How to know all threads that are currently running in the Java virtual machine? If you are curious, the Thread class provides a static method which can be used to list all active threads. Here’s the method:

static Map<Thread,StackTraceElement[]> getAllStackTraces()

This method returns a map with keys are the Thread objects, so we can get only the key set and iterate over its elements:

Set<Thread> threads = Thread.getAllStackTraces().keySet();

for (Thread t : threads) {
	// do something with each thread
}

The following code snippet will list all threads that are currently running in the JVM along with their information like name, state, priority, and daemon status:

Set<Thread> threads = Thread.getAllStackTraces().keySet();

for (Thread t : threads) {
	String name = t.getName();
	Thread.State state = t.getState();
	int priority = t.getPriority();
	String type = t.isDaemon() ? "Daemon" : "Normal";
	System.out.printf("%-20s \t %s \t %d \t %s\n", name, state, priority, type);
}

 

If you put this code snippet in a console program, the output would be something like this:

Finalizer                WAITING         8       Daemon
Attach Listener          RUNNABLE        5       Daemon
Signal Dispatcher        RUNNABLE        9       Daemon
Reference Handler        WAITING         10      Daemon
main                     RUNNABLE        5       Normal

You see, besides the main thread which runs the main program, there are 4 other threads - which are core and default threads necessary to run the JVM. Let’s understand these threads a little bit more:

• Reference Handler: this thread puts objects that are no longer needed to a queue which will be processed by the Finalizer thread below.

• Finalizer: this thread performs finalizations for objects that are no longer needed to release system resources. It executes each finalize() method of the objects (if implemented).

• Attach Listener: this thread is responsible for dynamic attach when another process injects a thread inside the JVM to query certain details about how the JVM is running. For example, the JConsole program is a tool that attaches to another Java program for the purpose of profiling and monitoring.

• Signal Dispatcher: this thread handles native signals sent by operating system to the JVM.

All of these core threads are daemon. They will be terminated if the main program exits.

 

In case you put the list threads code in a Swing program, there are more threads:

Attach Listener          RUNNABLE        5       Daemon
main                     RUNNABLE        5       Normal
AWT-EventQueue-0         RUNNABLE        6       Normal
Finalizer                WAITING         8       Daemon
Signal Dispatcher        RUNNABLE        9       Daemon
Reference Handler        WAITING         10      Daemon
Java2D Disposer          WAITING         10      Daemon
AWT-Windows              RUNNABLE        6       Daemon
AWT-Shutdown             WAITING         5       Normal

As you can see, besides the core threads, there are threads necessary for running the graphical user interface:

• Java2D Disposer: performs finalization on GUI objects that are no longer needed.

• AWT-Windows: handles native windows events.

• AWT-Shutdown: is responsible for terminating GUI programs.

• AWT-Event-Queue-0: handles GUI events.

 

And in case you put the list threads code snippet in a servlet running on an application server e.g. Tomcat, the list of threads would be longer like this:

http-nio-8080-ClientPoller-1             	 RUNNABLE 	 5 	 Daemon
Finalizer                                	 WAITING 	 8 	 Daemon
main                                     	 RUNNABLE 	 5 	 Normal
Reference Handler                        	 WAITING 	 10 	 Daemon
http-nio-8080-ClientPoller-0             	 RUNNABLE 	 5 	 Daemon
ajp-nio-8009-Acceptor-0                  	 RUNNABLE 	 5 	 Daemon
Signal Dispatcher                        	 RUNNABLE 	 9 	 Daemon
http-nio-8080-exec-1                     	 WAITING 	 5 	 Daemon
http-nio-8080-Acceptor-0                 	 RUNNABLE 	 5 	 Daemon
http-nio-8080-exec-2                     	 RUNNABLE 	 5 	 Daemon
ajp-nio-8009-ClientPoller-0              	 RUNNABLE 	 5 	 Daemon
ajp-nio-8009-ClientPoller-1              	 RUNNABLE 	 5 	 Daemon
NioBlockingSelector.BlockPoller-1        	 RUNNABLE 	 5 	 Daemon
GC Daemon                                	 TIMED_WAITING 	 2 	 Daemon
NioBlockingSelector.BlockPoller-2        	 RUNNABLE 	 5 	 Daemon
Attach Listener                          	 RUNNABLE 	 5 	 Daemon

As you can see, most of the additional threads are spawned by the server for handling HTTP requests/responses.

 

Related Java Thread Tutorials:

• How to use Threads in Java (create, start, pause, interrupt and join)
• Understanding Thread Group in Java
• Understanding Thread Priorities and Daemon Thread in Java
• Understanding Thread States (Thread Life Cycle) in Java

 

Other Java Concurrency Tutorials:

• Java Synchronization Tutorial
• Understanding Deadlock, Livelock and Starvation with Code Examples in Java
• Understanding Java Fork-Join Framework with Examples
• Understanding Java Thread Pool and Executors