2013-03-21

跟踪JVM在每个线程上分配的堆内存

JDK 6 update 25 里添加的一个新功能非常有趣，可以按照线程来跟踪（GC堆）内存的分配量。

JMX中，该功能由ThreadMXBean上新增的几个方法提供。详情可见下面例子。
ThreadMXBean.getThreadAllocatedBytes(long threadId)的用法基本上可以看成跟System.currentTimeMillis()用于计时的用法一样，在两点上记录并且求差即可。

import java.lang.management.ManagementFactory;
import java.text.DecimalFormat;
import java.util.ArrayList;
import java.util.List;

import com.sun.management.ThreadMXBean;

@SuppressWarnings("restriction")
public class TestThreadAllocateBytes {

    public static void main(String[] args) {

        for (int i = 0; i < 2; i++) {
            new Thread(new Task()).start();
        }

        while(true){

        }
    }

    public static class Task implements Runnable{

        @Override
        public void run() {

            ThreadMXBean threadMXBean = (ThreadMXBean) ManagementFactory.getThreadMXBean();
            long startSize = threadMXBean.getThreadAllocatedBytes(Thread.currentThread().getId());
            System.out.println(Thread.currentThread().getName() + " start " + startSize);

            String msg = "hello world";
            List<String> list = new ArrayList<>();
            for (int i = 0; i < 10000; i++) {
                list.add(msg + i);
            }

            long endSize = threadMXBean.getThreadAllocatedBytes(Thread.currentThread().getId());
            long costBytes = endSize - startSize;

            System.out.println(Thread.currentThread().getName() + " end " + endSize);
            System.out.println(Thread.currentThread().getName() + " 消耗了内存： " + readableFileSize(costBytes));


            try {
                Thread.sleep(1000 * 1000 * 1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

    }


    public static String readableFileSize(long size) {
        if(size <= 0) return "0";
        final String[] units = new String[] { "B", "KB", "MB", "GB", "TB" };
        int digitGroups = (int) (Math.log10(size)/Math.log10(1024));
        return new DecimalFormat("#,##0.#").format(size/Math.pow(1024, digitGroups)) + " " + units[digitGroups];
    }

}

输出

Thread-0 start 7568
Thread-0 end 1862032
Thread-1 start 48
Thread-1 end 1849304
Thread-1 消耗了内存： 1.8 MB
Thread-0 消耗了内存： 1.8 MB

通过jvisualvm 查看JVM的内存分配

1.查看每个对象占用的堆内存大小

#####2.查看每个线程占用的堆内存大小

参考资料