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The Java™ Tutorials
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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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# 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!");
        }
    }

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