JAVA入门[15]-过滤器filter

时间:2023-03-08 17:39:11

一、过滤器

过滤器是可用于 Servlet 编程的 Java 类,可以实现以下目的:

  • 在客户端的请求访问后端资源之前,拦截这些请求。
  • 在服务器的响应发送回客户端之前,处理这些响应。

参考:http://www.journaldev.com/1933/java-servlet-filter-example-tutorial

二、如何实现和配置过滤器

1.定义过滤器

过滤器实现接口: javax.servlet.Filter

示例:定义过滤器实现计数器,counterFilter实现Filter接口

public class counterFilter implements Filter {
ServletContext context;
int count;
public void init(FilterConfig filterConfig) throws ServletException {
context=filterConfig.getServletContext();
String initCount=filterConfig.getInitParameter("count");
count= Integer.valueOf(initCount);
} public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
context.log("===counterFilter do Filter====");
count++;
context.setAttribute("count",count);
filterChain.doFilter(servletRequest,servletResponse);
} public void destroy() {
}
}

  

jsp调用:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<div>欢迎,您是第 <%=application.getAttribute("count")%> 位访客</div>

  

2.配置web.xml

过滤器是可插拔的,通过web.xml来声明,然后映射到您的应用程序的部署描述符中的 Servlet 名称或 URL 模式。

当 Web 容器启动 Web 应用程序时,它会为部署描述符中声明的每一个过滤器创建一个实例。

web.xml 中的 filter-mapping 元素的顺序决定了 Web 容器应用过滤器到 Servlet 的顺序。若要反转过滤器的顺序,只需要在 web.xml 文件中反转 filter-mapping 元素即可。

<filter>
<filter-name>counterFilter</filter-name>
<filter-class>filter.counterFilter</filter-class>
<init-param>
<param-name>count</param-name>
<param-value>100</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>counterFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

  

3.如何给过滤器设置参数?

在web.xml中用init-param节点设置参数,然后在 init 方法使用 FilterConfig 对象获取参数。

context=filterConfig.getServletContext();
String initCount=filterConfig.getInitParameter("count");

  

三、使用注解@WebFilter定义过滤器

@WebFilter注解可以实现了javax.servlet.Filter接口的类定义为过滤器组件

@WebFilter(filterName = "filter1",initParams =@WebInitParam(name = "count",value = "100"),urlPatterns = "/*")
public class filter1 implements Filter {
ServletContext context;
public void destroy() {
} public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
context.log("filter1.doFilter()");
chain.doFilter(req, resp);
} public void init(FilterConfig config) throws ServletException {
context=config.getServletContext();
}
}

  

执行顺序:默认按照filter的名字排序,如果想调整顺序,还是要配置filter-mapping节点。

<filter-mapping>
<filter-name>filter2</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>filter1</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>

查看一下Tomcat Localhost Log:

02-Jun-2017 13:26:20.182 信息 [RMI TCP Connection(3)-127.0.0.1] org.apache.catalina.core.ApplicationContext.log Initializing Spring root WebApplicationContext
02-Jun-2017 13:26:21.302 信息 [RMI TCP Connection(3)-127.0.0.1] org.apache.catalina.core.ApplicationContext.log Initializing Spring FrameworkServlet 'springmvc'
02-Jun-2017 13:26:24.716 信息 [http-nio-8092-exec-1] org.apache.catalina.core.ApplicationContext.log ===counterFilter do Filter====
02-Jun-2017 13:26:24.716 信息 [http-nio-8092-exec-1] org.apache.catalina.core.ApplicationContext.log filter2.doFilter()
02-Jun-2017 13:26:24.716 信息 [http-nio-8092-exec-1] org.apache.catalina.core.ApplicationContext.log filter1.doFilter()

  

四、一个应用场景:过滤器防止中文编码

public class EncodeFilter implements Filter {
public void init(FilterConfig filterConfig) throws ServletException {
} public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
HttpServletRequest request=(HttpServletRequest)servletRequest;
HttpServletResponse response=(HttpServletResponse)servletResponse;
request.setCharacterEncoding("utf-8");
response.setCharacterEncoding("utf-8");
filterChain.doFilter(request,response);;
} public void destroy() { }
}