如题，在我多次点击标题下载，取消之后，java后台就报空指针异常，而且点击下载标题也点击不了了，该怎么解决。
应该是点击取消的时候，客户端和服务端通过socket也建立了链接吧。
重复1,2操作。
java.io.IOException: null
at org.apache.coyote.http11.InternalAprOutputBuffer.flushBuffer(InternalAprOutputBuffer.java:205) ~[tomcat-coyote.jar:7.0.69]
at org.apache.coyote.http11.InternalAprOutputBuffer.access$100(InternalAprOutputBuffer.java:37) ~[tomcat-coyote.jar:7.0.69]
at org.apache.coyote.http11.InternalAprOutputBuffer$SocketOutputBuffer.doWrite(InternalAprOutputBuffer.java:235) ~[tomcat-coyote.jar:7.0.69]
at org.apache.coyote.http11.filters.ChunkedOutputFilter.doWrite(ChunkedOutputFilter.java:119) ~[tomcat-coyote.jar:7.0.69]
at org.apache.coyote.http11.AbstractOutputBuffer.doWrite(AbstractOutputBuffer.java:192) ~[tomcat-coyote.jar:7.0.69]
at org.apache.coyote.Response.doWrite(Response.java:499) ~[tomcat-coyote.jar:7.0.69]
at org.apache.catalina.connector.OutputBuffer.realWriteBytes(OutputBuffer.java:402) ~[catalina.jar:7.0.69]
at org.apache.tomcat.util.buf.ByteChunk.flushBuffer(ByteChunk.java:480) ~[tomcat-coyote.jar:7.0.69]
at org.apache.tomcat.util.buf.ByteChunk.append(ByteChunk.java:310) ~[tomcat-coyote.jar:7.0.69]
at org.apache.catalina.connector.OutputBuffer.writeByte(OutputBuffer.java:451) ~[catalina.jar:7.0.69]
at org.apache.catalina.connector.CoyoteOutputStream.write(CoyoteOutputStream.java:77) ~[catalina.jar:7.0.69]
at com.thunisoft.gpip.modules.procuratorial.util.DownloadServlet.doGet(DownloadServlet.java:62) ~[classes/:na]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:624) ~[servlet-api.jar:na]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731) ~[servlet-api.jar:na]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303) [catalina.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.69]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat7-websocket.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) [catalina.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.69]
at com.thunisoft.summer.security.access.filter.ACFilter.doFilter(ACFilter.java:50) ~[summer-core-2.1.5.6.jar:2.1.5.6]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) [catalina.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.69]
at com.thunisoft.artery.support.filter.ThreadFilter.doFilter(ThreadFilter.java:56) [artery-platform-4.4.23.jar:na]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) [catalina.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.69]
at com.thunisoft.summer.web.filter.ParameterFilter.doFilterInternal(ParameterFilter.java:72) [summer-core-2.1.5.6.jar:2.1.5.6]
at com.thunisoft.summer.web.multipart.RequestWrapperFilter.doFilter(RequestWrapperFilter.java:138) [summer-core-2.1.5.6.jar:2.1.5.6]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) [catalina.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.69]
at com.thunisoft.summer.web.filter.CharsetFilter.doFilter(CharsetFilter.java:44) [summer-core-2.1.5.6.jar:2.1.5.6]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) [catalina.jar:7.0.69]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) [catalina.jar:7.0.69]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220) [catalina.jar:7.0.69]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122) [catalina.jar:7.0.69]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505) [catalina.jar:7.0.69]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169) [catalina.jar:7.0.69]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) [catalina.jar:7.0.69]
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956) [catalina.jar:7.0.69]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116) [catalina.jar:7.0.69]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:436) [catalina.jar:7.0.69]
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078) [tomcat-coyote.jar:7.0.69]
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625) [tomcat-coyote.jar:7.0.69]
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2517) [tomcat-coyote.jar:7.0.69]
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2506) [tomcat-coyote.jar:7.0.69]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_80]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_80]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-coyote.jar:7.0.69]
at java.lang.Thread.run(Thread.java:745) [na:1.7.0_80]
