JVM性能調優監控工具jps、jstack、jmap、jhat、jstat、hprof使用詳解 - 實例分析

原文 http://josh-persistence.iteye.com/blog/2161848

第一部分：工具介紹部分：

現實企業級Java開發中，有時候我們會碰到下面這些問題：

OutOfMemoryError，內存不足
內存泄露
線程死鎖
鎖爭用（Lock Contention）
Java進程消耗CPU過高
......

這些問題在日常開發中可能被很多人忽視（比如有的人遇到上面的問題只是重啓服務器或者調大內存，而不會深究問題根源），但能夠理解並解決這些問題是Java程序員進階的必備要求。

A、 jps(Java Virtual Machine Process Status Tool)

jps主要用來輸出JVM中運行的進程狀態信息。語法格式如下：

jps [options] [hostid]

如果不指定hostid就默認爲當前主機或服務器。

命令行參數選項說明如下：

-q 不輸出類名、Jar名和傳入main方法的參數
-m 輸出傳入main方法的參數
-l 輸出main類或Jar的全限名
-v 輸出傳入JVM的參數

比如下面：

root@ubuntu:/# jps -m -l
2458 org.artifactory.standalone.main.Main /usr/local/artifactory-2.2.5/etc/jetty.xml
29920 com.sun.tools.hat.Main -port 9998 /tmp/dump.dat
3149 org.apache.catalina.startup.Bootstrap start
30972 sun.tools.jps.Jps -m -l
8247 org.apache.catalina.startup.Bootstrap start
25687 com.sun.tools.hat.Main -port 9999 dump.dat
21711 mrf-center.jar

B、 jstack

jstack主要用來查看某個Java進程內的線程堆棧信息。語法格式如下：

jstack [option] pid
jstack [option] executable core
jstack [option] [server-id@]remote-hostname-or-ip

命令行參數選項說明如下：

-l long listings，會打印出額外的鎖信息，在發生死鎖時可以用jstack -l pid來觀察鎖持有情況
-m mixed mode，不僅會輸出Java堆棧信息，還會輸出C/C++堆棧信息（比如Native方法）

jstack可以定位到線程堆棧，根據堆棧信息我們可以定位到具體代碼，所以它在JVM性能調優中使用得非常多。下面我們來一個實例找出某個Java進程中最耗費CPU的Java線程並定位堆棧信息，用到的命令有ps、top、printf、jstack、grep。

第一步先找出Java進程ID，服務器上的Java應用名稱爲mrf-center：

root@ubuntu:/# ps -ef | grep mrf-center | grep -v grep
root     21711     1  1 14:47 pts/3    00:02:10 java -jar mrf-center.jar

得到進程ID爲21711，第二步找出該進程內最耗費CPU的線程，可以使用
1）ps -Lfp pid
2）ps -mp pid -o THREAD, tid, time
3）top -Hp pid
用第三個，輸出如下：

TIME列就是各個Java線程耗費的CPU時間，CPU時間最長的是線程ID爲21742的線程，用

printf "%x\n" 21742

得到21742的十六進制值爲54ee，下面會用到。

OK，下一步終於輪到jstack上場了，它用來輸出進程21711的堆棧信息，然後根據線程ID的十六進制值grep，如下：

root@ubuntu:/# jstack 21711 | grep 54ee
"PollIntervalRetrySchedulerThread" prio=10 tid=0x00007f950043e000 nid=0x54ee in Object.wait()

可以看到CPU消耗在PollIntervalRetrySchedulerThread這個類的Object.wait()，我找了下我的代碼，定位到下面的代碼：

// Idle wait
getLog().info("Thread [" + getName() + "] is idle waiting...");
schedulerThreadState = PollTaskSchedulerThreadState.IdleWaiting;
long now = System.currentTimeMillis();
long waitTime = now + getIdleWaitTime();
long timeUntilContinue = waitTime - now;
synchronized(sigLock) {
  try {
    if(!halted.get()) {
      sigLock.wait(timeUntilContinue);
    }
  } 
  catch (InterruptedException ignore) {
  }
}

它是輪詢任務的空閒等待代碼，上面的sigLock.wait(timeUntilContinue)就對應了前面的Object.wait()。

C、 jmap（Memory Map）和jhat（Java Heap Analysis Tool）

jmap用來查看堆內存使用狀況，一般結合jhat使用。

jmap語法格式如下：

jmap [option] pid
jmap [option] executable core
jmap [option] [server-id@]remote-hostname-or-ip

