Why is synchronized block better than synchronized method?

Synchronized Blocks vs. Synchronized Methods in Java: A Deep Dive

Explore the nuances of Java's synchronized keyword, comparing synchronized methods and synchronized blocks to understand why blocks often offer superior control and performance in multithreaded applications.

In Java's multithreading landscape, ensuring data consistency and preventing race conditions is paramount. The synchronized keyword is your primary tool for achieving this, providing intrinsic locks (monitors) to protect critical sections of code. While both synchronized methods and synchronized blocks serve the same fundamental purpose – to allow only one thread at a time to execute a specific piece of code – they differ significantly in their scope and flexibility. Understanding these differences is crucial for writing efficient, scalable, and bug-free concurrent applications.

Understanding Synchronized Methods

When you declare a method as synchronized, the entire method body becomes a critical section. For instance methods, the lock is acquired on the instance of the object (this). For static methods, the lock is acquired on the Class object itself. This approach is straightforward and easy to implement, making it a common choice for simple synchronization needs. However, its broad scope can sometimes lead to performance bottlenecks.

public class Counter {
    private int count = 0;

    // Synchronized method
    public synchronized void increment() {
        count++;
        // Potentially other non-critical operations here
    }

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

Example of a synchronized method in Java.

Understanding Synchronized Blocks

Synchronized blocks, on the other hand, offer finer-grained control over synchronization. You explicitly specify the object on which to acquire the lock. This allows you to protect only the critical section of code that truly needs synchronization, leaving other parts of the method free for concurrent execution. This targeted approach can significantly improve performance and scalability, especially in methods that perform both thread-safe and non-thread-safe operations.

public class FineGrainedCounter {
    private int count = 0;
    private final Object lock = new Object(); // A dedicated lock object

    // Synchronized block
    public void increment() {
        // Non-critical operations can happen here
        synchronized (lock) { // Acquire lock on 'lock' object
            count++;
        }
        // More non-critical operations
    }

    public int getCount() {
        synchronized (lock) { // Acquire lock on 'lock' object
            return count;
        }
    }
}

Example of a synchronized block using a dedicated lock object.

flowchart TD
    A[Thread Enters Method] --> B{Is Method Synchronized?}
    B -- Yes --> C[Acquire Lock on 'this' or Class]
    B -- No --> D{Is Synchronized Block Present?}
    D -- Yes --> E[Acquire Lock on Specified Object]
    D -- No --> F[Execute Code Concurrently]
    C --> G[Execute Critical Section]
    E --> G
    G --> H[Release Lock]
    H --> I[Thread Exits Method]

Decision flow for synchronization mechanisms.

Why Synchronized Blocks are Often Preferred

The primary advantage of synchronized blocks lies in their ability to minimize the scope of the lock. By locking only the essential code, you reduce the time threads spend waiting for a lock, thereby increasing throughput and responsiveness. This is particularly beneficial in methods that perform extensive computations or I/O operations alongside a small critical section. Furthermore, synchronized blocks allow you to lock on any arbitrary object, providing greater flexibility than synchronized methods, which are limited to this or the Class object.

Visualizing the scope difference between synchronized methods and blocks.