I will divide these exercises into sub topics and we will learn one by one. We will start with simple exercise first.
How to name a thread?
To set a name for a thread, you will need to call method setName(), then show the currently running thread using Thread.currentThread().getName(). See example below.
public class Minion implements Runnable {
private String name = null;
public Minion(String name) {
this.name = name;
}
@Override
public void run() {
Thread.currentThread().setName(name);
System.out.printf("minion %s eating banana", Thread.currentThread().getName());
}
}Another modified version as below. Note that we do not set thread name here, we set it in the class ThreadFactory. Read on to find out why later in this article.
import java.util.Random;
public class Minion implements Runnable {
private static String[] fruits = {"apple", "banana"};
public Minion() {
}
@Override
public void run() {
try {
// banana or apple
int idx = new Random().nextInt(fruits.length);
System.out.printf("minion %s eating %s %n", Thread.currentThread().getName(), fruits[idx]);
Thread.sleep((idx * 1000) + 10000);
StackTraceElement[] ste =Thread.currentThread().getStackTrace();
System.out.printf("minion %s done eat %s %n", Thread.currentThread().getName(), fruits[idx]);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
You can pass the task in the constructor as a parameter but for the sake of simplicity, I hard coded task within this worker thread, two fruits; apple and banana. Longer thread sleep because later in this article, we will using this class for getting heap dump and thread dump.
How to name the thread pool?
To change name of a thread pool, you will need to implement ThreadFactory and then override namePrefix in the constructor. There are some other libraries which is easier and do that for you but we will not look in other libraries but only java native libraries. Let's do the work, see example below.
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicInteger;
public class GruThreadFactory implements ThreadFactory {
static final AtomicInteger poolNumber = new AtomicInteger(1);
final ThreadGroup group;
final AtomicInteger threadNumber = new AtomicInteger(1);
final String namePrefix;
public GruThreadFactory() {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();
namePrefix = "GruPool-" + poolNumber.getAndIncrement() + "-thread-";
}
@Override
public Thread newThread(Runnable r) {
final Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
and the modified version. As you can see, minions are passed into GruThreadFactory constructor and then each time a newThread() is called, the thread name is named as random to the String array minions.
import java.util.Random;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicInteger;
public class GruThreadFactory implements ThreadFactory {
static final AtomicInteger poolNumber = new AtomicInteger(1);
final ThreadGroup group;
final AtomicInteger threadNumber = new AtomicInteger(1);
final String namePrefix;
final String[] minions;
public GruThreadFactory(String[] minions) {
SecurityManager s = System.getSecurityManager();
group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup();
namePrefix = "GruPool-" + poolNumber.getAndIncrement() + "-thread-";
this.minions = minions;
}
@Override
public Thread newThread(Runnable r) {
int index = new Random().nextInt(minions.length);
final Thread t = new Thread(group, r, namePrefix + minions[index], 0);
if (t.isDaemon())
t.setDaemon(false);
if (t.getPriority() != Thread.NORM_PRIORITY)
t.setPriority(Thread.NORM_PRIORITY);
return t;
}
}
and then out main app to bind the ThreadFactory and the workers.
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
public class MainApp {
/**
* @param args
* @throws InterruptedException
*/
public static void main(String[] args) throws InterruptedException {
ThreadFactory GruFactory = null;
ExecutorService exec = null;
try {
String[] minions = {"Dave", "Stuart", "Kevin", "Jerry", "Carl", "Phil", "Paul", "Mike", "Jorge"};
GruFactory = new GruThreadFactory(minions);
exec = Executors.newFixedThreadPool(minions.length, GruFactory);
for (int i = 0; i < 90; i++) {
Minion m = new Minion();
exec.execute(m);
}
exec.shutdown();
while (!exec.isTerminated()) {
}
} catch (Exception e) {
e.printStackTrace();
} finally {
System.out.println("done");
}
}
}
Now to check. Output below is from the modified version. Note that you may have different output than mine but that is as expected. I paste only a few lines below.
minion GruPool-1-thread-Mike eating banana
minion GruPool-1-thread-Phil eating banana
minion GruPool-1-thread-Mike eating apple
minion GruPool-1-thread-Stuart eating banana
minion GruPool-1-thread-Mike done eat apple
minion GruPool-1-thread-Paul eating banana
minion GruPool-1-thread-Stuart done eat banana
minion GruPool-1-thread-Stuart eating apple
minion GruPool-1-thread-Stuart done eat apple
minion GruPool-1-thread-Stuart eating apple
minion GruPool-1-thread-Stuart done eat apple
minion GruPool-1-thread-Stuart eating banana
..