如果運行在64位JVM上，可能需要指定-J-d64命令選項參數。

jmap -permstat pid

打印進程的類加載器和類加載器加載的持久代對象信息，輸出：類加載器名稱、對象是否存活（不可靠）、對象地址、父類加載器、已加載的類大小等信息，如下圖：

使用jmap -heap pid查看進程堆內存使用情況，包括使用的GC算法、堆配置參數和各代中堆內存使用情況。比如下面的例子

root@ubuntu:/# jmap -heap 21711
Attaching to process ID 21711, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 20.10-b01
 
using thread-local object allocation.
Parallel GC with 4 thread(s)
 
Heap Configuration:
  MinHeapFreeRatio = 40
  MaxHeapFreeRatio = 70
  MaxHeapSize		= 2067791872 (1972.0MB)
  NewSize			 = 1310720 (1.25MB)
  MaxNewSize		 = 17592186044415 MB
  OldSize			 = 5439488 (5.1875MB)
  NewRatio			= 2
  SurvivorRatio	 = 8
  PermSize			= 21757952 (20.75MB)
  MaxPermSize		= 85983232 (82.0MB)
 
Heap Usage:
PS Young Generation
Eden Space:
  capacity = 6422528 (6.125MB)
  used	  = 5445552 (5.1932830810546875MB)
  free	  = 976976 (0.9317169189453125MB)
  84.78829520089286% used
From Space:
  capacity = 131072 (0.125MB)
  used	  = 98304 (0.09375MB)
  free	  = 32768 (0.03125MB)
  75.0% used
To Space:
  capacity = 131072 (0.125MB)
  used	  = 0 (0.0MB)
  free	  = 131072 (0.125MB)
  0.0% used
PS Old Generation
  capacity = 35258368 (33.625MB)
  used	  = 4119544 (3.9287033081054688MB)
  free	  = 31138824 (29.69629669189453MB)
  11.683876009235595% used
PS Perm Generation
  capacity = 52428800 (50.0MB)
  used	  = 26075168 (24.867218017578125MB)
  free	  = 26353632 (25.132781982421875MB)
  49.73443603515625% used
  ....

使用jmap -histo[:live] pid查看堆內存中的對象數目、大小統計直方圖，如果帶上live則只統計活對象，如下：

root@ubuntu:/# jmap -histo:live 21711 | more
 
 num	  #instances			#bytes  class name
----------------------------------------------
  1:			38445		  5597736  <constMethodKlass>
  2:			38445		  5237288  <methodKlass>
  3:			 3500		  3749504  <constantPoolKlass>
  4:			60858		  3242600  <symbolKlass>
  5:			 3500		  2715264  <instanceKlassKlass>
  6:			 2796		  2131424  <constantPoolCacheKlass>
  7:			 5543		  1317400  [I
  8:			13714		  1010768  [C
  9:			 4752		  1003344  [B
  10:			 1225			639656  <methodDataKlass>
  11:			14194			454208  java.lang.String
  12:			 3809			396136  java.lang.Class
  13:			 4979			311952  [S
  14:			 5598			287064  [[I
  15:			 3028			266464  java.lang.reflect.Method
  16:			  280			163520  <objArrayKlassKlass>
  17:			 4355			139360  java.util.HashMap$Entry
  18:			 1869			138568  [Ljava.util.HashMap$Entry;
  19:			 2443			 97720  java.util.LinkedHashMap$Entry
  20:			 2072			 82880  java.lang.ref.SoftReference
  21:			 1807			 71528  [Ljava.lang.Object;
  22:			 2206			 70592  java.lang.ref.WeakReference
  23:			  934			 52304  java.util.LinkedHashMap
  24:			  871			 48776  java.beans.MethodDescriptor
  25:			 1442			 46144  java.util.concurrent.ConcurrentHashMap$HashEntry
  26:			  804			 38592  java.util.HashMap
  27:			  948			 37920  java.util.concurrent.ConcurrentHashMap$Segment
  28:			 1621			 35696  [Ljava.lang.Class;
  29:			 1313			 34880  [Ljava.lang.String;
  30:			 1396			 33504  java.util.LinkedList$Entry
  31:			  462			 33264  java.lang.reflect.Field
  32:			 1024			 32768  java.util.Hashtable$Entry
  33:			  948			 31440  [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;

class name是對象類型，說明如下:

