Java Thread Join

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Another functionality with threads in Java is the possibility to combine threads together using the join() method.

How to Join Threads in Java?

Java's Thread class provides an overloaded join() method, which allows one Thread to wait/pause/hold until another thread is complete. 

This is very helpful when you want to use multi-threading to complete a number of non-sequential tasks but then need to do something in sequential order once those threads are complete.

Three Types of the Join Method

There are three overloaded join functions.

1. join(): It will put the current Thread on hold until the Thread on which it is called has been completed.
2. join(long millis): It will put the current Thread on hold until the Thread on which it is called is complete - but it will only wait for as long as you specify in milliseconds. If the Thread is not completed in that time, the current Thread will move on without it.
3. join(long millis, int nanos): It will put the current Thread on hold until the Thread on which it is called is complete - but it will only wait for as long as you specify in milliseconds + nanoseconds. If the Thread is not completed in that time, the current Thread will move on without it.

Example Thread Join in Java

In the example below, you create a Thread object named thrd. After starting that Thread, you invoke the join() method on that object, and this way, the main Thread will wait until thrd completes before moving forward.

Calling thrd.join() will make sure that thrd is terminated before moving forward with the program.

// Java program to explain the concept of joining a thread (author: Nitsdheerendra)
import java.io.*; 

// Creating thread by creating the objects of that class 
class ThreadJoining extends Thread { 

  @Override
  public void run() { 
    for (int i = 0; i < 2; i++) {
      try { 
        Thread.sleep(500); 
        System.out.println("Current Thread: " 
          + Thread.currentThread().getName()); 
      } catch(Exception ex) { 
        System.out.println(
          "Exception has been caught" + ex); 
      } 
      System.out.println(i); 
    } 
  } 
} 

class GFG {
  public static void main (String[] args) { 
    ThreadJoining thrd0 = new ThreadJoining(); 
    ThreadJoining thrd1 = new ThreadJoining(); 
    ThreadJoining thrd2 = new ThreadJoining(); 

    // thread thrd0 starts 
    thrd0.start(); 

    // starts second thread after when 
    // first thread "thrd0" has died. 
    try { 
      System.out.println("Current Thread: " 
        + Thread.currentThread().getName()); 
      thrd0.join(); 
    } catch(Exception ex) { 
      System.out.println(
        "Exception has been caught" + ex); 
    } 

    // thrd1 starts 
    thrd1.start(); 

    try { 
      System.out.println("Current Thread: " 
        + Thread.currentThread().getName()); 
      thrd1.join(); 
    } catch(Exception ex)  { 
      System.out.println(
        "Exception has been  caught" + ex); 
    } 

    // start thrd2 after when thread thrd1 has died.   
    thrd2.start(); 
  } 
} 

/* OUTPUT:

Current Thread: main
Current Thread: Thread-0
0
Current Thread: Thread-0
1
Current Thread: main
Current Thread: Thread-1
0
Current Thread: Thread-1
1
Current Thread: Thread-2
0
Current Thread: Thread-2
1 

Summary: What is a Thread Join?

• Threads can be joined together in Java, putting them in sequential order
• A joined Thread will only execute once its previous Thread has been completed
• Threads are joined using the join() method

Three Types of the Join Method

1. join()
2. join(long millis)
3. join(long millis, int nanos)