..
minion GruPool-1-thread-Paul done eat apple
minion GruPool-1-thread-Jorge eating apple
minion GruPool-1-thread-Jorge done eat apple
minion GruPool-1-thread-Mike done eat banana
minion GruPool-1-thread-Paul done eat banana
minion GruPool-1-thread-Mike done eat banana
minion GruPool-1-thread-Jerry done eat banana
minion GruPool-1-thread-Stuart done eat banana
minion GruPool-1-thread-Mike done eat banana
done
So far so good, our app work.
How to get a stack dump?
In order to get a stack dump, you can use jstack. When you install jdk, jstack came with it. To get the stack dump,
jstack -F 19557 &> jstack.log
where 19557 is the pid of the java process and we redirected stdout and stderr to a file. There are several command to get the process id, you can either use ps or java jps. Output as attached.
Attaching to process ID 19557, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 20.6-b01
Deadlock Detection:
No deadlocks found.
Thread 19586: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19585: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19584: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19583: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19582: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19581: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19580: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19579: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19578: (state = BLOCKED)
- java.lang.Thread.sleep(long) @bci=0 (Interpreted frame)
- Minion.run() @bci=55, line=18 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.runTask(java.lang.Runnable) @bci=59, line=886 (Interpreted frame)
- java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=28, line=908 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=662 (Interpreted frame)
Thread 19573: (state = BLOCKED)
Thread 19572: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=0 (Interpreted frame)
- java.lang.ref.ReferenceQueue.remove(long) @bci=44, line=118 (Interpreted frame)
- java.lang.ref.ReferenceQueue.remove() @bci=2, line=134 (Interpreted frame)
- java.lang.ref.Finalizer$FinalizerThread.run() @bci=3, line=159 (Interpreted frame)
Thread 19571: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=0 (Interpreted frame)
- java.lang.Object.wait() @bci=2, line=485 (Interpreted frame)
- java.lang.ref.Reference$ReferenceHandler.run() @bci=46, line=116 (Interpreted frame)
Thread 19564: (state = IN_JAVA)
- MainApp.main(java.lang.String[]) @bci=119, line=28 (Compiled frame; information may be imprecise)
jstack output show
- it has 9 threads with Minion.run().
- no deadlocks
- individual thread state was blocked because it was sleeping.
- main thread state is in IN_JAVA which is running in Java or in stub code.
Another useful information could probably be thread dump. With visualvm, you can actually see the thread running over time, I recorded a video and you should try to watch it.
http://www.youtube.com/watch?v=REoi6sbcIUY
Also see the thread dump capture using VisualVM
2014-10-07 22:58:32
Full thread dump Java HotSpot(TM) 64-Bit Server VM (20.6-b01 mixed mode):
"RMI TCP Connection(2)-127.0.0.1" daemon prio=10 tid=0x00007fd5c0003000 nid=0x646c runnable [0x00007fd618af8000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:218)
at java.io.BufferedInputStream.read(BufferedInputStream.java:237)
- locked <0x00000007d7ece620> (a java.io.BufferedInputStream)
at java.io.FilterInputStream.read(FilterInputStream.java:66)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:517)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d79e3908> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"JMX server connection timeout 22" daemon prio=10 tid=0x00007fd5b4008000 nid=0x6464 in Object.wait() [0x00007fd618bf9000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000007d7b6ddb8> (a [I)
at com.sun.jmx.remote.internal.ServerCommunicatorAdmin$Timeout.run(ServerCommunicatorAdmin.java:150)
- locked <0x00000007d7b6ddb8> (a [I)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- None
"RMI Scheduler(0)" daemon prio=10 tid=0x00007fd5b4004800 nid=0x6463 waiting on condition [0x00007fd618cfa000]
java.lang.Thread.State: TIMED_WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000007d799dc18> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:196)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2025)
at java.util.concurrent.DelayQueue.take(DelayQueue.java:164)
at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:609)
at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:602)
at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:947)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- None
"RMI TCP Connection(1)-127.0.0.1" daemon prio=10 tid=0x00007fd5c0001800 nid=0x6462 runnable [0x00007fd618efc000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at java.io.BufferedInputStream.fill(BufferedInputStream.java:218)
at java.io.BufferedInputStream.read(BufferedInputStream.java:237)
- locked <0x00000007d7b1e8f8> (a java.io.BufferedInputStream)
at java.io.FilterInputStream.read(FilterInputStream.java:66)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:517)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d79e3430> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"RMI TCP Accept-0" daemon prio=10 tid=0x00007fd5bc424800 nid=0x6460 runnable [0x00007fd618ffd000]