B  byte
C  char
D  double
F  float
I  int
J  long
Z  boolean
[  數組，如[I表示int[]
[L+類名 其他對象

還有一個很常用的情況是：用jmap把進程內存使用情況dump到文件中，再用jhat分析查看。jmap進行dump命令格式如下：

jmap -dump:format=b,file=dumpFileName pid

我一樣地對上面進程ID爲21711進行Dump：

root@ubuntu:/# jmap -dump:format=b,file=/tmp/dump.dat 21711     
Dumping heap to /tmp/dump.dat ...
Heap dump file created

dump出來的文件可以用MAT、VisualVM等工具查看，這裏用jhat查看：

root@ubuntu:/# jhat -port 9998 /tmp/dump.dat
Reading from /tmp/dump.dat...
Dump file created Tue Jan 28 17:46:14 CST 2014
Snapshot read, resolving...
Resolving 132207 objects...
Chasing references, expect 26 dots..........................
Eliminating duplicate references..........................
Snapshot resolved.
Started HTTP server on port 9998
Server is ready.

注意如果Dump文件太大，可能需要加上-J-Xmx512m這種參數指定最大堆內存，即jhat -J-Xmx512m -port 9998 /tmp/dump.dat。然後就可以在瀏覽器中輸入主機地址:9998查看了：

上面紅線框出來的部分大家可以自己去摸索下，最後一項支持OQL（對象查詢語言）。

D、 jstat（JVM統計監測工具）

語法格式如下：

jstat [ generalOption | outputOptions vmid [interval[s|ms] [count]] ]

vmid是Java虛擬機ID，在Linux/Unix系統上一般就是進程ID。interval是採樣時間間隔。count是採樣數目。比如下面輸出的是GC信息，採樣時間間隔爲250ms，採樣數爲4：

root@ubuntu:/# jstat -gc 21711 250 4
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       PC     PU    YGC     YGCT    FGC    FGCT     GCT   
192.0  192.0   64.0   0.0    6144.0   1854.9   32000.0     4111.6   55296.0 25472.7    702    0.431   3      0.218    0.649
192.0  192.0   64.0   0.0    6144.0   1972.2   32000.0     4111.6   55296.0 25472.7    702    0.431   3      0.218    0.649
192.0  192.0   64.0   0.0    6144.0   1972.2   32000.0     4111.6   55296.0 25472.7    702    0.431   3      0.218    0.649
192.0  192.0   64.0   0.0    6144.0   2109.7   32000.0     4111.6   55296.0 25472.7    702    0.431   3      0.218    0.649

要明白上面各列的意義，先看JVM堆內存佈局：

可以看出：

堆內存 = 年輕代 + 年老代 + 永久代
年輕代 = Eden區 + 兩個Survivor區（From和To）

現在來解釋各列含義：

S0C、S1C、S0U、S1U：Survivor 0/1區容量（Capacity）和使用量（Used）
EC、EU：Eden區容量和使用量
OC、OU：年老代容量和使用量
PC、PU：永久代容量和使用量
YGC、YGT：年輕代GC次數和GC耗時
FGC、FGCT：Full GC次數和Full GC耗時
GCT：GC總耗時

E、hprof（Heap/CPU Profiling Tool）

hprof能夠展現CPU使用率，統計堆內存使用情況。

語法格式如下：

java -agentlib:hprof[=options] ToBeProfiledClass
java -Xrunprof[:options] ToBeProfiledClass
javac -J-agentlib:hprof[=options] ToBeProfiledClass

完整的命令選項如下：

Option Name and Value  Description                    Default
---------------------  -----------                    -------
heap=dump|sites|all    heap profiling                 all
cpu=samples|times|old  CPU usage                      off
monitor=y|n            monitor contention             n
format=a|b             text(txt) or binary output     a
file=<file>            write data to file             java.hprof[.txt]
net=<host>:<port>      send data over a socket        off
depth=<size>           stack trace depth              4
interval=<ms>          sample interval in ms          10
cutoff=<value>         output cutoff point            0.0001
lineno=y|n             line number in traces?         y
thread=y|n             thread in traces?              n
doe=y|n                dump on exit?                  y
msa=y|n                Solaris micro state accounting n
force=y|n              force output to <file>         y
verbose=y|n            print messages about dumps     y

