What is the ReentrantLock class in Java multithreading and how do you use it?

1 ReentrantLock basics

The ReentrantLock class is part of the java.util.concurrent.locks package. It implements the Lock interface, which defines the basic operations of a lock: lock, unlock, tryLock, and lockInterruptibly. A ReentrantLock object has two main characteristics: it is reentrant and it is fair. Reentrant means that a thread can acquire the same lock multiple times without blocking itself. Fair means that the lock grants access to the longest-waiting thread, rather than the one that arrives first.

3 How to use ReentrantLock

To use the ReentrantLock class, you need to create an instance of it and pass a boolean argument to indicate if it is fair or unfair. Then, you need to call the lock method before accessing the shared resource, and the unlock method after releasing it. You also need to handle any exceptions that may occur, such as InterruptedException. For example, suppose you have a shared counter that needs to be incremented by multiple threads. You can use a ReentrantLock to synchronize the increment operation as follows:

import java.util.concurrent.locks.ReentrantLock;
public class Counter {
  private int value;
  private ReentrantLock lock;
  public Counter(boolean fair) {
    value = 0;
    lock = new ReentrantLock(fair);
  }
  public void increment() {
    lock.lock();
    try {
      value++;
    } finally {
      lock.unlock();
    }
  }
  public int getValue() {
    return value;
  }
}        

4 Benefits of ReentrantLock

The ReentrantLock class offers several benefits compared to the synchronized keyword. It enables you to create more complex locking logic, such as conditional, nested, and timed locking. Additionally, it can improve the performance and responsiveness of your program by reducing the contention and blocking of threads, as well as allowing thread interruption and cancellation. Furthermore, it allows you to have more control and visibility over the state of the lock by exposing methods like isLocked, isHeldByCurrentThread, and getHoldCount.

5 Drawbacks of ReentrantLock

The ReentrantLock class has some drawbacks in comparison to the synchronized keyword. These include requiring more code and boilerplate, as you need to explicitly create, lock, and unlock the lock object and handle any exceptions. Additionally, it is more prone to errors and bugs, as you may forget to unlock the lock or release it in the wrong order. It may also cause deadlocks or livelocks if you use multiple locks without proper coordination or unfair locks in a highly contended scenario.