Java ReentrantLock hasQueuedThreads() Method

The hasQueuedThreads() returns a boolean value true or false depending upon whether any threads are waiting to acquire this lock. Note that because cancellations may occur at any time, a true return does not guarantee that any other thread will ever acquire this lock.

Syntax

Parameter

No parameter is passed.

Returns

true if there may be other threads waiting to acquire the lock

Throws

No Exception is thrown.

Example 1

import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockhasQueuedThreadsExample1  {
    public static void main(String[] args) {
 
           ReentrantLock lock=new ReentrantLock();
           MyRunnableClass33 myRnbl=new MyRunnableClass33(lock);
           new Thread(myRnbl,"Thread-1").start();
                     
    }
}
  
class MyRunnableClass33 implements Runnable{
    
    ReentrantLock lock;
    public MyRunnableClass33(ReentrantLock lock) { 
           this.lock=lock;
    }
    
    public void run(){
           
           System.out.println(Thread.currentThread().getName()
                        +" is Waiting to get the lock");
           
           lock.lock();
 
           System.out.println(Thread.currentThread().getName()
                        +" has got the lock.");
           
           try {
                  Thread.sleep(2000);
           } catch (InterruptedException e) {
                  e.printStackTrace();
           }
 
 
           System.out.println(" has queued Threads = "+lock.hasQueuedThreads());
           
           System.out.println(Thread.currentThread().getName()
                        +" has released the lock.");
           
           lock.unlock();    //read explanation for 5sec
    }
}

Test it Now

Output:

Thread-1 is Waiting to get the lock
Thread-1 has got the lock.
 has queued Threads = false
Thread-1 has released the lock.

Example 2

import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockhasQueuedThreadsExample2  {
    public static void main(String[] args) {
 
           ReentrantLock lock=new ReentrantLock();
           MyRunnableClass44 myRnbl=new MyRunnableClass44(lock);
           new Thread(myRnbl,"Thread-1").start();
           new Thread(myRnbl,"Thread-2").start();
           new Thread(myRnbl,"Thread-3").start();
           
    }
}
  
 class MyRunnableClass44 implements Runnable{
    
    ReentrantLock lock;
    public MyRunnableClass44(ReentrantLock lock) { 
           this.lock=lock;
    }
    
    public void run(){
           
           System.out.println(Thread.currentThread().getName()
                        +" is Waiting to get the lock");
           
           lock.lock();
 
           System.out.println(Thread.currentThread().getName()
                        +" has got the  lock.");
           
           try {
                  Thread.sleep(2000);
           } catch (InterruptedException e) {
                  e.printStackTrace();
           }
 
 
           System.out.println(" has queued Threads = "+lock.hasQueuedThreads());
           
           System.out.println(Thread.currentThread().getName()
                        +" has released  the lock.");
           
           lock.unlock();    //read explanation for 5sec
    }
}

Test it Now

Output:

Thread-1 is Waiting to get the lock
Thread-1 has got the  lock.
Thread-2 is Waiting to get the lock
Thread-3 is Waiting to get the lock
 has queued Threads = true
Thread-1 has released  the lock.
Thread-2 has got the  lock.
 has queued Threads = true
Thread-2 has released  the lock.
Thread-3 has got the  lock.
 has queued Threads = false
Thread-3 has released  the lock.

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