來幾個官方指南上的實例。

CPU Usage Sampling Profiling(cpu=samples)的例子：

CPU Usage Sampling Profiling(cpu=samples)的例子：
  
java -agentlib:hprof=cpu=samples,interval=20,depth=3 Hello

  上面每隔20毫秒採樣CPU消耗信息，堆棧深度爲3，生成的profile文件名稱是java.hprof.txt，在當前目錄。 

  CPU Usage Times Profiling(cpu=times)的例子，它相對於CPU Usage Sampling Profile能夠獲得更加細粒度的CPU消耗信息，能夠細到每個方法調用的開始和結束，它的實現使用了字節碼注入技術（BCI）：

  
javac -J-agentlib:hprof=cpu=times Hello.java

  Heap Allocation Profiling(heap=sites)的例子：

  
javac -J-agentlib:hprof=heap=sites Hello.java

  Heap Dump(heap=dump)的例子，它比上面的Heap Allocation Profiling能生成更詳細的Heap Dump信息：

javac -J-agentlib:hprof=heap=dump Hello.java

  雖然在JVM啓動參數中加入-Xrunprof:heap=sites參數可以生成CPU/Heap Profile文件，但對JVM性能影響非常大，不建議在線上服務器環境使用。

第二部分：實例部分：

1、使用jstack來分析死鎖問題：

上面說明中提到jstack 是一個可以返回在應用程序上運行的各種各樣線程的一個完整轉儲的實用程序,您可以使用它查明問題。jstack [-l] <pid>,jpid可以通過使用jps命令來查看當前Java程序的jpid值,-l是可選參數,它可以顯示線程阻塞/死鎖情況

/**
 * Dead lock example
 * 
 * @author Josh Wang(Sheng)
 *
 * @email  josh_wang23@hotmail.com
 */
public class DeadLock2Live {  
  
  public static void main(String[] args) {  
    System.out.println(" start the example ----- ");
    final Object obj_1 = new Object(), obj_2 = new Object();  
      
    Thread t1 = new Thread("t1"){  
      @Override  
      public void run() {  
        synchronized (obj_1) {  
          try {  
            Thread.sleep(3000);  
          } catch (InterruptedException e) {}  
            
          synchronized (obj_2) {  
            System.out.println("thread t1 done.");  
          }  
        }  
      }  
    };  
      
    Thread t2 = new Thread("t2"){  
      @Override  
      public void run() {  
        synchronized (obj_2) {  
          try {  
            Thread.sleep(3000);  
          } catch (InterruptedException e) {}  
            
          synchronized (obj_1) {  
            System.out.println("thread t2 done.");  
          }  
        }  
      }  
    };  
      
    t1.start();  
    t2.start();  
  }  
    
}

以上DeadLock類是一個死鎖的例子,假使在我們不知情的情況下,運行DeadLock後,發現等了N久都沒有在屏幕打印線程完成信息。這個時候我們就可以使用jps查看該程序的jpid值和使用jstack來生產堆棧結果問題。

java -cp deadlock.jar DeadLock &

$ jps  
  3076 Jps  
  448 DeadLock  
$ jstack -l 448 > deadlock.jstack

結果文件deadlock.jstack內容如下：

2014-11-29 13:31:06
Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.65-b04 mixed mode):

"Attach Listener" daemon prio=5 tid=0x00007fd9d4002800 nid=0x440b waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"DestroyJavaVM" prio=5 tid=0x00007fd9d4802000 nid=0x1903 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"t2" prio=5 tid=0x00007fd9d30ac000 nid=0x5903 waiting for monitor entry [0x000000011da46000]
  java.lang.Thread.State: BLOCKED (on object monitor)
  at DeadLock$2.run(DeadLock.java:38)
  - waiting to lock <0x00000007aaba7e58> (a java.lang.Object)
  - locked <0x00000007aaba7e68> (a java.lang.Object)

