拦截器（Interceptor）是一种常用的设计模式，在软件工程领域被广泛应用。拦截器通常被用来处理各种请求和响应，可以在请求发送前或响应返回后进行一系列的操作和处理。拦截器可以用于请求身份认证、日志记录、性能优化、权限控制、数据加密等等，是很多框架和中间件中常用的一种组件。

在Java中，拦截器一般是通过实现javax.servlet.Filter接口或Spring框架中的HandlerInterceptor接口来实现的。拦截器可以被用于Web应用程序、消息队列、RPC调用等各种情况。

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;public class MyInterceptor implements Filter {@Overridepublic void init(FilterConfig filterConfig) throws ServletException {// 初始化操作，可以为空}@Overridepublic void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {HttpServletRequest httpRequest = (HttpServletRequest) request;HttpServletResponse httpResponse = (HttpServletResponse) response;// 在此处可以进行各种拦截操作，例如身份认证、权限控制、日志记录等等// 这里只是一个简单的示例：在请求头中添加一个自定义的字段httpResponse.setHeader("My-Interceptor", "Hello World!");chain.doFilter(request, response);}@Overridepublic void destroy() {// 销毁操作，可以为空}
}

<filter><filter-name>MyInterceptor</filter-name><filter-class>com.example.MyInterceptor</filter-class>
</filter><filter-mapping><filter-name>MyInterceptor</filter-name><url-pattern>*</url-pattern>
</filter-mapping>

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;public class MyServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String header = req.getHeader("My-Interceptor");resp.getWriter().write(header != null ? header : "Header Not Set");}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doGet(req, resp);}
}

文件上下载

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;public class FileUploader {private static final String UPLOAD_URL = "http://example.com/upload"; // 上传文件的URL地址private static final String FILE_PATH = "path/to/file"; // 要上传的文件路径public static void main(String[] args) throws IOException {File file = new File(FILE_PATH);if (!file.exists()) {System.out.println("文件不存在");return;}URL url = new URL(UPLOAD_URL);HttpURLConnection conn = (HttpURLConnection) url.openConnection();conn.setRequestMethod("POST");conn.setDoOutput(true);conn.setRequestProperty("Content-Type", "application/octet-stream"); // 文件类型conn.setRequestProperty("filename", file.getName()); // 文件名InputStream in = new FileInputStream(file);byte[] buffer = new byte[1024];int len;while ((len = in.read(buffer)) != -1) {conn.getOutputStream().write(buffer, 0, len);}in.close();if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {System.out.println("上传成功");} else {System.out.println("上传失败");}conn.disconnect();}
}

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;public class FileDownloader {private static final String DOWNLOAD_URL = "http://example.com/download"; // 下载文件的URL地址private static final String FILE_PATH = "path/to/save/file"; // 要保存的文件路径public static void main(String[] args) throws IOException {URL url = new URL(DOWNLOAD_URL);HttpURLConnection conn = (HttpURLConnection) url.openConnection();conn.setRequestMethod("GET");conn.setDoOutput(true);InputStream in = conn.getInputStream();FileOutputStream out = new FileOutputStream(FILE_PATH);byte[] buffer = new byte[1024];int len;while ((len = in.read(buffer)) != -1) {out.write(buffer, 0, len);}in.close();out.close();if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {System.out.println("下载成功");} else {System.out.println("下载失败");}conn.disconnect();}
}