英文原文：http://blogs.manageengine.com/appmanager/2011/02/09/identify-java-code-consuming-high-cpu-in-linux-linking-jvm-thread-and-linux-pid

在Linux中，我們可以非常方便地找出佔用了大量CPU的有問題的Java程式碼。讓我通過下面的例子詳細解釋一下具體的操作步驟：

package test; public class ThreadDumpTest { public void test() { for (int i = 0; i < 10; i++) { Thread th = new Thread(new TR(i)); th.setName("MyThread-" + (1000 + i)); th.start(); } } public static void main(String[] args) { ThreadDumpTest t = new ThreadDumpTest(); t.test(); } private class TR implements Runnable { int ins = 0; TR(int i) { ins = i; } public void run() { while (true) { if (ins != 5) { try { Thread.sleep(10000); } catch (Exception e) { e.printStackTrace(); } } } } } }

在上面的例子中，所有執行緒都在執行while迴圈。除了MyThread-1005 之外，其它執行緒都會不停地在迴圈中sleep 10秒鐘。MyThread-1005 執行緒不會進入sleep部分的程式碼，所以會不停地迴圈而不做sleep。因為有while迴圈，所以MyThread-1005 執行緒會導致很高的CPU利用率。

如何找到佔用了大量CPU的執行緒？

步驟1：

在命令列中執行'top'命令。你會看到一個PID為7074的'java'程序佔用了97%的CPU。

步驟2：

'top'命令顯示瞭如上圖所示的程序列表。按'shift + h'，等幾秒鐘。你會看到上面游標的位置出現'Show threads on'的訊息字樣。現在，你可以看到執行緒級別的CPU/Memory利用率的詳細情況。你可以看到一個java執行緒，PID是7087使用了97%的CPU。

步驟3：

有問題的執行緒的PID（7087）是十進位制的。把它轉換成十六進位制 。7087相應的十六進位制的值是1BAF。然後轉換成小寫（1baf）。

步驟4：

做一個Thread Dump（kill -3 7074)，然後在Thread Dump中查詢轉換出來的十六進位制的執行緒PID（1baf）。你可以找到一個'0x'開頭的'nid '值為1baf（20到22行）。你可以看到正是例子程式中有問題的執行緒MyThread-1005 。從這個執行緒的stack trace中，你可以找到有問題的程式碼然後修復它。

[rameshj@rameshj thread]$ java test/ThreadDumpTest ^/Full thread dump Java HotSpot(TM) Client VM (1.5.0_06-b05 mixed mode): "DestroyJavaVM" prio=1 tid=0x083d2d78 nid=0x1ba2 waiting on condition [0x00000000..0xbff4bf60] "MyThread-1009" prio=1 tid=0x08510a88 nid=0x1bb3 waiting on condition [0xa9cca000..0xa9ccb020] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1008" prio=1 tid=0x08510318 nid=0x1bb2 waiting on condition [0xa9d4b000..0xa9d4bea0] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1007" prio=1 tid=0x0850f298 nid=0x1bb1 waiting on condition [0xa9dcc000..0xa9dccf20] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1006" prio=1 tid=0x0850d818 nid=0x1bb0 waiting on condition [0xa9e4d000..0xa9e4dda0] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1005" prio=1 tid=0x0850c9f8 nid=0x1baf runnable [0xa9ece000..0xa9ecee20] at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:43) at java.lang.Thread.run(Thread.java:595) "MyThread-1004" prio=1 tid=0x0850b8e8 nid=0x1bae waiting on condition [0xa9f4f000..0xa9f500a0] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1003" prio=1 tid=0x0850aac8 nid=0x1bad waiting on condition [0xa9fd0000..0xa9fd1120] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1002" prio=1 tid=0x085099b8 nid=0x1bac waiting on condition [0xaa051000..0xaa051fa0] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1001" prio=1 tid=0x08508a28 nid=0x1bab waiting on condition [0xaa0d2000..0xaa0d3020] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "MyThread-1000" prio=1 tid=0x08508520 nid=0x1baa waiting on condition [0xaa153000..0xaa153ea0] at java.lang.Thread.sleep(Native Method) at test.ThreadDumpTest$TR.run(ThreadDumpTest.java:45) at java.lang.Thread.run(Thread.java:595) "Low Memory Detector" daemon prio=1 tid=0x0843cf58 nid=0x1ba8 runnable [0x00000000..0x00000000] "CompilerThread0" daemon prio=1 tid=0x0843b930 nid=0x1ba7 waiting on condition [0x00000000..0xaa3eb808] "Signal Dispatcher" daemon prio=1 tid=0x0843a9d0 nid=0x1ba6 waiting on condition [0x00000000..0x00000000] "Finalizer" daemon prio=1 tid=0x08430740 nid=0x1ba5 in Object.wait() [0xaa52c000..0xaa52d120] at java.lang.Object.wait(Native Method) - waiting on <0xaa6b0848> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:116) - locked <0xaa6b0848> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:132) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159) "Reference Handler" daemon prio=1 tid=0x0842e898 nid=0x1ba4 in Object.wait() [0xaa5ad000..0xaa5adfa0] at java.lang.Object.wait(Native Method) - waiting on <0xaa6b0758> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:474) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116) - locked <0xaa6b0758> (a java.lang.ref.Reference$Lock) "VM Thread" prio=1 tid=0x0842bb68 nid=0x1ba3 runnable "VM Periodic Task Thread" prio=1 tid=0x0843e538 nid=0x1ba9 waiting on condition

- Ramesh