Why does a Thread Remain Alive After Setting its Reference to Null in Java?

Java Thread Garbage Collection

In the given code snippet, a new thread is created using an anonymous class and started in line A. The thread reference "t" is then set to null in line B, effectively removing all references to the thread within the main thread.

Why the Thread Stays Alive

Contrary to your assumption, the thread does not get garbage collected after line B. This is because a running thread is considered a garbage collection root.

Garbage Collection Roots

Garbage collection roots are objects that are either directly reachable by the main thread or are reachable by another accessible object. Any object that is reachable from a garbage collection root is considered "live" and is not eligible for garbage collection.

Implications for Thread Garbage Collection

Since a running thread is a garbage collection root, the thread object remains alive and active even after the main thread has no references to it. This ensures that the thread continues running until it finishes its task or is manually stopped.

Behavior with Main Thread

Similarly, the main thread itself is considered a garbage collection root because it is the entry point of the program. Therefore, it also remains active until the program terminates, even though there are no references to it from outside the program.

Conclusion

In summary, a running Java thread is considered a garbage collection root, ensuring that it remains active and running even if the main thread has no references to it. This is necessary to ensure that threads can continue executing tasks without being prematurely terminated by garbage collection.

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