  Locked ownable synchronizers:
  - None

"t1" prio=5 tid=0x00007fd9d30ab800 nid=0x5703 waiting for monitor entry [0x000000011d943000]
  java.lang.Thread.State: BLOCKED (on object monitor)
  at DeadLock$1.run(DeadLock.java:23)
  - waiting to lock <0x00000007aaba7e68> (a java.lang.Object)
  - locked <0x00000007aaba7e58> (a java.lang.Object)

  Locked ownable synchronizers:
  - None

"Service Thread" daemon prio=5 tid=0x00007fd9d2809000 nid=0x5303 runnable [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"C2 CompilerThread1" daemon prio=5 tid=0x00007fd9d304e000 nid=0x5103 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"C2 CompilerThread0" daemon prio=5 tid=0x00007fd9d2800800 nid=0x4f03 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"Signal Dispatcher" daemon prio=5 tid=0x00007fd9d3035000 nid=0x4d03 runnable [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"Finalizer" daemon prio=5 tid=0x00007fd9d2013000 nid=0x3903 in Object.wait() [0x000000011d18d000]
  java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(Native Method)
  - waiting on <0x00000007aaa85608> (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
  - locked <0x00000007aaa85608> (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
  at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

  Locked ownable synchronizers:
  - None

"Reference Handler" daemon prio=5 tid=0x00007fd9d2012000 nid=0x3703 in Object.wait() [0x000000011d08a000]
  java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(Native Method)
  - waiting on <0x00000007aaa85190> (a java.lang.ref.Reference$Lock)
  at java.lang.Object.wait(Object.java:503)
  at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
  - locked <0x00000007aaa85190> (a java.lang.ref.Reference$Lock)

  Locked ownable synchronizers:
  - None

"VM Thread" prio=5 tid=0x00007fd9d5011000 nid=0x3503 runnable 

"GC task thread#0 (ParallelGC)" prio=5 tid=0x00007fd9d200b000 nid=0x2503 runnable 

"GC task thread#1 (ParallelGC)" prio=5 tid=0x00007fd9d200b800 nid=0x2703 runnable 

"GC task thread#2 (ParallelGC)" prio=5 tid=0x00007fd9d200c800 nid=0x2903 runnable 

"GC task thread#3 (ParallelGC)" prio=5 tid=0x00007fd9d200d000 nid=0x2b03 runnable 

"GC task thread#4 (ParallelGC)" prio=5 tid=0x00007fd9d200d800 nid=0x2d03 runnable 

"GC task thread#5 (ParallelGC)" prio=5 tid=0x00007fd9d200e000 nid=0x2f03 runnable 

"GC task thread#6 (ParallelGC)" prio=5 tid=0x00007fd9d200f000 nid=0x3103 runnable 

"GC task thread#7 (ParallelGC)" prio=5 tid=0x00007fd9d200f800 nid=0x3303 runnable 

"VM Periodic Task Thread" prio=5 tid=0x00007fd9d3033800 nid=0x5503 waiting on condition 

JNI global references: 114


Found one Java-level deadlock:
=============================
"t2":
  waiting to lock monitor 0x00007fd9d30aebb8 (object 0x00000007aaba7e58, a java.lang.Object),
  which is held by "t1"
"t1":
  waiting to lock monitor 0x00007fd9d28128b8 (object 0x00000007aaba7e68, a java.lang.Object),
  which is held by "t2"

Java stack information for the threads listed above:
===================================================
"t2":
  at DeadLock$2.run(DeadLock.java:38)
  - waiting to lock <0x00000007aaba7e58> (a java.lang.Object)
  - locked <0x00000007aaba7e68> (a java.lang.Object)
"t1":
  at DeadLock$1.run(DeadLock.java:23)
  - waiting to lock <0x00000007aaba7e68> (a java.lang.Object)
  - locked <0x00000007aaba7e58> (a java.lang.Object)

Found 1 deadlock.

