In the Java programming language, managing access to shared resources by multiple threads is a critical task that requires effective synchronization techniques. The synchronized keyword and ReentrantLock are two essential tools provided by the Java concurrency framework to handle this task. Understanding why one might choose ReentrantLock over synchronized can influence the design and performance of multithreaded applications.

Synchronized Mechanism

The synchronized keyword in Java is a built-in synchronization mechanism. By using synchronized, developers can ensure that only one thread executes a code block at a time for a given object monitor. It is straightforward and easy to use, as shown in the following example:

1public class Counter {
2    private int count = 0;
3
4    public synchronized void increment() {
5        count++;
6    }
7
8    public synchronized int getCount() {
9        return count;
10    }
11}

Key Characteristics of synchronized:

1. Ease of Use: Simple to implement locking on methods or code blocks.
2. Implicit Locking: Automatically acquires and releases locks, reducing the risk of programming errors.
3. Fairness: Lacks fairness in acquiring locks, potentially leading to thread starvation.
4. Monitor Association: Locks are associated with the specific object it synchronizes on.

ReentrantLock Class

The ReentrantLock class is part of the java.util.concurrent.locks package and provides more sophisticated locking capabilities compared to synchronized. It allows for greater flexibility in handling locks, which can be beneficial in complex concurrent scenarios.

Key Advantages of ReentrantLock:

1. Lock Flexibility & Control: Offers methods such as lockInterruptibly(), tryLock(), and unlock() for advanced lock management.
2. Condition Variables: Supports multiple Condition objects for finer-grained control over thread coordination.
3. Fairness Policies: Can be configured with fairness settings to ensure first-come-first-serve lock granting.
4. Lock State Inquiry: Provides the ability to monitor lock state, such as checking if a lock is held by the current thread through methods like isHeldByCurrentThread().

Technical Example:

1import java.util.concurrent.locks.ReentrantLock;
2
3public class Counter {
4    private final ReentrantLock lock = new ReentrantLock();
5    private int count = 0;
6
7    public void increment() {
8        lock.lock();
9        try {
10            count++;
11        } finally {
12            lock.unlock();
13        }
14    }
15
16    public int getCount() {
17        lock.lock();
18        try {
19            return count;
20        } finally {
21            lock.unlock();
22        }
23    }
24}

Detailed Comparison

The choice between synchronized and ReentrantLock often depends on the specific requirements of the application. Below is a detailed comparison:

Advanced Features of ReentrantLock

Fairness Policy

By default, ReentrantLock does not implement any fairness policy, meaning that thread scheduling might be arbitrary. However, ReentrantLock can be initialized with a fairness parameter:

ReentrantLock fairLock = new ReentrantLock(true);

This ensures that the longest-waiting thread is granted the lock first, reducing thread starvation.

Interruptible Locking

With methods like lockInterruptibly(), threads can acquire a lock while also being able to respond to interruption requests, which is not possible with synchronized.

lock.lockInterruptibly();

Conditions

ReentrantLock supports condition variables, enabling more granular control over thread communication compared to synchronized.

Condition condition = lock.newCondition();

Conclusion

While synchronized remains a robust and easy-to-use mechanism for many basic concurrency tasks, ReentrantLock offers additional capabilities and fine-grained control that can be advantageous in complex multithreading situations. Choosing between them involves weighing factors like performance, code complexity, and the specific synchronization requirements of the application. Understanding and leveraging the strengths of each mechanism is key to developing efficient and reliable concurrent applications.