IOUtils.copy(inputStream,outputStream)；

        <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.4</version>
        </dependency>

org.apache.commons.io.IOUtils;

 案例

@WebServlet("/servletA")
public class ServletA extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        FileInputStream fis = new FileInputStream("D://a.jpg");
        ServletOutputStream os = resp.getOutputStream();
        /*byte[] buff = new byte[1024];
        int len = 0;
        while ((len = fis.read(buff))!=-1){
            os.write(buff,0,len);
        }*/
        /* 上面代码替换为IOUtils.copy(fis,os);
        需添加依赖，使用import org.apache.commons.io.IOUtils;中的IOUtils.copy();
        * <dependency>
            <groupId>commons-io</groupId>
            <artifactId>commons-io</artifactId>
            <version>2.4</version>
        </dependency>
        */
        IOUtils.copy(fis,os);
        fis.close();
    }
 
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}