How to Get a Reference to a Java Thread

Running code under Java threads is a powerful way to get a lot done in a short time. To monitor and control the threads, you need to set up a thread object tied to your thread-enabled code. This gives you full control over the threads you created.

Things You'll Need

• Java project with runnable threads in the code

Instructions

1. Get the thread ID with this command inside the mandatory run() function that executes your thread:

   Thread.currentThread().getId()

2. Assign the thread object to a public variable if you need to control the thread from other parts of the program, or print it out directly if you just want to know what's running:

   public int myThreadId = 0;

   public void run () {

   System.out.println("Thread Name: " + Thread.currentThread().getName(); // Printing the thread name

   myThreadId = Thread.currentThread().getId(); // Assigning the thread ID to a public variable

   }

3. Create a new thread object to control or monitor a thread from other parts of the program. You can also grab the object from an outside thread, such as the main process, by building a complete tree of running threads and thread groups. Call this from the main() function of your Java class:

   // Walk down the tree to find the root group

   ThreadGroup myRootGroup = Thread.currentThread( ).getThreadGroup( );

   ThreadGroup parentGroup;

   while ( ( parentGroup = myRootGroup.getParent() ) != null ) {

   rootGroup = parentGroup;

   }

   // Walk back up from the root, creating a Thread object for every thread in your process

   Threads[] myThreads = new Thread[ myRootGroup.activeCount() ];

   while ( rootGroup.enumerate( myThreads, true ) == myThreads.length ) {

   myThreads = new Thread[ myThreads.length * 2 ];

   }

   Now you can access every thread in your process one by one. For example:

   System.out.println("Thread Name: " + myThreads[0].currentThread().getName();

   myThreadId = myThreads[0].currentThread().getId();

References

• Oracle: Thread Class Overview

Resources

• Nadeau Software: How to List and Find Threads and Thread Groups