Java Thread Join Example

In Java, using Thread.join() causes the current thread to wait until the specified thread dies. Using this method allows us to impose an order such that we can make one thread wait until the other completes doing what it needed to do, such as completing a calculation.

This method sometimes causes some confusion among Java developers at all levels from novice to more senior folks. It is important to note that the calling thread will wait until the thread being called completes or the specified duration has elapsed. For illustration, suppose we have several threads mainThread, calcThread, and jmsThread. Suppose now that mainThread needs to wait on calcThread to get the required calculation, but calcThread needs to wait until jmsThread returns the required message from the queue before it can perform the required calculations.

What needs to happen is that inside of calcThread there must be a jsmThread.join(), and inside of mainThread there must be a calcThread.join() such that mainThread waits for calcThread and calcThread waits for jmsThread.

In the example below, I will demonstrate setting up and starting five threads with and without the join() method to help you better understand the concept.

Also, please be aware that Java contains three overloaded methods for join:

Join Methods

MethodDescription
join()Waits until the thread that it is attached it dies
join(long millis)Waits at most millis until the thread that it is attached it dies
join(long millis, int nanos)Waits at most millis plus nanos until the thread that it is attached it dies

ThreadJoinExample (without join)

In the example, you will notice from the output below that the main program actually ends before all the threads have even started up. This is because there is a little bit of overhead that takes place when starting threads and the t.start() return almost instantly from the call. Since I am using the default constructor the threads only sleep for 500ms (or half a second).

package com.avaldes.tutorials;

import java.util.ArrayList;

public class ThreadJoinExample implements Runnable {
  int sleepTime = 0;
  
  public ThreadJoinExample(int sleepTime) {
    setSleepTime(sleepTime);
  }
  
  public ThreadJoinExample() {
    setSleepTime(500); 
  }
  
  public long getSleepTime() {
    return sleepTime;
  }

  public void setSleepTime(int sleepTime) {
    this.sleepTime = sleepTime;
  }

  public void run() {
    try {
      System.out.format("Thread %s started...\n", Thread.currentThread().getName());
      Thread.sleep(sleepTime);
      System.out.format("Thread %s ended...\n", Thread.currentThread().getName());
    } catch (InterruptedException e) {
      System.out.format("Thread %s interrupted...", Thread.currentThread().getName());
    }
  }
  
  public static void main(String[] args) throws InterruptedException {
    
    System.out.println("Starting ThreadJoinExample without wait...");
    for (int i = 1; i< 5; i++) {
      Thread t = new Thread(new ThreadJoinExample(), "Thread_" + i);
      t.start();
    }

    System.out.println("Ending ThreadJoinExample...");
  }
}

Output

Starting ThreadJoinExample without wait...
Thread Thread_1 started...
Thread Thread_4 started...
Ending ThreadJoinExample...
Thread Thread_3 started...
Thread Thread_2 started...
Thread Thread_4 ended...
Thread Thread_3 ended...
Thread Thread_1 ended...
Thread Thread_2 ended...

ThreadJoinExample (with join)

In this second example, I have added an ArrayList so I can add all five threads to it as they get created. I will use this list later in the main program with the join() method. Now you will see that the “main” thread actually will wait until all five threads have completed before completing….

package com.avaldes.tutorials;

import java.util.ArrayList;

public class ThreadJoinExample implements Runnable {
  int sleepTime = 0;
  
  public ThreadJoinExample(int sleepTime) {
    setSleepTime(sleepTime);
  }
  
  public ThreadJoinExample() {
    setSleepTime(500); 
  }
  
  public long getSleepTime() {
    return sleepTime;
  }

  public void setSleepTime(int sleepTime) {
    this.sleepTime = sleepTime;
  }

  public void run() {
    try {
      System.out.format("Thread %s started...\n", Thread.currentThread().getName());
      Thread.sleep(sleepTime);
      System.out.format("Thread %s ended...\n", Thread.currentThread().getName());
    } catch (InterruptedException e) {
      System.out.format("Thread %s interrupted...", Thread.currentThread().getName());
    }
  }
  
  public static void main(String[] args) throws InterruptedException {
    ArrayList<Thread> threadList = new ArrayList<Thread>();
    
    System.out.println("Starting ThreadJoinExample with wait...");
    for (int i = 1; i< 5; i++) {
      Thread t = new Thread(new ThreadJoinExample(), "Thread_" + i);
      threadList.add(t);
      t.start();
    }

    for (Thread t: threadList) {
      t.join();
    }
    System.out.println("Ending ThreadJoinExample...");
  }
}

Output

Starting ThreadJoinExample with wait...
Thread Thread_1 started...
Thread Thread_4 started...
Thread Thread_3 started...
Thread Thread_2 started...
Thread Thread_3 ended...
Thread Thread_2 ended...
Thread Thread_4 ended...
Thread Thread_1 ended...
Ending ThreadJoinExample...

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