Understanding the names of java Thread

Question

I ran this code for my app

Map<Thread, StackTraceElement[]> myMap = Thread.getAllStackTraces();
Log.d("threads", myMap.toString());

I got the following results

{Thread[pool-1-thread-1,5,main]=[Ljava.lang.StackTraceElement;@e21f0dc, Thread[queued-work-looper,5,main]=[Ljava.lang.StackTraceElement;@1fec5e5, Thread[FinalizerWatchdogDaemon,5,system]=[Ljava.lang.StackTraceElement;@de438ba, Thread[process reaper,10,system]=[Ljava.lang.StackTraceElement;@d51036b, Thread[GoogleApiHandler,5,main]=[Ljava.lang.StackTraceElement;@a4b3ec8, Thread[Thread-2,3,main]=[Ljava.lang.StackTraceElement;@5dadb61, Thread[HeapTaskDaemon,5,system]=[Ljava.lang.StackTraceElement;@7d7a51c, Thread[ReferenceQueueDaemon,5,system]=[Ljava.lang.StackTraceElement;@b0d2686, Thread[FinalizerDaemon,5,system]=[Ljava.lang.StackTraceElement;@7fa6b47, Thread[main,5,main]=[Ljava.lang.StackTraceElement;@2397f74}`

I think the text Thread[pool-1-thread-1,5,main] conveys very important information, but I am not quite sure what exactly it means. Could anyone please explain what does it mean?

UPDATE: For another app, I got output {Thread[Thread-7,5,main] =.. so Thread-7 seems to indicate something different from pool-1-thread-1 , so I want to understand what the names of threads indicate?

Answer 1

UPDATE

Thread names indicate whatever the person who wrote the code that created the thread decided. There is no simple answer to that question.

However, some names seem self-explanatory, eg the names listed in the formatted output below. Names like:

• main - The main thread
• Finalizer - The thread responsible for executing finalize() methods.
• . . .

Other names are documented. Eg the javadoc of new Thread() says:

Allocates a new Thread object. This constructor has the same effect as Thread (null, null, gname) , where gname is a newly generated name. Automatically generated names are of the form "Thread-"+n , where n is an integer.

So Thread-7 would appear to be the thread created by the 8th call to new Thread(...) that didn't specify a name.

A thread name like pool-1-thread-1 would then also be an auto-generate name, for Thread #1 in Thread Pool #1.

---

To print the result of calling Thread.getAllStackTraces() in an easily readable format, use code like this:

Map<Thread, StackTraceElement[]> myMap = Thread.getAllStackTraces();
for (Entry<Thread, StackTraceElement[]> entry : myMap.entrySet()) {
    System.out.println(entry.getKey());
    for (StackTraceElement elem : entry.getValue())
        System.out.println("    " + elem);
}

Example Output (Java 8)

Thread[Finalizer,8,system]
    java.lang.Object.wait(Native Method)
    java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:144)
    java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:165)
    java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:216)
Thread[Attach Listener,5,system]
Thread[Signal Dispatcher,9,system]
Thread[Reference Handler,10,system]
    java.lang.Object.wait(Native Method)
    java.lang.Object.wait(Object.java:502)
    java.lang.ref.Reference.tryHandlePending(Reference.java:191)
    java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
Thread[main,5,main]
    java.lang.Thread.dumpThreads(Native Method)
    java.lang.Thread.getAllStackTraces(Thread.java:1610)
    Test8.main(Test8.java:7)

Example Output (Java 15)

Thread[Finalizer,8,system]
    java.base@15.0.1/java.lang.Object.wait(Native Method)
    java.base@15.0.1/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:155)
    java.base@15.0.1/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:176)
    java.base@15.0.1/java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:170)
Thread[Attach Listener,5,system]
Thread[Signal Dispatcher,9,system]
Thread[Notification Thread,9,system]
Thread[Common-Cleaner,8,InnocuousThreadGroup]
    java.base@15.0.1/java.lang.Object.wait(Native Method)
    java.base@15.0.1/java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:155)
    java.base@15.0.1/jdk.internal.ref.CleanerImpl.run(CleanerImpl.java:148)
    java.base@15.0.1/java.lang.Thread.run(Thread.java:832)
    java.base@15.0.1/jdk.internal.misc.InnocuousThread.run(InnocuousThread.java:134)
Thread[main,5,main]
    java.base@15.0.1/java.lang.Thread.dumpThreads(Native Method)
    java.base@15.0.1/java.lang.Thread.getAllStackTraces(Thread.java:1649)
    app//Test.main(Test.java:9)
Thread[Reference Handler,10,system]
    java.base@15.0.1/java.lang.ref.Reference.waitForReferencePendingList(Native Method)
    java.base@15.0.1/java.lang.ref.Reference.processPendingReferences(Reference.java:241)
    java.base@15.0.1/java.lang.ref.Reference$ReferenceHandler.run(Reference.java:213)