java.lang.Thread.State: RUNNABLE
at java.net.PlainSocketImpl.socketAccept(Native Method)
at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:408)
- locked <0x00000007d79ac788> (a java.net.SocksSocketImpl)
at java.net.ServerSocket.implAccept(ServerSocket.java:462)
at java.net.ServerSocket.accept(ServerSocket.java:430)
at sun.management.jmxremote.LocalRMIServerSocketFactory$1.accept(LocalRMIServerSocketFactory.java:34)
at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.executeAcceptLoop(TCPTransport.java:369)
at sun.rmi.transport.tcp.TCPTransport$AcceptLoop.run(TCPTransport.java:341)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- None
"Attach Listener" daemon prio=10 tid=0x00007fd5f4001000 nid=0x645e waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"GruPool-1-thread-Paul" prio=10 tid=0x00007fd61c0f8000 nid=0x6451 waiting on condition [0x00007fd620898000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706f6c8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Carl" prio=10 tid=0x00007fd61c0f6000 nid=0x6450 waiting on condition [0x00007fd620999000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706f3f0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Jorge" prio=10 tid=0x00007fd61c0f4000 nid=0x644f waiting on condition [0x00007fd620a9a000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706f168> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Dave" prio=10 tid=0x00007fd61c0f2000 nid=0x644e waiting on condition [0x00007fd620b9b000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706eee0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Phil" prio=10 tid=0x00007fd61c0f0000 nid=0x644d waiting on condition [0x00007fd620c9c000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706ec58> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Jorge" prio=10 tid=0x00007fd61c0ee000 nid=0x644c waiting on condition [0x00007fd620d9d000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706e9a0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Carl" prio=10 tid=0x00007fd61c0ec800 nid=0x644b waiting on condition [0x00007fd620e9e000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706e718> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Dave" prio=10 tid=0x00007fd61c0eb000 nid=0x644a waiting on condition [0x00007fd620f9f000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706e490> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"GruPool-1-thread-Mike" prio=10 tid=0x00007fd61c0e9800 nid=0x6449 waiting on condition [0x00007fd6210a0000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Minion.run(Minion.java:18)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x00000007d706d8e8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"Low Memory Detector" daemon prio=10 tid=0x00007fd61c094000 nid=0x6447 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"C2 CompilerThread1" daemon prio=10 tid=0x00007fd61c092000 nid=0x6446 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"C2 CompilerThread0" daemon prio=10 tid=0x00007fd61c08f000 nid=0x6445 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"Signal Dispatcher" daemon prio=10 tid=0x00007fd61c08d000 nid=0x6444 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
Locked ownable synchronizers:
- None
"Finalizer" daemon prio=10 tid=0x00007fd61c071000 nid=0x6443 in Object.wait() [0x00007fd62182f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000007d7001300> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
- locked <0x00000007d7001300> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)
Locked ownable synchronizers:
- None
"Reference Handler" daemon prio=10 tid=0x00007fd61c06f000 nid=0x6442 in Object.wait() [0x00007fd621930000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000007d70011d8> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:485)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
- locked <0x00000007d70011d8> (a java.lang.ref.Reference$Lock)
Locked ownable synchronizers:
- None
"main" prio=10 tid=0x00007fd61c00a800 nid=0x643c runnable [0x00007fd623a7f000]
java.lang.Thread.State: RUNNABLE
at MainApp.main(MainApp.java:28)
Locked ownable synchronizers:
- None
"VM Thread" prio=10 tid=0x00007fd61c068800 nid=0x6441 runnable
"GC task thread#0 (ParallelGC)" prio=10 tid=0x00007fd61c01d800 nid=0x643d runnable
"GC task thread#1 (ParallelGC)" prio=10 tid=0x00007fd61c01f800 nid=0x643e runnable
"GC task thread#2 (ParallelGC)" prio=10 tid=0x00007fd61c021000 nid=0x643f runnable
"GC task thread#3 (ParallelGC)" prio=10 tid=0x00007fd61c023000 nid=0x6440 runnable
"VM Periodic Task Thread" prio=10 tid=0x00007fd61c09e800 nid=0x6448 waiting on condition
JNI global references: 948
How to get a heap dump?
To get a heap dump, we will use jmap. Just like jstack, jmap also come with jdk installation. To get a heap dump, run the command such as below.
user@localhost:~$ jmap -dump:format=b,file=myapp.hprof 26266
Dumping heap to /home/user/myapp.hprof ...
Heap dump file created
Then you can inspect using visualvm. See the following screenshot.
As you can see, the classes and instance give a lot of information about how much instances it created and it occupied for the size.With all these method written out, I hope the next time this exception happen to you, get know what are the tools and steps needed to start investigating into the cause.
Good luck!
