Using the Synchronized Keyword in Java Multithreading

Java’s multithreading capabilities offer tremendous power and flexibility, but they come with challenges. One of the most fundamental concepts that I frequently rely on to ensure thread safety is the synchronized keyword. It’s the backbone of many concurrency solutions in Java and an essential tool to prevent data races and inconsistent states.

In this article, I’ll walk through how the synchronized keyword works, the scenarios where it shines, its relationship with monitors and locks, common pitfalls to watch out for, and best practices that have served me well in my programming journey.

The Role of Synchronization in Multithreaded Environments

When multiple threads operate on shared data, problems arise if one thread reads or writes that data while another is modifying it. Such situations lead to race conditions, data corruption, or unexpected behaviors.

To avoid these issues, access to shared resources must be controlled. Synchronization provides a mechanism to ensure that only one thread at a time can execute a block of code or method that manipulates shared state.

How synchronized Works

At its core, the synchronized keyword in Java enforces mutual exclusion. It leverages monitors, which are internal locks tied to objects. When a thread enters a synchronized block or method, it acquires the monitor associated with that object. Other threads attempting to enter any synchronized block or method guarded by the same monitor must wait until the lock is released.

This guarantee serializes access to critical sections of code, preventing simultaneous modifications.

Synchronizing Methods

One of the simplest ways to synchronize access is by declaring methods as synchronized:

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

Here, the lock is tied to the current object instance (this). If multiple threads call this method on the same object, only one thread can execute it at a time. Others will block until the lock becomes available.

If the method is static, the lock applies to the Class object itself, ensuring synchronization across all instances.

Synchronizing Blocks

Sometimes, synchronizing an entire method is unnecessary or inefficient. Narrowing the scope to a smaller block improves concurrency and reduces contention.

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

You can synchronize on any non-null object reference, allowing fine-grained control. This flexibility lets me protect just the critical section of code instead of the entire method.

Synchronizing on Different Objects

Choosing the right lock object is vital. Synchronizing on this is common but not always ideal, especially if exposing the object to external code that might cause deadlocks or unwanted blocking.

I often create private final lock objects dedicated solely for synchronization:

java private final Object lock = new Object();

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

This approach encapsulates locking, reducing risks from external interference.

Intrinsic Locks and Monitor Concept

Java’s synchronized keyword relies on intrinsic locks (also called monitors) implemented at the JVM level.

When a thread enters a synchronized block or method, it requests ownership of the monitor. If another thread already holds it, the new thread blocks until the monitor is released.

Exiting the synchronized code releases the monitor, allowing waiting threads to proceed.

Visibility and Happens-Before Guarantees

Synchronization does more than just mutual exclusion; it establishes memory visibility guarantees.

Changes made by a thread inside a synchronized block are flushed to main memory when it exits the block. Any thread entering a synchronized block guarded by the same monitor will see those changes.

This behavior ensures consistent views of shared data across threads.

Deadlocks: The Hidden Danger

One of the major hazards when working with synchronized blocks is deadlocks.

Deadlocks occur when two or more threads are waiting indefinitely for locks held by each other, creating a cycle of dependencies that never resolves.

For example:

java synchronized (lock1) {
    synchronized (lock2) {
        // do work
    }
}

synchronized (lock2) {
    synchronized (lock1) {
        // do work
    }
}

If Thread A acquires lock1 and waits for lock2, while Thread B holds lock2 and waits for lock1, both threads block forever.

I always watch for potential deadlocks when acquiring multiple locks and follow consistent lock ordering to prevent them.

Reentrant Locks and synchronized

The intrinsic locks used by synchronized are reentrant, meaning a thread can acquire the same lock multiple times without blocking itself.

This feature allows synchronized methods to call other synchronized methods on the same object safely.

Without reentrancy, recursive calls or method chains involving synchronization would deadlock.

Performance Considerations

While synchronized provides safety, it comes with performance costs.

Acquiring and releasing locks involves overhead, and threads blocking while waiting reduce parallelism.

However, modern JVMs have optimized synchronized significantly through techniques like biased locking and lock coarsening, so it’s often efficient enough for many use cases.

Still, I avoid overusing synchronized blocks, especially wide ones, and strive to keep critical sections short.

Alternatives to synchronized

Though synchronized is powerful, Java offers other concurrency utilities that sometimes provide better scalability or flexibility:

• ReentrantLock: Part of the java.util.concurrent.locks package, providing explicit lock control, timed waits, and interruptibility.
• Atomic Variables: Classes like AtomicInteger offer lock-free thread-safe operations.
• Concurrent Collections: Classes such as ConcurrentHashMap handle synchronization internally for better performance.

I use these tools when synchronized doesn’t fit the problem or when I need advanced features.

Practical Example: Thread-Safe Counter

Consider a simple counter shared among threads:

java public class Counter {
    private int count = 0;

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

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

Using synchronized methods, I ensure that increments and reads are atomic and consistent. Without synchronization, concurrent access could lead to lost updates.

Synchronizing Static Methods and Class-Level Locks

Declaring a static method synchronized locks on the Class object:

java public static synchronized void staticMethod() {
    // critical section
}

This means all threads accessing any synchronized static methods of that class share the same lock, preventing concurrent access at the class level.

I use static synchronization when shared static resources or states need protection.

Nested Synchronization and Monitor Escalation

Nested synchronized blocks can be used to acquire multiple locks:

java synchronized(lock1) {
    synchronized(lock2) {
        // critical section
    }
}

The JVM efficiently manages monitors, but deep nesting or acquiring many locks can complicate design and increase risk of deadlocks.

Monitor escalation occurs when lightweight locks are promoted to heavier OS-level locks under contention, impacting performance. I aim to minimize lock contention to avoid this.

Volatile vs synchronized

The volatile keyword ensures visibility of changes to variables but does not provide atomicity or mutual exclusion.

In contrast, synchronized guarantees exclusive access and memory consistency.

I use volatile for simple flags or state indicators where atomicity is not required, and synchronized when protecting complex operations or multiple variables.

Common Mistakes with synchronized

Some pitfalls I’ve encountered include:

• Synchronizing on mutable or publicly accessible objects, leading to unpredictable locking behavior.
• Holding locks longer than necessary, causing contention.
• Neglecting to consider exceptions within synchronized blocks, which still release locks but may leave state inconsistent.
• Mixing synchronized with other concurrency controls improperly, leading to subtle bugs.

Being mindful of these improves program correctness and maintainability.

Debugging Synchronization Issues

Deadlocks and race conditions can be challenging to debug.

Thread dumps, available through tools like jstack, reveal thread states and lock ownership.

I also rely on logging and thread naming to trace synchronization flow during execution.

Unit testing with multiple threads and tools like Thread Sanitizer help catch concurrency issues early.

Best Practices for Using synchronized

Through experience, I follow these guidelines:

• Keep synchronized blocks as small as possible.
• Prefer synchronizing on private final objects, not this.
• Avoid nested synchronization unless necessary and follow consistent lock ordering.
• Consider alternatives like ReentrantLock or atomic classes when appropriate.
• Always handle exceptions carefully to maintain consistent state.
• Document the locking strategy clearly for maintainers.

Conclusion

The synchronized keyword remains a fundamental tool for managing concurrency in Java. By enforcing exclusive access and providing visibility guarantees, it helps avoid many concurrency pitfalls.

While modern Java concurrency offers more advanced alternatives, mastering synchronized provides a strong foundation for understanding thread safety and designing robust multithreaded applications.

Using it wisely, combined with good design and debugging practices, has helped me create stable and performant concurrent programs time and again.