Thread Synchronization and Locks in Java

In concurrent programming, where multiple threads access shared resources, it is crucial to ensure synchronization to avoid data corruption and inconsistent results. Java provides built-in mechanisms for thread synchronization using locks to handle such scenarios safely. In this article, we will explore thread synchronization and locks in Java and understand how they help in coordinating thread execution.

Thread Synchronization

Thread synchronization is the process of controlling the order of thread execution to prevent race conditions and maintain data integrity. In Java, synchronization is achieved by using the synchronized keyword. This keyword can be applied to methods or code blocks to ensure that only one thread can execute the synchronized part at a time.

Synchronized Methods

To synchronize an entire method, you can declare it as synchronized. When a thread enters a synchronized method, it acquires the intrinsic lock (also known as monitor lock) associated with the object on which the method is called. This lock allows only one thread to execute the synchronized method while other threads wait until the lock is released.

public synchronized void synchronizedMethod() {
    // Synchronized method code
}

By synchronizing methods, you ensure that only one thread can access the critical section at a time, preventing data inconsistencies.

Synchronized Blocks

Instead of synchronizing entire methods, you can also synchronize specific code blocks within a method using synchronized blocks. A synchronized block is enclosed within curly braces and takes an object as a parameter. The specified object represents the lock, and only one thread holding the lock can execute the synchronized block.

public void methodWithSynchronizedBlock() {
    // Non-synchronized code

    synchronized (lockObject) {
        // Synchronized block code
    }

    // Non-synchronized code
}

In the above example, when a thread reaches the synchronized block, it acquires the lock on the lockObject and executes the block. Other threads attempting to enter the synchronized block will be blocked until the lock is released.

Locks in Java

Apart from using the synchronized keyword, Java provides explicit lock objects to manage thread synchronization. The java.util.concurrent.locks package contains various lock implementations that offer more flexibility and control than traditional synchronization.

ReentrantLock

One commonly used lock implementation is the ReentrantLock. It provides a reentrant lock that allows a thread to repeatedly acquire the lock without deadlocking itself. This means that a thread can acquire the lock multiple times, as long as it releases it an equal number of times.

private Lock lock = new ReentrantLock();

public void methodWithLock() {
    // Non-synchronized code

    lock.lock();
    try {
        // Code under lock
    } finally {
        lock.unlock();
    }

    // Non-synchronized code
}

Using ReentrantLock, you explicitly acquire the lock using lock() and release it using unlock() within a try-finally block to ensure proper handling, even if an exception occurs.

ReadWriteLock

Another lock implementation provided by Java is ReadWriteLock. It allows multiple threads to read a shared resource simultaneously, as long as no thread is writing to it. This can be beneficial when the majority of thread operations are reads, enabling better performance.

private ReadWriteLock lock = new ReentrantReadWriteLock();

public void readMethod() {
    lock.readLock().lock();
    try {
        // Read operation
    } finally {
        lock.readLock().unlock();
    }
}

public void writeMethod() {
    lock.writeLock().lock();
    try {
        // Write operation
    } finally {
        lock.writeLock().unlock();
    }
}

In the above example, multiple threads can access readMethod() simultaneously as they acquire the read lock. However, writeMethod() requires an exclusive write lock, allowing only one thread to execute the write operation while preventing other threads from both reading and writing.

Conclusion

Thread synchronization and locks are essential concepts in concurrent programming to avoid data races and maintain consistency. Java provides built-in support for thread synchronization using the synchronized keyword, as well as more advanced lock implementations like ReentrantLock and ReadWriteLock. Understanding and properly utilizing these mechanisms ensures safe and efficient execution of multi-threaded applications in Java.