Java ReentrantLock hasQueuedThread() Method

The hasQueuedThread(Thread thread) method of ReentrantLock class returns true if there is some thread is waiting for the this lock. Note that because cancellations may occur at any time, a true return does not guarantee that this thread will ever acquire this lock. This method is designed primarily to use in monitoring of the system state.

Syntax

Parameter

thread - the thread

Returns

true if the given thread is queued waiting for this lock

Throws

NullPointerException - if the thread is null.

Example 1

//import statement
  
package com.app.Rentrant;

 
 import java.util.concurrent.locks.ReentrantLock;
 
 public class ReentrantLockhasQueuedThreadExample1  {
     public static void main(String[] args) {
  
            ReentrantLock lock=new ReentrantLock();
            MyRunnableClass3 myRnbl=new MyRunnableClass3(lock);
            new Thread(myRnbl,"Thread-1").start();
                        
     }
 }
   
 class MyRunnableClass3 implements Runnable{
     
     ReentrantLock lock;
     public MyRunnableClass3(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 Thread = "+lock.hasQueuedThread(Thread.currentThread()));
            
            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 Thread = false
Thread-1 has released the lock.

Example 2

//import statements
package com.app.Rentrant;
 
 import java.util.concurrent.locks.ReentrantLock;
 
 public class ReentrantLockhasQueuedThreadExample2  {
     public static void main(String[] args) {
  
            ReentrantLock lock=new ReentrantLock();
            MyRunnableClass4 myRnbl=new MyRunnableClass4(lock);
            new Thread(myRnbl,"Thread-1").start();
            new Thread(myRnbl,"Thread-2").start();
            new Thread(myRnbl,"Thread-3").start();
            
     }
 }
    
  class MyRunnableClass4 implements Runnable{
     
     ReentrantLock lock;
     public MyRunnableClass4(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 Thread = "+lock.hasQueuedThread(Thread.currentThread()));
            
            System.out.println(Thread.currentThread().getName()
                         +" has released  the lock.");
            
            lock.unlock();    //read explanation for 5sec
     }
 }

Test it Now

Output:

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

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