Java - 过滤器有哪些作用和用法？

Myth丶恋晨 2022-06-06 06:40 237阅读 0赞

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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，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：

package chimomo.learning.java.code.jsp;
    
    import javax.servlet.*;
    import javax.servlet.annotation.WebFilter;
    import javax.servlet.annotation.WebInitParam;
    import java.io.IOException;
    
    /**
     * @author Created by Chimomo
     */
    @WebFilter(urlPatterns = {"*"}, initParams = {@WebInitParam(name = "encoding", value = "utf-8")})
    public class CodingFilter implements Filter {
    
        private String defaultEncoding = "utf-8";
    
        @Override
        public void init(FilterConfig config) {
            String encoding = config.getInitParameter("encoding");
            if (encoding != null) {
                defaultEncoding = encoding;
            }
        }
    
        @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 destroy() {
        }
    
    }

下载计数过滤器的例子：

package chimomo.learning.java.code.jsp;
    
    import javax.servlet.*;
    import javax.servlet.annotation.WebFilter;
    import javax.servlet.http.HttpServletRequest;
    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;
    
    /**
     * @author Created by Chimomo
     */
    @WebFilter(urlPatterns = {"/*"})
    public class DownloadCounterFilter implements Filter {
    
        private ExecutorService executorService = Executors.newSingleThreadExecutor();
        private Properties downloadLog;
        private File logFile;
    
        @Override
        public void init(FilterConfig config) {
            String appPath = config.getServletContext().getRealPath("/");
            logFile = new File(appPath, "DownloadLog.txt");
            if (!logFile.exists()) {
                try {
                    boolean b = logFile.createNewFile();
                    if (b) {
                        System.out.println("Log file is successfully created");
                    } else {
                        System.out.println("Log file already exists");
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            downloadLog = new Properties();
            try {
                downloadLog.load(new FileReader(logFile));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    
        @Override
        public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
                throws IOException, ServletException {
            HttpServletRequest request = (HttpServletRequest) req;
            final String uri = request.getRequestURI();
            executorService.execute(() -> {
                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 destroy() {
            executorService.shutdown();
        }
    
    }

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。

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