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;
}
}