How to Properly Stop a Java Thread

In Java multithreading programming, properly stopping a Java thread is a crucial issue. Incorrect ways to stop a thread can lead to resource leaks, data inconsistencies, and other potential problems. This article will introduce some proper methods to stop a thread to help developers avoid potential issues when writing multithreaded applications.

Common Ways to Stop a Thread

Using a Flag to Stop a Thread

One common way to stop a thread is to use a flag to control the thread’s execution. The thread checks the flag’s status to decide whether to continue executing. When you need to stop the thread, set the flag to the appropriate status, and the thread will detect the change in the flag’s status on the next loop iteration and exit voluntarily.

For example, you can use a boolean flag to stop a thread:

public class MyThread extends Thread {
    private volatile boolean flag = true;
    public void stopThread() {
        flag = false;
    }
    @Override
    public void run() {
        while (flag) {
            // Thread execution logic
        }
    }
}

In the example above, the thread is stopped by setting the flag to false. The thread checks the flag’s status at the beginning of each loop iteration and exits the loop if it is false, stopping the thread. However, this method is not recommended for the following reasons:

• Visibility issues: Shared variables may have visibility issues, meaning a modification to a shared variable by one thread might not be immediately visible to other threads. This is due to thread caching and optimization mechanisms, which can cause a thread not to see changes to a shared variable promptly, preventing the thread from stopping in time.

• Race conditions: When multiple threads simultaneously access and modify a shared variable, race conditions can occur, leading to unpredictable behavior and erroneous results. For example, one thread is deciding whether to stop based on the shared variable, while another thread is modifying that variable, which might cause the stop decision to fail or be inaccurate.

• Deadlock risk: Using locks during thread stopping can pose a deadlock risk. For example, if one thread holds a lock and waits for a specific value of a shared variable, while another thread tries to modify that shared variable, it can lead to a deadlock.

• Maintenance and debugging difficulties: Using shared variables to control thread stopping can increase code complexity, making the code harder to understand, maintain, and debug. Especially when multiple threads share and modify the variable, it increases code fragility and the likelihood of errors.

Using Thread.interrupted() Method

Another common way to stop a thread is to use the interrupt() method of the Thread class. This method sends an interrupt signal to the thread, which can decide whether to continue executing by checking if it has been interrupted.

public class MyThread extends Thread {
    @Override
    public void run() {
        while (!Thread.interrupted()) {
            // Thread execution logic
        }
    }
}

In the example above, the thread checks if it has been interrupted at the beginning of each loop iteration and exits the loop if it has, stopping the thread.

In Java, the best way to stop a thread is to use the interrupt mechanism, but this only notifies the thread to stop running. The interrupted thread itself has the final say (deciding whether and when to stop), depending on a well-agreed coding convention between the requester and the requested thread.

Related interrupt Methods

Before we start, let’s understand the interrupt mechanism in Java. An interrupt is a collaborative mechanism that requests a thread to stop execution by setting its interrupt status. The interrupted thread needs to check the interrupt status and decide whether to stop execution accordingly. Java provides the Thread.interrupt() method to send an interrupt request. When you need to stop a thread, call the interrupt() method of the target thread to send an interrupt request.

• Thread.interrupt() This method interrupts the thread by setting its interrupt status to “interrupted,” effectively sending an interrupt request to the target thread. If the target thread is in a waiting state (like Thread.sleep() or Object.wait()), it throws an InterruptedException. If the target thread is running, it can decide whether to stop execution by checking its interrupt status.

• Thread.isInterrupted() This method checks the thread’s interrupt status. Calling the isInterrupted() method on a thread returns a boolean indicating whether the thread has been interrupted.

• Thread.interrupted() This method checks the current thread’s interrupt status and clears the interrupt status. Calling the interrupted() method checks the current thread’s interrupt status, returns a boolean indicating whether the thread has been interrupted, and clears the interrupt status.

Best Practices for Using interrupt

• Prefer Propagating Interrupts: Add a throws InterruptedException to the method signature, passing the received exception to the caller to handle the exception.
• Restore Interrupt Status if Propagation is Not Possible: In some cases, you might not be able or willing to propagate the interrupt to other threads. This could be because you cannot control other thread’s code, or you need to continue executing specific logic after handling the interrupt. In this case, the best practice is to restore the interrupt status. After catching the InterruptedException, manually restore the interrupt status so other code can detect the interrupt request.

try {
    // Code that might throw InterruptedException
} catch (InterruptedException e) {
    // Restore interrupt status
    Thread.currentThread().interrupt();
    // Continue executing other logic
}

By calling Thread.currentThread().interrupt(), you restore the interrupt status. This way, other threads can correctly detect the interrupt request when they call Thread.currentThread().isInterrupted().

• Avoid Swallowing Interrupts:

  Sometimes, we might inadvertently swallow interrupts, preventing proper detection of interrupt requests. Best practice is to avoid swallowing interrupts. Swallowing interrupts typically occurs in blocking operations like Thread.sleep() or Object.wait() that throw InterruptedException. When catching this exception, avoid empty catches or not handling the exception properly, and handle the interrupt request appropriately.

Best Practice Example

/**
 * Description: Best practice: When catching InterruptedException, the preferred option is to throw it in the method signature, forcing try/catch in run().
 */
public class RightWayStopThreadInProd implements Runnable {
    @Override
    public void run() {
        while (true && !Thread.currentThread().isInterrupted()) {
            System.out.println("go");
            try {
                throwInMethod();
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
                // Log and stop the program
                System.out.println("Logging");
                e.printStackTrace();
            }
        }
    }
    private void throwInMethod() throws InterruptedException {
        Thread.sleep(2000);
    }
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new RightWayStopThreadInProd());
        thread.start();
        Thread.sleep(1000);
        thread.interrupt();
    }
}

Summary

In Java, using the interrupt mechanism (Thread.interrupt()) is the best way to stop a thread. An interrupt is a collaborative mechanism where the thread needs to check its interrupt status and decide whether to stop executing. To ensure the correctness of the interrupt mechanism, developers should follow these best practices:

• Prefer Propagating Interrupts: Add a throws InterruptedException to the method signature.
• Restore Interrupt Status if Propagation is Not Possible: Manually restore the interrupt status after catching InterruptedException.
• Avoid Swallowing Interrupts: Ensure proper handling of interrupt requests without swallowing them.

By following these practices, you can write more robust and maintainable multithreaded applications.