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Trail: Essential Classes
Lesson: Concurrency
Section: Thread Objects
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
Concurrent Collections
Atomic Variables
For Further Reading
Questions and Exercises
Home Page > Essential Classes > Concurrency
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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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