Threads – Interrupts

Interrupts

An interrupt is an indication to a thread that it should stop what it is doing and do something else. It’s up to the programmer to decide exactly how a thread responds to an interrupt, but it is very common for the thread to terminate. This is the usage emphasized in this lesson.A thread sends an interrupt by invoking interrupt on the Thread object for the thread to be interrupted. For the interrupt mechanism to work correctly, the interrupted thread must support its own interruption.

Supporting Interruption

How does a thread support its own interruption? This depends on what it’s currently doing. If the thread is frequently invoking methods that throw InterruptedException, it simply returns from the run method after it catches that exception. For example, suppose the central message loop in the SleepMessages example were in the run method of a thread’s Runnable object. Then it might be modified as follows to support interrupts:

for (int i = 0; i < importantInfo.length; i++) {
//Pause for 4 seconds
try {
Thread.sleep(4000);
} catch (InterruptedException e) {
//We’ve been interrupted: no more messages.
return;
}
//Print a message
System.out.println(importantInfo[i]);
}

Many methods that throw InterruptedException, such as sleep, are designed to cancel their current operation and return immediately when an interrupt is received.

What if a thread goes a long time without invoking a method that throws InterruptedException? Then it must periodically invoke Thread.interrupted, which returns true if an interrupt has been received. For example:

for (int i = 0; i < inputs.length; i++) {
heavyCrunch(inputs[i]);
if (Thread.interrupted()) {
//We’ve been interrupted: no more crunching.
return;
}
}

In this simple example, the code simply tests for the interrupt and exits the thread if one has been received. In more complex applications, it might make more sense to throw an InterruptedException:

if (Thread.interrupted()) {
throw new InterruptedException();
}

This allows interrupt handling code to be centralized in a catch clause.

The Interrupt Status Flag

The interrupt mechanism is implemented using an internal flag known as the interrupt status. Invoking Thread.interrupt sets this flag. When a thread checks for an interrupt by invoking the static method Thread.interrupted, interrupt status is cleared. The non-static Thread.isInterrupted, which is used by one thread to query the interrupt status of another, does not change the interrupt status flag.

By convention, any method that exits by throwing an InterruptedException clears interrupt status when it does so. However, it’s always possible that interrupt status will immediately be set again, by another thread invoking interrupt.

Joins

The join method allows one thread to wait for the completion of another. If t is a Thread object whose thread is currently executing,

        t.join();

causes the current thread to pause execution until t’s thread terminates. Overloads of join allow the programmer to specify a waiting period. However, as with sleep, join is dependent on the OS for timing, so you should not assume that join will wait exactly as long as you specify.

 Like sleep, join responds to an interrupt by exiting with an InterruptedException.

Finally, an example:

The SimpleThreads Example

    The following example brings together some of the concepts of this section. SimpleThreads consists of two threads. The first is the main thread that every Java application has. The main thread creates a new thread from the Runnable object, MessageLoop, and waits for it to finish. If the MessageLoop thread takes too long to finish, the main thread interrupts it.

    The MessageLoop thread prints out a series of messages. If interrupted before it has printed all its messages, the MessageLoop thread prints a message and exits.

        public class SimpleThreads {

            //Display a message, preceded by the name of the current thread
            static void threadMessage(String message) {
                String threadName = Thread.currentThread().getName();
                System.out.format(“%s: %s%n”, threadName, message);
            }

            private static class MessageLoop implements Runnable {
                public void run() {
                    String importantInfo[] = {
                        “Mares eat oats”,
                        “Does eat oats”,
                        “Little lambs eat ivy”,
                        “A kid will eat ivy too”
                    };
                    try {
                        for (int i = 0; i < importantInfo.length; i++) {
                            //Pause for 4 seconds
                            Thread.sleep(4000);
                            //Print a message
                            threadMessage(importantInfo[i]);
                        }
                    } catch (InterruptedException e) {
                        threadMessage(“I wasn’t done!”);
                    }
                }
            }

            public static void main(String args[]) throws InterruptedException {

                //Delay, in milliseconds before we interrupt MessageLoop
                //thread (default one hour).
                long patience = 1000 * 60 * 60;

                //If command line argument present, gives patience in seconds.
                if (args.length > 0) {
                    try {
                        patience = Long.parseLong(args[0]) * 1000;
                    } catch (NumberFormatException e) {
                        System.err.println(“Argument must be an integer.”);
                        System.exit(1);
                    }

                }

                threadMessage(“Starting MessageLoop thread”);
                long startTime = System.currentTimeMillis();
                Thread t = new Thread(new MessageLoop());
                t.start();

                threadMessage(“Waiting for MessageLoop thread to finish”);

                //loop until MessageLoop thread exits
                while (t.isAlive()) {
                    threadMessage(“Still waiting…”);
                    //Wait maximum of 1 second for MessageLoop thread to
                    //finish.
                    t.join(1000);
                    if (((System.currentTimeMillis() – startTime) > patience) &&
                            t.isAlive()) {
                        threadMessage(“Tired of waiting!”);
                        t.interrupt();
                        //Shouldn’t be long now — wait indefinitely
                        t.join();
                    }

                }
                threadMessage(“Finally!”);
            }
        }