Java：GarbageCollectorMXBean getCollectionCount 抛出 java.io.IOException：客户端已关闭

Question

我正在编写一个 Java 应用程序，我使用 Java GarbageCollectorMXBean API 定期（每 5 秒）获取收集计数。下面是我为完成任务而编写的程序。

import java.io.IOException;
import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.util.HashMap;
import java.util.Map;

import javax.management.MBeanServerConnection;
import javax.management.remote.JMXConnector;
import javax.management.remote.JMXConnectorFactory;
import javax.management.remote.JMXServiceURL;

public class JMXTest {
    public static final String GC_NAME = "java.lang:name=MarkSweepCompact,type=GarbageCollector";
    private static GarbageCollectorMXBean garbageCollectorMXBean;
    private static JMXConnector jmxConnector;
    private static MBeanServerConnection mbsc;

    public static void main(String[] args) throws Exception {
        String rmiHostname = "jmxserver";
        String defaultUrl = "service:jmx:rmi:///jndi/rmi://" + rmiHostname + ":1999/jmxrmi";
        JMXServiceURL jmxServiceURL = new JMXServiceURL(defaultUrl);

        Map<String,Object> jmxCredentials = new HashMap<String,Object>();
        String[] credentials = new String[]{"jmxusername", "jmxpassword"};
        jmxCredentials.put("jmx.remote.credentials", credentials);

       boolean run = true;
       while(run){
         try {
            if(garbageCollectorMXBean == null){
                if (mbsc == null){
                    jmxConnector = JMXConnectorFactory.connect(jmxServiceURL, jmxCredentials);
                    mbsc = jmxConnector.getMBeanServerConnection();
                }
                garbageCollectorMXBean = ManagementFactory.newPlatformMXBeanProxy(mbsc, GC_NAME,GarbageCollectorMXBean.class);
            }
            long count = garbageCollectorMXBean.getCollectionCount();
            System.out.println("Garbage Collector count = " + count);
        } catch (Exception e) {
            e.printStackTrace();
            garbageCollectorMXBean = null;
            if (jmxConnector != null)
            {
                try
                {
                    jmxConnector.close();
                } catch (IOException ioe) {}
                jmxConnector = null;
            }
            mbsc = null;
        }
        Thread.currentThread().sleep(5000);
    }
}

}

程序运行良好，但有时它会在每个循环中重复出现以下 IOException。

Exception in thread "main" java.io.IOException: The client has been closed.
at java.util.TimerThread.run(Timer.java:505)
at com.sun.jmx.remote.internal.ClientCommunicatorAdmin.restart(ClientCommunicatorAdmin.java:94)
at com.sun.jmx.remote.internal.ClientCommunicatorAdmin.gotIOException(ClientCommunicatorAdmin.java:54)
at javax.management.remote.rmi.RMIConnector$RMIClientCommunicatorAdmin.gotIOException(RMIConnector.java:1470)
at javax.management.remote.rmi.RMIConnector$RemoteMBeanServerConnection.getAttribute(RMIConnector.java:906)
at com.ibm.lang.management.OpenTypeMappingIHandler$6.run(OpenTypeMappingIHandler.java:506)
at java.security.AccessController.doPrivileged(AccessController.java:330)
at com.ibm.lang.management.OpenTypeMappingIHandler.invokeAttributeGetter(OpenTypeMappingIHandler.java:501)
at com.ibm.lang.management.OpenTypeMappingIHandler.invoke(OpenTypeMappingIHandler.java:121)
at com.sun.proxy.$Proxy112.getCollectionCount(Unknown Source)
at JMXTest.main(JMXTest.java:48)

查看代码，任何异常都会在 catch 块中捕获，其中所有字段都将初始化为 null，并且在下一个循环中，所有字段都将重新初始化。但是，查看日志，一旦异常开始出现，我只能在每个循环中的getCollectionCount() 调用中获得上述异常。我想知道即使对象被重新初始化，每次我得到同样的异常。

我正在从上述信息中查看以下内容

1. 在所有情况下，我们都会在上述场景中得到这个异常java.io.IOException: The client has been closed.。我知道，如果我们调用jmxConnector.close() 然后使用已经创建的garbageCollectorMXBean 对象来获取集合计数，我们就会得到这个。但我的代码不遵循这条路。
2. 对于上述问题，jmxserver 远程 JMX 服务器有贡献吗？我尝试通过停止/重新启动远程 JMX 服务器来重现，但无法做到。

Answer 1

从堆栈跟踪最后一行来看：

at com.sun.proxy.$Proxy112.getCollectionCount(Unknown Source)
at JMXTest.main(JMXTest.java:48)

看来问题出在代码行上

if (jmxConnector != null)
            {
                try
                {
                    jmxConnector.close();
                } catch (IOException ioe) {}
                jmxConnector = null;   //...line no.48
            }
            mbsc = null;    //......this is probably causing the issue

@raghavendra ，当您从 "JMXConnector" 获取 "MBeanServerConnection" 对象时，您应该按该顺序关闭对象/取消它们，即将您的代码更改为

if (jmxConnector != null)
            {
                try
                {
                    mbsc = null;    //...object handle assigned null before closing the connector 
                    jmxConnector.close();
                } catch (IOException ioe) {}
                jmxConnector = null;
            }