Reetrant Lock in Java

ReetrantLock is a class that implements Lock Interface. It provides the synchronization feature with great flexibility which is why it is the most used lock class in Java. It is necessary for the reliable and fair working of thread. Here, threads are small sub-processes of a big operation. In this article, we are going to learn ReetrantLock and how they manage threads so that they can work efficiently.

Working of ReetrantLock

When multiple threads try to access a shared resource then, ReetrantLock restricts access to a single thread at a time through ‘lock()’ and ‘unlock()’ methods. Suppose there are three people trying to book a train ticket. At the same time, all three people will try to access the booking system, it may happen that two people end up booking the same seat. Reetrant Lock can handle this situation.

First, all three people will request to acquire the booking system through ‘tryLock()’ method. When one acquires the booking system then, it restricts the particular seat booking through ‘lock()’ method. After booking, the person will call the ‘unlock()’ method to release the acquired lock. Till the resources are busy other people will wait in a queue for their turn and after the release of lock they will come in a runnable state.

ReetrantLock tries to provide locks in a fair manner. We can set for how long a thread can acquire lock and also, it ensures that a thread with the longest wait time may get access to lock first. By default the locks are unfair, to make it fair we need to pass Boolean value ‘true’ in its constructor.


ReentrantLock nameOflock = new  ReentrantLock(); 
// by default false
ReentrantLock nameOflock = new  ReentrantLock(true); 
// we can make it true

The locks are explicit and can lock or unlock in any order. A single thread can ask for the lock multiple times that’s the reason the name of lock is Reentrant. We can count the number of times a lock is acquired by using ‘getHoldCount()’ method.


The following example illustrates the use of Reetrant Lock.

Working of Code

  • Create a class ‘Thrd’ and inside this thread define an object of ReentrantLock.

  • Define a method ‘operation()’ along with a parameter of type integer. Store the Boolean value of ‘tryLock()’ method to a variable named ‘lockAcquired’ which will check if lock gets acquired by any thread or not.

  • If the lock is acquired give the lock to that thread using ‘lock()’ method and let the thread perform the given task.

  • The task will be performed in the try block and the lock will be released in the finally block using ‘unlock()’ method.

  • Now create three thread classes and call the ‘operation()’ method.

  • In the main() method, define three objects of thread class and call its ‘start()’ method to start the execution of threads.

import java.util.concurrent.*;
import java.util.concurrent.locks.*;
class Thrd {
   // creating object of ReentrantLock class
   private static ReentrantLock lockr = new  ReentrantLock();
   static void operation(int data) {
     // give access to lock
     boolean lockAcquired = lockr.tryLock(); 
     if (lockAcquired) {
       try {
         // giving lock to thread
         for(int i = 1; i <= 4; i++) {
         // checking lock count
         System.out.println("Count of Lock: " + lockr.getHoldCount());
       } finally {
         // unlocking the lock 
     } else {
       System.out.println("I am in else block");
class Thrd1 extends Thread {
   // thread number 1 
   public void run() {
     // method calling  
class Thrd2 extends Thread {
   // thread number 2 
   public void run() {
     // method calling
class Thrd3 extends Thread {
   // thread number 3
   public void run() {
     // method calling
public class ThrdExecution {
   public static void main(String args[]) {
     // creating object for thread class
     Thrd1 oprt1 = new Thrd1();
     Thrd2 oprt2 = new Thrd2();  
     Thrd3 oprt3 = new Thrd3();
     // Starting the thread operation


I am in else block
I am in else block
Count of Lock: 2


In this article, we have learned ReentrantLock class and the use of its inbuilt methods like ‘lock()’, ‘unlock()’ and ‘trylock()’. Also, we created a Java program in which we have seen the practical implementation of ReentrantLock class and its methods.

Updated on: 16-May-2023


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