從這個結果文件我們一看到發現了一個死鎖,具體是線程t2在等待線程t1,而線程t1在等待線程t2造成的,同時也記錄了線程的堆棧和代碼行數,通過這個堆棧和行數我們就可以去檢查對應的代碼塊,從而發現問題和解決問題。

可通過下面的代碼解決死鎖問題：

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Dead lock example
 * 
 * @author Josh Wang(Sheng)
 *
 * @email  josh_wang23@hotmail.com
 */
public class DeadLock2Live {  
  
  public static void main(String[] args) {  
    System.out.println(" start the example ----- ");
    final Lock lock = new ReentrantLock(); 
      
    Thread t1 = new Thread("t1") {  
      @Override  
      public void run() {  
        try {  
         	lock.lock();
          Thread.sleep(3000); 
          System.out.println("thread t1 done.");
        } catch (InterruptedException e) {
          e.printStackTrace();
        } finally {
          lock.unlock();
        }
      }
      };  
      
    Thread t2 = new Thread("t2") {  
      @Override  
      public void run() {  
        try {  
          lock.lock();
          Thread.sleep(3000);
          System.out.println("thread t2 done.");
           

        }  catch (InterruptedException e) {
          e.printStackTrace();
        } finally {
          lock.unlock();
        }
      }  
    };  
      
    t1.start();  
    t2.start();  
  
}
    
}

2、繼續使用jstack來分析HashMap在多線程情況下的死鎖問題：

對於如下代碼，使用10個線程來處理提交的2000個任務，每個任務會分別循環往hashmap中分別存入和取出1000個數，通過測試發現，程序並不能完整執行完成。[PS:該程序能不能成功執行完，有時也取決於所使用的服務器的運行狀況，我在筆記本上測試的時候，大多時候該程序不能成功執行完成，後者會出現CPU轉速加快，發熱等情況]

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 
 */

/**
 * @author Josh Wang(Sheng)
 *
 * @email  josh_wang23@hotmail.com
 */
public class HashMapDeadLock implements Callable<Integer> {
  
  private static ExecutorService threadPool = Executors.newFixedThreadPool(10);
  
  private static Map<Integer, Integer> results = new HashMap<>();

  @Override
  public Integer call() throws Exception {
    results.put(1, 1);
    results.put(2, 2);
    results.put(3, 3);
    
    for (int i = 0; i < 1000; i++) {
      results.put(i, i);
    }
    
    Thread.sleep(1000);
    
    for (int i= 0; i < 1000; i++) {
      results.remove(i);
    }
    
    System.out.println(" ---- " + Thread.currentThread().getName()  + "		" + results.get(0));
    
    return results.get(1);
  }
  
  
  public static void main(String[] args) throws InterruptedException, ExecutionException {
    try {
      for (int i = 0; i < 2000; i++) {
          HashMapDeadLock hashMapDeadLock  = new HashMapDeadLock();
//					Future<Integer> future = threadPool.submit(hashMapDeadLock);
//					future.get();
          threadPool.submit(hashMapDeadLock);
        }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      threadPool.shutdown();
    }
  
    
    
    
  }

  
}

1）使用jps查看線程可得：

43221 Jps
30056 
43125 HashMapDeadLock

2）使用jstack導出多線程棧區信息：

jstack -l 43125 > hash.jstack

3) hash.jstack的內容如下：

2014-11-29 18:14:22
Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.65-b04 mixed mode):

"Attach Listener" daemon prio=5 tid=0x00007f83ee08a000 nid=0x5d07 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"DestroyJavaVM" prio=5 tid=0x00007f83eb016800 nid=0x1903 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"pool-1-thread-10" prio=5 tid=0x00007f83ec80a000 nid=0x6903 runnable [0x000000011cd19000]
  java.lang.Thread.State: RUNNABLE
  at java.util.HashMap.transfer(HashMap.java:601)
  at java.util.HashMap.resize(HashMap.java:581)
  at java.util.HashMap.addEntry(HashMap.java:879)
  at java.util.HashMap.put(HashMap.java:505)
  at HashMapDeadLock.call(HashMapDeadLock.java:30)
  at HashMapDeadLock.call(HashMapDeadLock.java:1)
  at java.util.concurrent.FutureTask.run(FutureTask.java:262)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
  at java.lang.Thread.run(Thread.java:745)

  Locked ownable synchronizers:
  - <0x00000007aaba84c8> (a java.util.concurrent.ThreadPoolExecutor$Worker)

"Service Thread" daemon prio=5 tid=0x00007f83eb839800 nid=0x5303 runnable [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"C2 CompilerThread1" daemon prio=5 tid=0x00007f83ee002000 nid=0x5103 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"C2 CompilerThread0" daemon prio=5 tid=0x00007f83ee000000 nid=0x4f03 waiting on condition [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"Signal Dispatcher" daemon prio=5 tid=0x00007f83ec04c800 nid=0x4d03 runnable [0x0000000000000000]
  java.lang.Thread.State: RUNNABLE

