Synchronized blocks are a fundamental feature of Java's concurrency model, utilized to control access to a block of code by multiple threads. The synchronized keyword can be applied to methods or blocks within methods to enforce mutual exclusion based on a specific object's monitor. While synchronization is essential to avoid race conditions and ensure thread safety, improper use of synchronization can lead to performance bottlenecks or, in some scenarios, unintended behavior. One specific best practice is to avoid using synchronized(this) and instead use alternative strategies for locking. This article delves into the technical nuances of why synchronized(this) should be avoided, proposes alternatives, and concludes with a summary table.

Understanding Synchronization in Java

In Java, synchronization is used to control the access of multiple threads to shared resources. The synchronized keyword can be used with methods or blocks to ensure that the code within can only be executed by one thread at a time.

Method Synchronization

public synchronized void exampleMethod() {
    // method code
}

When a method is declared synchronized, only one thread can execute this method on the same instance of the class at a time.

Block Synchronization

1public void exampleMethod() {
2    synchronized(this) {
3        // synchronized block code
4    }
5}

This block synchronization locks the current instance (this) for the synchronized section.

Issues with synchronized(this)

Using synchronized(this) has several caveats:

1. Encapsulation Violation: Locking on this exposes the lock object to the outside world. External code can use the same lock to interfere with the synchronized block inside your class, leading to unpredicated locking policies.
2. Deadlock Risks: If external code obtains the lock on this, the internal synchronized block might end up waiting indefinitely, resulting in a deadlock situation.
3. Interference with Subclassing: When other classes extend your class and override methods, using synchronized(this) can inadvertently lead to subclass interactions that break assumptions about synchronized behavior.
4. Decreased Modularity: Relying on this for synchronization ties the locking policy directly to the instance of the class, making it less modular and harder to refactor.

Using Alternative Lock Objects

To avoid the above issues, it's beneficial to use an explicit lock object instead of this. This practice enhances encapsulation and modularity.

Example with an Explicit Lock Object

1public class MyClass {
2    private final Object lock = new Object();
3
4    public void exampleMethod() {
5        synchronized(lock) {
6            // synchronized block code
7        }
8    }
9}

Benefits of Using a Private Lock Object

1. Encapsulation: Since the lock is private, it isn't exposed to external code, thus preserving encapsulation.
2. Reduced Risk of Deadlocks: By not sharing the lock object with external code, you lower the risk of unintended deadlocks.
3. Control Over Synchronization Granularity: Different operations within the same class may use different lock objects, allowing fine-tuned control over concurrency.
4. Improved Modularity: Because the lock object isn't tied to the class instance (this), it becomes easier to change how synchronization is handled internally without affecting external behavior.

Alternatives to Synchronization

Java provides higher-level concurrency utilities that can often replace traditional synchronized blocks, offering more scalable and flexible solutions:

java.util.concurrent.locks

The Lock interface and its implementations, such as ReentrantLock, provide more sophisticated locking mechanisms, including try-lock operations and timed locks.

1import java.util.concurrent.locks.Lock;
2import java.util.concurrent.locks.ReentrantLock;
3
4public class MyClass {
5    private final Lock lock = new ReentrantLock();
6
7    public void exampleMethod() {
8        lock.lock();
9        try {
10            // critical section
11        } finally {
12            lock.unlock();
13        }
14    }
15}

java.util.concurrent

Java's concurrent package provides other utilities, such as:

• Executor Framework: Manages concurrent execution of tasks without explicit synchronization.
• Concurrent Collections: Automatically handle synchronization internally, reducing the need for explicit locks.

1import java.util.concurrent.ExecutorService;
2import java.util.concurrent.Executors;
3
4public class MyClass {
5    private final ExecutorService executor = Executors.newFixedThreadPool(10);
6
7    public void submitTask(Runnable task) {
8        executor.submit(task);
9    }
10}

Summary Table

In conclusion, while synchronized(this) can be used for simple synchronization tasks, it comes with several pitfalls that could impact the robustness, efficiency, and maintainability of your code. Implementing alternative synchronization strategies, like using private lock objects or leveraging concurrent utilities, can better meet the demands of modern, concurrent Java applications.