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Concurrency
Processes and Threads
Thread Objects
Defining and Starting a Thread
Pausing Execution with Sleep
Interrupts
Joins
The SimpleThreads Example
Synchronization
Thread Interference
Memory Consistency Errors
Synchronized Methods
Intrinsic Locks and Synchronization
Atomic Access
Liveness
Deadlock
Starvation and Livelock
Guarded Blocks
Immutable Objects
A Synchronized Class Example
A Strategy for Defining Immutable Objects
High Level Concurrency Objects
Lock Objects
Executors
Executor Interfaces
Thread Pools
Fork/Join
Concurrent Collections
Atomic Variables
Concurrent Random Numbers
For Further Reading
Questions and Exercises
Trail: Essential Classes
Lesson: Concurrency
Section: Thread Objects
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# 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 `isInterrupted` method, 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`.

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