  Locked ownable synchronizers:
  - None

"Finalizer" daemon prio=5 tid=0x00007f83eb836800 nid=0x3903 in Object.wait() [0x000000011bc58000]
  java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(Native Method)
  - waiting on <0x00000007aaa85608> (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
  - locked <0x00000007aaa85608> (a java.lang.ref.ReferenceQueue$Lock)
  at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
  at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)

  Locked ownable synchronizers:
  - None

"Reference Handler" daemon prio=5 tid=0x00007f83eb01a800 nid=0x3703 in Object.wait() [0x000000011bb55000]
  java.lang.Thread.State: WAITING (on object monitor)
  at java.lang.Object.wait(Native Method)
  - waiting on <0x00000007aaa85190> (a java.lang.ref.Reference$Lock)
  at java.lang.Object.wait(Object.java:503)
  at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
  - locked <0x00000007aaa85190> (a java.lang.ref.Reference$Lock)

  Locked ownable synchronizers:
  - None

"VM Thread" prio=5 tid=0x00007f83ed808800 nid=0x3503 runnable 

"GC task thread#0 (ParallelGC)" prio=5 tid=0x00007f83ec80d800 nid=0x2503 runnable 

"GC task thread#1 (ParallelGC)" prio=5 tid=0x00007f83ec80e000 nid=0x2703 runnable 

"GC task thread#2 (ParallelGC)" prio=5 tid=0x00007f83ec001000 nid=0x2903 runnable 

"GC task thread#3 (ParallelGC)" prio=5 tid=0x00007f83ec002000 nid=0x2b03 runnable 

"GC task thread#4 (ParallelGC)" prio=5 tid=0x00007f83ec002800 nid=0x2d03 runnable 

"GC task thread#5 (ParallelGC)" prio=5 tid=0x00007f83ec003000 nid=0x2f03 runnable 

"GC task thread#6 (ParallelGC)" prio=5 tid=0x00007f83ec003800 nid=0x3103 runnable 

"GC task thread#7 (ParallelGC)" prio=5 tid=0x00007f83ec004800 nid=0x3303 runnable 

"VM Periodic Task Thread" prio=5 tid=0x00007f83ec814800 nid=0x5503 waiting on condition 

JNI global references: 134

4）從紅色高亮部分可看出，代碼中的30行出問題了，即往hashmap中寫入數據出問題了：

results.put(i, i);

很快就明白因爲Hashmap不是線程安全的，所以問題就出在這個地方，我們可以使用線程安全的map即

ConcurrentHashMap後者HashTable來解決該問題：

import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 
 */

/**
 * @author Josh Wang(Sheng)
 *
 * @email  josh_wang23@hotmail.com
 */
public class HashMapDead2LiveLock implements Callable<Integer> {
  
  private static ExecutorService threadPool = Executors.newFixedThreadPool(10);
  
  private static Map<Integer, Integer> results = new ConcurrentHashMap<>();

  @Override
  public Integer call() throws Exception {
    results.put(1, 1);
    results.put(2, 2);
    results.put(3, 3);
    
    for (int i = 0; i < 1000; i++) {
      results.put(i, i);
    }
    
    Thread.sleep(1000);
    
    for (int i= 0; i < 1000; i++) {
      results.remove(i);
    }
    
    System.out.println(" ---- " + Thread.currentThread().getName()  + "		" + results.get(0));
    
    return results.get(1);
  }
  
  
  public static void main(String[] args) throws InterruptedException, ExecutionException {
    try {
      for (int i = 0; i < 2000; i++) {
          HashMapDead2LiveLock hashMapDeadLock  = new HashMapDead2LiveLock();
//					Future<Integer> future = threadPool.submit(hashMapDeadLock);
//					future.get();
          threadPool.submit(hashMapDeadLock);
        }
    } catch (Exception e) {
      e.printStackTrace();
    } finally {
      threadPool.shutdown();
    }
  
    
    
    
  }

  
}

改成ConcurrentHashMap後，重新執行該程序，你會發現很快該程序就執行完了。

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