Synchronized Blocks vs Synchronized Methods in Java: Key Differences and Best Practices

Concurrency introduces risks like race conditions and data corruption. Java provides built-in support for synchronization through synchronized blocks and synchronized methods. Understanding the difference is essential for writing thread-safe, performant Java code.

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🧠 What Is Synchronization in Java?

Synchronization ensures that only one thread can access a shared resource at a time. It prevents inconsistent or corrupted states in concurrent environments.

Java supports synchronization using the synchronized keyword in two forms:

• Synchronized Methods
• Synchronized Blocks

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🧵 Thread Lifecycle Relevance

Synchronization is important in the RUNNING and BLOCKED states:

• Threads enter BLOCKED if a required monitor (lock) is held by another thread.
• They resume RUNNING once the lock becomes available.

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🔒 Synchronized Methods

Syntax

public synchronized void increment() {
    count++;
}

• Locks on the current object (this) if non-static.
• Locks on the class object if static.

Pros

• Simple and readable
• Ensures full method protection

Cons

• Can block more code than necessary
• Less flexible than blocks

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🔐 Synchronized Blocks

Syntax

public void increment() {
    synchronized (this) {
        count++;
    }
}

• Locks only a section of code.
• You can choose any object as a lock (not just this).

synchronized (someOtherObject) {
    // custom lock
}

Pros

• More fine-grained control
• Better performance under contention

Cons

• Slightly more verbose
• Easier to misuse lock objects

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✅ When to Use What?

Scenario	Use
Full method needs protection	Synchronized method
Only part of the method is critical	Synchronized block
Need to use a custom lock	Synchronized block
Static method requiring lock	Static synchronized method

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🧪 Example: Synchronized Method

public class Counter {
    private int count = 0;

    public synchronized void increment() {
        count++;
    }

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

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🧪 Example: Synchronized Block

public class Counter {
    private int count = 0;
    private final Object lock = new Object();

    public void increment() {
        synchronized (lock) {
            count++;
        }
    }

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

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📉 Performance Considerations

• Synchronized blocks reduce the scope of the lock and improve throughput.
• Over-synchronization can lead to contention and degraded performance.
• Use profiling tools to evaluate lock impact.

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📌 What's New in Java Concurrency (8–21)

• Java 8: CompletableFuture, lambdas for threads
• Java 9: StackWalker, improved monitoring tools
• Java 11: var inference in lambda parameters
• Java 21: Structured concurrency, virtual threads, ScopedValue

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🛠 Best Practices

• Use synchronized blocks for finer control
• Prefer immutability where possible
• Keep synchronized sections short and fast
• Avoid locking on this or interned Strings
• Always release locks in finally blocks

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❓ FAQ

1. What’s the difference between synchronized block and method?
   Block locks part of the code; method locks the entire method.

2. Which is better for performance?
   Synchronized blocks generally, because they lock less code.

3. Can I use multiple locks in one method?
   Yes — each block can lock a different object.

4. Can static methods be synchronized?
   Yes — they lock on the class object.

5. Is synchronization reentrant?
   Yes — the same thread can reacquire the same lock.

6. Are synchronized blocks thread-safe?
   Yes, when used correctly with shared resources.

7. Can I synchronize on any object?
   Yes, but avoid locking on mutable or externally accessible objects.

8. Do synchronized methods block all threads?
   Only those trying to access the same monitor.

9. Can virtual threads use synchronization?
   Yes — but structured concurrency and ScopedValue are preferred.

10. Is synchronized faster than ReentrantLock?
    It depends — synchronized has improved significantly in modern JVMs.

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🧾 Conclusion and Key Takeaways

• Use synchronized to guard critical sections from concurrent access.
• Prefer blocks over methods for better control and scalability.
• Understand object-level vs class-level locking.
• Combine synchronization with modern APIs for clean, maintainable code.

Mastering synchronized blocks vs methods enables you to write safe and efficient Java multithreaded applications.