Oracle几天前发布的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。这个功能在VM核心、解释器、C1编译器、C2编译器以及GC中都有代码支持，并且通过JMX API暴露出来。 
不过新加的这功能不是加在java.开头的包里，而是加在com.sun.management.ThreadMXBean这个接口上，要用的话还得cast一下。 

当然，这么底层的功能不可能没有代价。添加这个功能后，在GC堆上分配空间的slow-path会比以前稍微慢一些。但希望对整体性能的影响并不大吧。 
Fast-path是在TLAB上分配空间的，而TLAB的分配数据是在TLAB refill的时候才批量更新，所以这个功能对fast-path的执行效率基本上没影响，以稍微放宽数据精准性为代价。 

相关链接： 
​​​Bug ID 7003271: Hotspot should track cumulative Java heap bytes allocated on a per-thread basis​​​ 
​​​changeset​​​ 

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。 

不知道为什么JVMTI没得到这个更新，或许是因为更新JVMTI spec涉及committee stuff？ 

============================================================== 

直接拿一段Groovy脚本来演示吧： 

先看JDK 6 update 24的情况 

Groovysh代码 

 

1. D:\sdk\groovy-1.7.2\bin>groovysh  
2. Groovy Shell (1.7.2, JVM: 1.6.0_24)  
3. Type 'help' or '\h' for help.  
4. -------------------------------------------------------------------------------  
5. groovy:000> import java.lang.management.*  
6. ===> [import java.lang.management.*]  
7. groovy:000> tb = ManagementFactory.threadMXBean  
8. ===> sun.management.ThreadImpl@8b677f  
9. groovy:000> tb.class.methods.name.unique().sort()  
10. ===> [dumpAllThreads, equals, findDeadlockedThreads, findMonitorDeadlockedThreads, getAllThreadIds, getClass, getCurrentThreadCpuTime, getCurrentThreadUserTime, getDaemonThreadCount, getPeakThreadCount, getThreadCount, getThreadCpuTime, getThreadInfo, getThreadUserTime, getTotalStartedThreadCount, hashCode, isCurrentThreadCpuTimeSupported, isObjectMonitorUsageSupported, isSynchronizerUsageSupported, isThreadContentionMonitoringEnabled, isThreadContentionMonitoringSupported, isThreadCpuTimeEnabled, isThreadCpuTimeSupported, notify, notifyAll, resetPeakThreadCount, setThreadContentionMonitoringEnabled, setThreadCpuTimeEnabled, toString, wait]  
11. groovy:000> tb.class.methods.findAll { it.name =~ /Alloc/}.each { println it }; null  
12. ===> null

这个时候ThreadMXBean上还没有跟alloc相关的方法。 
 

Groovysh代码 

1. D:\sdk\groovy-1.7.2\bin>groovysh  
2. Groovy Shell (1.7.2, JVM: 1.6.0_25)  
3. Type 'help' or '\h' for help.  
4. -------------------------------------------------------------------------------  
5. groovy:000> import java.lang.management.*  
6. ===> [import java.lang.management.*]  
7. groovy:000> tb = ManagementFactory.threadMXBean  
8. ===> sun.management.ThreadImpl@9b1670  
9. groovy:000> tb.class.methods.name.unique().sort()  
10. ===> [dumpAllThreads, equals, findDeadlockedThreads, findMonitorDeadlockedThreads, getAllThreadIds, getClass, getCurrentThreadCpuTime, getCurrentThreadUserTime, getDaemonThreadCount, getPeakThreadCount, getThreadAllocatedBytes, getThreadCount, getThreadCpuTime, getThreadInfo, getThreadUserTime, getTotalStartedThreadCount, hashCode, isCurrentThreadCpuTimeSupported, isObjectMonitorUsageSupported, isSynchronizerUsageSupported, isThreadAllocatedMemoryEnabled, isThreadAllocatedMemorySupported, isThreadContentionMonitoringEnabled, isThreadContentionMonitoringSupported, isThreadCpuTimeEnabled, isThreadCpuTimeSupported, notify, notifyAll, resetPeakThreadCount, setThreadAllocatedMemoryEnabled, setThreadContentionMonitoringEnabled, setThreadCpuTimeEnabled, toString, wait]  
11. groovy:000> tb.class.methods.findAll { it.name =~ /Alloc/}.each { println it };  
12. null  
13. public boolean sun.management.ThreadImpl.isThreadAllocatedMemoryEnabled()  
14. public boolean sun.management.ThreadImpl.isThreadAllocatedMemorySupported()  
15. public long[] sun.management.ThreadImpl.getThreadAllocatedBytes(long[])  
16. public long sun.management.ThreadImpl.getThreadAllocatedBytes(long)  
17. public void sun.management.ThreadImpl.setThreadAllocatedMemoryEnabled(boolean)  
18. ===> null  
19. groovy:000> tb.threadAllocatedMemoryEnabled  
20. ===> true  
21. groovy:000> tid = Thread.currentThread().id  
22. ===> 1  
23. groovy:000> tb.getThreadAllocatedBytes(tid)  
24. ===> 48106672  
25. groovy:000> tb.getThreadAllocatedBytes(tid)  
26. ===> 48751520  
27. groovy:000> tb.getThreadAllocatedBytes(tid)  
28. ===> 49384752  
29. groovy:000> tb.getThreadAllocatedBytes(tid)  
30. ===> 50086240  
31. groovy:000> quit