Is a Thread Garbage Collected When its Reference is Set to Null in Java?

Java Thread: Garbage Collected or Not?

In this Java code snippet, a new thread is created and started with t.start(). However, after starting the thread, the thread reference t is set to null with t = null. This raises the question of whether or not the thread will be garbage collected without an active reference to it.

Garbage Collection in Java

When the garbage collector runs in Java, it identifies and removes objects that are no longer reachable from the active parts of the program. In this case, the thread is created as an anonymous class that keeps running. Even though the t reference in the main thread is set to null, the thread itself remains active and is not immediately garbage collected.

Why the Thread Remains Active

A running thread is considered a garbage collection root, meaning that it is considered reachable by the garbage collector. As long as the thread is still running, it prevents the objects it references from being garbage collected.

In this case, the thread is referencing its own internal data structures, such as the stack trace, thread state, and shared memory. By being a garbage collection root, the thread ensures that these internal objects remain accessible.

Main Thread as Garbage Collection Root

Similarly, the main thread also serves as a garbage collection root. Even though there may not be any explicit references to the main thread, it is still active and running. As a result, it prevents itself and any objects it references from being garbage collected.

Conclusion

Therefore, in this code snippet, the thread will continue running indefinitely because it is considered a garbage collection root. The fact that it doesn't have an explicit reference from the main thread does not matter as it remains active and keeps the objects it references reachable. The garbage collector will only collect the thread when it terminates and is no longer considered a garbage collection root.

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