关于java.net.SocketException: Socket closed

下面这段代码的------>  out.write(buf, 0, len)报异常
java.net.SocketException: Socket closed
at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:99)
at java.net.SocketOutputStream.write(SocketOutputStream.java:136)
at weblogic.servlet.internal.ChunkOutput.writeChunkNoTransfer(ChunkOutput.java:548)
at weblogic.servlet.internal.ChunkOutput.writeChunks(ChunkOutput.java:505)
at weblogic.servlet.internal.ChunkOutput.flush(ChunkOutput.java:382)
at weblogic.servlet.internal.ChunkOutput.checkForFlush(ChunkOutput.java:469)
at weblogic.servlet.internal.ChunkOutput.write(ChunkOutput.java:304)
at weblogic.servlet.internal.ChunkOutputWrapper.write(ChunkOutputWrapper.java:137)
at weblogic.servlet.internal.ServletOutputStreamImpl.write(ServletOutputStreamImpl.java:168)
at jp.co.nes.necle.common.util.ServletUtils.download(ServletUtils.java:126)
at jp.co.nes.necle.common.util.ServletUtils.download(ServletUtils.java:97)
at jp.co.nes.necle.goods.web.PublishingOutPutCSVAllAction.showPublishingOutPutCSVAll(PublishingOutPutCSVAllAction.java:535)
at jp.co.nes.necle.goods.web.PublishingOutPutCSVAllAction.perform(PublishingOutPutCSVAllAction.java:83)
at org.apache.struts.action.ActionServlet.processActionPerform(ActionServlet.java:1787)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1586)
at jp.co.nes.necle.common.web.BasicActionServlet.process(BasicActionServlet.java:87)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:510)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:226)
at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:124)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:283)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:175)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3370)
at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
at weblogic.security.service.SecurityManager.runAs(Unknown Source)
at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2117)
at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2023)
at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1359)
at weblogic.work.ExecuteThread.execute(ExecuteThread.java:200)
at weblogic.work.ExecuteThread.run(ExecuteThread.java:172)
java.net.SocketException: Socket closed
at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:99)
at java.net.SocketOutputStream.write(SocketOutputStream.java:136)
at weblogic.servlet.internal.ChunkOutput.writeChunkNoTransfer(ChunkOutput.java:548)
at weblogic.servlet.internal.ChunkOutput.writeChunks(ChunkOutput.java:505)
at weblogic.servlet.internal.ChunkOutput.flush(ChunkOutput.java:382)
at weblogic.servlet.internal.ChunkOutput.checkForFlush(ChunkOutput.java:469)
at weblogic.servlet.internal.ChunkOutput.write(ChunkOutput.java:304)
at weblogic.servlet.internal.ChunkOutputWrapper.write(ChunkOutputWrapper.java:137)
at weblogic.servlet.internal.ServletOutputStreamImpl.write(ServletOutputStreamImpl.java:168)
at jp.co.nes.necle.common.util.ServletUtils.download(ServletUtils.java:126)
at jp.co.nes.necle.common.util.ServletUtils.download(ServletUtils.java:97)
at jp.co.nes.necle.goods.web.PublishingOutPutCSVAllAction.showPublishingOutPutCSVAll(PublishingOutPutCSVAllAction.java:535)
at jp.co.nes.necle.goods.web.PublishingOutPutCSVAllAction.perform(PublishingOutPutCSVAllAction.java:83)
at org.apache.struts.action.ActionServlet.processActionPerform(ActionServlet.java:1787)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1586)
at jp.co.nes.necle.common.web.BasicActionServlet.process(BasicActionServlet.java:87)
at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:510)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:226)
at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:124)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:283)
at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:175)
at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3370)
at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
at weblogic.security.service.SecurityManager.runAs(Unknown Source)
at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2117)
at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2023)
at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1359)
at weblogic.work.ExecuteThread.execute(ExecuteThread.java:200)
at weblogic.work.ExecuteThread.run(ExecuteThread.java:172)
不知道什么原因，希望各位高手解答
代码：
 private static void download(HttpServletResponse response,
            InputStream in, int contentLength, String fileName)
            throws IOException {
        response.reset();
        response.setContentLength(contentLength);
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition",
                "attachment; filename=\"" + fileName + "\"");
        OutputStream out = null;
        try {
            out = response.getOutputStream();
            byte[] buf = new byte[512];
            int len;
            while ((len = in.read(buf)) != -1) {
              ------>  out.write(buf, 0, len);
            }
        } catch (Exception e) {
         e.printStackTrace();
        } finally {
            try {
                in.close();
            } catch (IOException e) {
             e.printStackTrace();
            }
            if (out != null) {
                try {
                    out.close();
                } catch (IOException e) {
                 e.printStackTrace();
                }
            }
        }
    }