*</url-pattern>
			</filter-mapping>
		
			// 控制动态资源(Servlet JSP)不要缓存的过滤器
			public class NoCacheFilter implements Filter {

				public void destroy() {
					// TODO Auto-generated method stub
				}

				public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) 
					throws IOException, ServletException {

					HttpServletRequest request = (HttpServletRequest)req;
					HttpServletResponse response = (HttpServletResponse)resp;
					response.setDateHeader("Expires", 0);
					response.setHeader("Cache-Control", "no-cache");
					response.setHeader("Pragma", "no-cache");
					chain.doFilter(request, response);
				}

				public void init(FilterConfig filterConfig) throws ServletException {
				
				}
			}
			3）控制html、css、js等静态资源的缓存时间的过滤器
				web.xml中的配置
				<filter>
					<filter-name>NeedCacheFilter</filter-name>
					<filter-class>cn.jxn.filter.example.NeedCacheFilter</filter-class>
					<init-param>
						<param-name>html</param-name>
						<param-value>1</param-value><!-- 单位是小时 -->
					</init-param>
					<init-param>
						<param-name>css</param-name>
						<param-value>2</param-value><!-- 单位是小时 -->
					</init-param>
					<init-param>
						<param-name>js</param-name>
						<param-value>3</param-value><!-- 单位是小时 -->
					</init-param>
				</filter>
				<filter-mapping>
					<filter-name>NeedCacheFilter</filter-name>
					<url-pattern>*.html</url-pattern>
				</filter-mapping>
				<filter-mapping>
					<filter-name>NeedCacheFilter</filter-name>
					<url-pattern>*.css</url-pattern>
				</filter-mapping>
				<filter-mapping>
					<filter-name>NeedCacheFilter</filter-name>
					<url-pattern>*.js</url-pattern>
				</filter-mapping>
			
			public class NeedCacheFilter implements Filter {
			
				private FilterConfig filterConfig;
				
				public void destroy() {
					// TODO Auto-generated method stub
				}

				public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) 
					throws IOException, ServletException {

					System.out.println("控制缓存时间的过滤器过滤了");
					HttpServletRequest request = (HttpServletRequest)req;
					HttpServletResponse response = (HttpServletResponse)resp;
					//获取html、css、js各自的缓存时间。假设没有。给个默认值是1小时
					int time = 1;
					String uri = request.getRequestURI();//   /day19/1.html
					String extendName = uri.substring(uri.lastIndexOf(".")+1);
					
					if("html".equals(extendName)){
						//訪问的html资源
						String value = filterConfig.getInitParameter("html");
						if(value!=null){
							time = Integer.parseInt(value);
						}
					}
					if("css".equals(extendName)){
						//訪问的css资源
						String value = filterConfig.getInitParameter("css");
						if(value!=null){
							time = Integer.parseInt(value);
						}
					}
					if("js".equals(extendName)){
						//訪问的js资源
						String value = filterConfig.getInitParameter("js");
						if(value!=null){
							time = Integer.parseInt(value);
						}
					}
					response.setDateHeader("Expires", System.currentTimeMillis()+time*60*60*1000);
					chain.doFilter(request, response);
				}

				public void init(FilterConfig filterConfig) throws ServletException {
					this.filterConfig = filterConfig;
				}
			}