Synchronized Keyword and Intrinsic Locks in Java

Concurrency is an important aspect of modern software development, especially in the context of multi-threaded applications. Managing shared resources and ensuring thread safety can be a challenging task. In Java, the synchronized keyword and intrinsic locks provide a mechanism for coordinating access to shared resources, allowing multiple threads to safely operate on them. In this article, we will explore the usage and benefits of the synchronized keyword and intrinsic locks in Java.

The synchronized Keyword

In Java, the synchronized keyword is used to create mutually exclusive regions, or critical sections, within a code block or method. It enforces thread synchronization by acquiring and releasing an intrinsic lock associated with an object or class.

Synchronized Blocks

A synchronized block is a section of code marked with the synchronized keyword, surrounded by curly braces. Only one thread can enter a synchronized block at a time, ensuring that the shared resource accessed within the block is protected from concurrent modification.

public void synchronizedMethod() {
  // non-critical section

  synchronized (this) {
    // critical section
    // accessed by only one thread at a time
  }

  // non-critical section
}

In the example above, the synchronized block is enclosed within a method. The keyword this denotes the intrinsic lock associated with the current instance of the object. When a thread enters the synchronized block, it acquires the lock, and other threads attempting to enter the block will wait until the lock is released. This ensures that only one thread can access the critical section at a time.

Synchronized Methods

Alternatively, you can use the synchronized keyword directly on a method declaration. When a method is marked as synchronized, it behaves the same way as if it were enclosed within a synchronized block using the object instance lock.

public synchronized void synchronizedMethod() {
  // critical section
  // accessed by only one thread at a time
}

Here, the entire method body is considered the critical section, and only one thread can execute the method at a time due to the acquired lock. Other threads will have to wait until the lock is released.

Intrinsic Locks

Behind the scenes, the synchronized keyword operates on an intrinsic lock, also known as a monitor lock. Every Java object and class has an associated intrinsic lock. When a thread encounters a synchronized block or method, it attempts to acquire the intrinsic lock of the associated object or class.

When a lock is acquired, it blocks other threads from entering the same synchronized block or method until the lock is released. This way, intrinsic locks provide mutual exclusion and synchronization between threads.

Benefits of synchronized Keyword and Intrinsic Locks

The synchronized keyword and intrinsic locks play a crucial role in managing thread safety and ensuring proper synchronization of shared resources in Java. Some of the benefits include:

1. Automatic thread synchronization: By using synchronized blocks or methods, you delegate the responsibility of handling thread synchronization to the Java runtime. This helps prevent data corruption and race conditions that can occur when multiple threads access shared resources simultaneously.

2. Built-in fairness: The synchronized keyword provides built-in fairness, ensuring that threads waiting to acquire a lock are granted access in the order they requested it. This helps prevent thread starvation and promotes fairness in resource allocation.

3. Simplicity: Using the synchronized keyword and intrinsic locks is relatively simple and easy to understand. It allows you to protect critical sections of your code without having to implement complex synchronization mechanisms manually.

4. Inter-thread communication: Intrinsic locks can also be used in conjunction with other features such as wait() and notify() methods to facilitate inter-thread communication, allowing threads to coordinate their activities efficiently.

Conclusion

In Java, the synchronized keyword and intrinsic locks are powerful tools for managing thread synchronization and ensuring thread safety in concurrent programs. They provide automatic thread synchronization, fairness, simplicity, and support for inter-thread communication.

However, it is important to note that the synchronized keyword has some overhead, as acquiring and releasing locks may incur a performance cost. In certain scenarios, other concurrency mechanisms like java.util.concurrent package classes or Lock interfaces may be more suitable.

Understanding how to effectively utilize the synchronized keyword and intrinsic locks is crucial for writing robust and efficient multi-threaded applications in Java. Mastering this concept will help you develop scalable and thread-safe software solutions.