运行JVM的GC参数是什么？

问题描述

我仍然在调查我使用GC调整的问题（请参阅

以下示例将输出选项的值以及值是DEFAULT还是VM_CREATION：

  import java.lang.management.ManagementFactory; 
 
 import javax.management.ObjectName; 
 import javax.management.openmbean.CompositeData; 
 import javax.management.openmbean.CompositeDataSupport; 
 
 public class HotSpotTest {
 
 public static void main（String [] args）throws Exception {
 printHotSpotOption（MaxHeapFreeRatio）; 
 printHotSpotOption（SurvivorRatio）; 
 printHotSpotOptions（）; 
} 
 
 private static void printHotSpotOption（String option）throws Exception {
 ObjectName name = new ObjectName（com.sun.management:type=HotSpotDiagnostic）; 
 String operationName =getVMOption; 
 Object [] params = new Object [] {option}; 
 String [] signature = new String [] {String.class.getName（）}; 
 Object result = ManagementFactory.getPlatformMBeanServer（）。invoke（name，operationName，params，signature）; 
 CompositeDataSupport data =（CompositeDataSupport）结果; 
 
 System.out.println（option）; 
 System.out.println（ -  Value：+ data.get（value））; 
 System.out.println（ -  Origin：+ data.get（origin））; 
} 
 
 private static void printHotSpotOptions（）throws Exception {
 ObjectName name = new ObjectName（com.sun.management：type = HotSpotDiagnostic）; 
 String attributeName =DiagnosticOptions; 
 Object result = ManagementFactory.getPlatformMBeanServer（）。getAttribute（name，attributeName）; 
 CompositeData [] array =（CompositeData []）result; 
 for（CompositeData d：array）{
 System.out.println（d.get（name））; 
 System.out.println（ -  Value：+ d.get（value））; 
 System.out.println（ -  Origin：+ d.get（origin））; 
} 
} 
} 
  

I'm still investigating issues I have with GC tuning (see prior question), which involves lots of reading and experimentation.

Sun Java5+ JVMs attempt to automatically select the optimal GC strategy and parameters based on their environment, which is great, but I can't figure out how to query the running JVM to find out what those parameters are.

Ideally, I'd like to see what values of the various GC-related -XX options are being used, as selected automatically by the VM. If I had that, I could have a baseline to begin tweaking.

Anyone know to recover these values from a running VM?

解决方案

Check out the HotSpotDiagnosticMBean

The following example will print out the value of an option as well as whether the value is DEFAULT or VM_CREATION:

import java.lang.management.ManagementFactory;

import javax.management.ObjectName;
import javax.management.openmbean.CompositeData;
import javax.management.openmbean.CompositeDataSupport;

public class HotSpotTest {

    public static void main(String [] args) throws Exception {
        printHotSpotOption("MaxHeapFreeRatio");
        printHotSpotOption("SurvivorRatio");
        printHotSpotOptions();
    }

    private static void printHotSpotOption(String option) throws Exception {
        ObjectName name = new ObjectName("com.sun.management:type=HotSpotDiagnostic");
        String operationName = "getVMOption";
        Object [] params = new Object [] {option};
        String [] signature = new String[] {String.class.getName()};
        Object result = ManagementFactory.getPlatformMBeanServer().invoke(name, operationName, params, signature);
        CompositeDataSupport data = (CompositeDataSupport) result;

        System.out.println(option);
        System.out.println("- Value: "+data.get("value"));
        System.out.println("- Origin: "+data.get("origin"));
    }

    private static void printHotSpotOptions() throws Exception {
        ObjectName name = new ObjectName("com.sun.management:type=HotSpotDiagnostic");
        String attributeName = "DiagnosticOptions";
        Object result = ManagementFactory.getPlatformMBeanServer().getAttribute(name, attributeName);
        CompositeData [] array = (CompositeData[]) result;
        for (CompositeData d : array) {
            System.out.println(d.get("name"));
            System.out.println("- Value: "+d.get("value"));
            System.out.println("- Origin: "+d.get("origin"));
        }
    }
}

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