🔐 Understanding the Synchronized Keyword in Java: Concurrency Control Made Easy

Concurrency is a cornerstone of modern programming. With Java's multithreading capabilities, multiple threads can be executed concurrently to improve application performance. However, when multiple threads access shared resources or critical sections of code, synchronization becomes necessary to avoid race conditions, deadlocks, and other concurrency issues. One of the primary tools in Java for achieving synchronization is the synchronized keyword.

In this blog post, we will dive deep into the synchronized keyword in Java: how it works, when to use it, and best practices for using it to manage multithreading and concurrency in your applications.

⚙️ What is the Synchronized Keyword?

The synchronized keyword in Java is used to control access to a critical section of code by multiple threads. When a method or block of code is marked as synchronized, only one thread can access that method or block at any given time. This prevents other threads from executing the synchronized code concurrently, ensuring that shared resources or variables are accessed in a controlled, thread-safe manner.

The synchronized keyword can be used in two main ways:

Synchronized Methods
Synchronized Blocks

Let's look at both forms in detail.

1. 🛠️ Synchronized Methods

You can declare an entire method as synchronized. This means that only one thread can execute the method at any given time.

Example of Synchronized Method:

class Counter {
    private int count = 0;

    // Synchronized method
    public synchronized void increment() {
        count++; // Critical section
    }

    public synchronized int getCount() {
        return count;
    }
}

In this example, the method increment() is synchronized. If multiple threads are trying to call increment() on the same Counter object, only one thread can execute it at a time, ensuring thread safety.

How Synchronized Methods Work

The synchronized keyword applies to the instance method or class method.
When a thread enters a synchronized method, it acquires a monitor lock on the object.
If another thread is already executing a synchronized method on the same object, other threads attempting to access that method are blocked until the lock is released.

2. 🧹 Synchronized Blocks

In some cases, you may not want to synchronize an entire method. You can synchronize a specific block of code within a method instead. This provides more fine-grained control over the synchronization.

Example of Synchronized Block:

class Counter {
    private int count = 0;

    public void increment() {
        // Synchronizing only this critical section
        synchronized (this) {
            count++; // Critical section
        }
    }
}

Here, only the critical section inside the synchronized block is protected from concurrent access, while the rest of the method remains unsynchronized. This approach minimizes the scope of synchronization and improves performance.

Synchronizing on Specific Objects

You can synchronize on any object, not just this.
For example, synchronizing on a class object can be useful for static methods.

class Counter {
    private static int count = 0;

    public static void increment() {
        synchronized (Counter.class) {
            count++; // Critical section
        }
    }
}

🔐 How the Synchronized Keyword Works

The Locking Mechanism

The synchronized keyword operates by acquiring a lock (also known as a monitor) on an object. When a thread enters a synchronized method or block, it holds the lock for that object until it exits the method or block.

This is known as mutual exclusion (mutex). If another thread attempts to access the synchronized method or block while the first thread is holding the lock, it will be blocked until the lock is released.

In Java, there are two types of locks:

Instance-level lock: When synchronizing an instance method, the lock is acquired on the instance of the object (using this).
Class-level lock: When synchronizing a static method or block, the lock is acquired on the class (using ClassName.class).

Example of Locking Mechanism

class SharedResource {
    private int count = 0;

    // Instance lock
    public synchronized void increment() {
        count++; // Critical section
    }
}

In the example above, the lock is acquired on the SharedResource object instance. No other thread can call increment() on the same object until the first thread finishes executing.

🔄 Reentrant Locking in Java

A critical feature of synchronized methods and blocks in Java is reentrancy. Reentrancy means that a thread that already holds a lock can enter the synchronized block or method again without causing a deadlock.

Example of Reentrant Locking

class Counter {
    private int count = 0;

    public synchronized void increment() {
        // Reentrant lock
        incrementAgain(); // Calling another synchronized method
        count++; // Critical section
    }

    public synchronized void incrementAgain() {
        count++; // Critical section
    }
}

In this example, the thread can call incrementAgain() inside increment() because the lock is reentrant. The thread does not need to release the lock when entering incrementAgain().

This feature prevents deadlocks in recursive or nested synchronized calls.

⚠️ Common Pitfalls of Synchronized Keyword

While synchronization is essential for ensuring thread safety, it comes with a few potential pitfalls:

1. Deadlocks

Deadlocks occur when two or more threads are blocked forever, each waiting for the other to release a lock. This can happen if two threads are each holding a lock and waiting on the other thread’s lock.

How to prevent deadlocks:

Always acquire locks in a consistent order.
Use tryLock() with a timeout instead of locking indefinitely.

2. Thread Contention

If many threads are waiting for the same lock, it can cause thread contention and performance degradation. The more threads you have trying to acquire a lock, the longer they have to wait.

How to manage contention:

Use more fine-grained locks (synchronize only the critical section).
Use higher-level concurrency constructs such as ReentrantLock or Semaphore to manage complex synchronization patterns.

3. Improper Use of Synchronization

Unnecessary synchronization can lead to performance overhead. It can make the program slower by blocking threads unnecessarily.

Best practices:

Synchronize only what’s necessary. Avoid synchronizing code that doesn’t access shared resources.
Minimize the scope of synchronized blocks.

💡 Best Practices for Using Synchronized in Java

Minimize the Scope of Synchronization

Always synchronize only the critical section of code that needs protection, not the entire method.

Use Fine-Grained Locks

Prefer synchronized blocks over synchronized methods to avoid unnecessarily locking large sections of code.

Avoid Synchronization on Objects with High Contention

Don’t synchronize on frequently accessed resources, like collections or data structures. Use thread-safe alternatives such as ConcurrentHashMap or CopyOnWriteArrayList.

Consider Using High-Level Concurrency Utilities

Consider using tools from the java.util.concurrent package, such as ReentrantLock, Semaphore, and CountDownLatch, for more advanced control over synchronization.

🚀 Conclusion

The synchronized keyword is a powerful tool in Java for ensuring thread safety in multithreaded environments. It helps prevent concurrency issues like race conditions, deadlocks, and inconsistent data access. However, it comes with the responsibility of managing thread contention and deadlocks effectively.

By understanding how the synchronized keyword works, following best practices, and leveraging Java’s concurrency utilities, you can create robust, efficient, and thread-safe applications that scale well in multi-threaded environments.