Java Web开发中的过滤器(filter)是从Servlet 2.3规范开始增加的功能,并在Servlet 2.4规范中得到增强。对Web应用来说,过滤器是一个驻留在服务器端的Web组件,它可以截取客户端和服务器之间的请求与响应信息,并对这些信息进行过滤。
当Web容器接受到一个对资源的请求时,它将判断是否有过滤器与这个资源相关联。如果有,那么容器将把请求交给过滤器进行处理。在过滤器中,你可以改变请求的内容,或者重新设置请求的报头信息,然后再将请求发送给目标资源。当目标资源对请求作出响应时候,容器同样会将响应先转发给过滤器,在过滤器中你可以对响应的内容进行转换,然后再将响应发送到客户端。
常见的过滤器用途主要包括:对用户请求进行统一认证、对用户的访问请求进行记录和审核、对用户发送的数据进行过滤或替换、转换图象格式、对响应内容进行压缩以减少传输量、对请求或响应进行加解密处理、触发资源访问事件、对XML的输出应用XSLT等。
和过滤器相关的接口主要有:Filter、FilterConfig和FilterChain。
编码过滤器的例子:
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; import javax.servlet.annotation.WebFilter; import javax.servlet.annotation.WebInitParam; @WebFilter(urlPatterns = { "*" }, initParams = {@WebInitParam(name="encoding", value="utf-8")}) public class CodingFilter implements Filter { private String defaultEncoding = "utf-8"; @Override public void destroy() { } @Override public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException { req.setCharacterEncoding(defaultEncoding); resp.setCharacterEncoding(defaultEncoding); chain.doFilter(req, resp); } @Override public void init(FilterConfig config) throws ServletException { String encoding = config.getInitParameter("encoding"); if (encoding != null) { defaultEncoding = encoding; } } }
下载计数过滤器的例子:
import java.io.File; import java.io.FileReader; import java.io.FileWriter; import java.io.IOException; import java.util.Properties; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.annotation.WebFilter; import javax.servlet.http.HttpServletRequest; @WebFilter(urlPatterns = {"/*"}) public class DownloadCounterFilter implements Filter { private ExecutorService executorService = Executors.newSingleThreadExecutor(); private Properties downloadLog; private File logFile; @Override public void destroy() { executorService.shutdown(); } @Override public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException { HttpServletRequest request = (HttpServletRequest) req; final String uri = request.getRequestURI(); executorService.execute(new Runnable() { @Override public void run() { String value = downloadLog.getProperty(uri); if(value == null) { downloadLog.setProperty(uri, "1"); } else { int count = Integer.parseInt(value); downloadLog.setProperty(uri, String.valueOf(++count)); } try { downloadLog.store(new FileWriter(logFile), ""); } catch (IOException e) { e.printStackTrace(); } } }); chain.doFilter(req, resp); } @Override public void init(FilterConfig config) throws ServletException { String appPath = config.getServletContext().getRealPath("/"); logFile = new File(appPath, "downloadLog.txt"); if(!logFile.exists()) { try { logFile.createNewFile(); } catch(IOException e) { e.printStackTrace(); } } downloadLog = new Properties(); try { downloadLog.load(new FileReader(logFile)); } catch (IOException e) { e.printStackTrace(); } } }
说明:这里使用了Servlet 3规范中的注解来部署过滤器,当然也可以在web.xml中使用<filter>和<filter-mapping>标签部署过滤器,如108题中所示。