How Does Java's `Thread.interrupt()` Work and How Should Threads Respond?

What does java.lang.Thread.interrupt() do and how does it work?

The java.lang.Thread.interrupt() method ensures that the target thread's interrupted flag/status is set. This action triggers the target thread to check its interrupted status and respond appropriately.

Interruption in Java is a collaborative process requiring cooperation from both threads:

• Setting the Interrupted Flag: Thread.interrupt() sets the interrupted flag on the target thread, indicating that there may be a pending interruption.
• Polling the Flag: To respond to the interrupt, the target thread must periodically check its interrupted status using the Thread.interrupted() method. This method both retrieves and clears the interrupted flag.
• Responding to Interruption: Upon detecting an interrupted status, the thread might handle it specifically, for instance, by throwing an InterruptedException.

Pre-emption vs. Cooperation

Unlike in certain other programming languages, interruption in Java is cooperative. That is, both threads must cooperate for the interrupt to be processed successfully. If the target thread never checks its interrupted status, the interrupt will be effectively ignored.

Built-in Interrupt Handling

Certain API methods come with built-in interrupt handling, including:

• Object.wait(), Thread.sleep(), and Thread.join()
• Most java.util.concurrent constructs
• Java NIO (using ClosedByInterruptException, not InterruptedException)

Interruption as a Nudging Mechanism

Thread interruption serves as a gentle and polite way to request that a thread gracefully exits. Unlike Thread.stop(), which abruptly terminates the thread, interruption allows the thread to clean up and finish executing any tasks before exiting.

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