What are locks and monitors?

In concurrent programming, locks and monitors are fundamental synchronization primitives used to manage access to shared resources by multiple threads. They prevent race conditions and ensure data integrity. This tutorial explains locks and monitors in Java, focusing on their concepts, implementation, and usage.

Introduction to Locks and Monitors

Locks are synchronization mechanisms that allow only one thread to access a shared resource at any given time. When a thread acquires a lock, other threads attempting to acquire the same lock are blocked until the lock is released.

Monitors are higher-level constructs that provide a mechanism for threads to have both mutual exclusion and the ability to wait for a certain condition to become true. In Java, every object has an intrinsic monitor associated with it, accessible via the synchronized keyword and the wait(), notify(), and notifyAll() methods.

Java's Intrinsic Locks (Monitors)

Java's intrinsic locks, or monitors, are implemented using the synchronized keyword. When a thread enters a synchronized block or method, it acquires the lock associated with the object. Only one thread can hold the lock at a time.

In this example, the increment() and getCount() methods are synchronized. This means that only one thread can execute either of these methods on the same Counter object at a time, preventing race conditions.

public class Counter {
    private int count = 0;

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

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

ReentrantLock

The ReentrantLock class provides a more flexible locking mechanism than intrinsic locks. It allows a thread to re-enter a lock it already holds without blocking. It must be explicitly acquired and released.

In this example, we use a ReentrantLock to protect the count variable. The lock() method acquires the lock, and the unlock() method releases it. The try-finally block ensures that the lock is always released, even if an exception occurs.

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Counter {
    private int count = 0;
    private Lock lock = new ReentrantLock();

    public void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }

    public int getCount() {
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }
}

Concepts Behind the Snippets

• Mutual Exclusion: Only one thread can hold a lock at a time, preventing concurrent access to shared resources.
• Reentrancy: A thread can reacquire a lock it already holds. This is crucial for avoiding deadlocks in recursive or nested synchronized methods/blocks.
• Fairness: Locks can be fair or unfair. Fair locks grant access to the longest-waiting thread. Unfair locks may grant access to any waiting thread, potentially improving performance but risking starvation. ReentrantLock allows specifying fairness.
• Condition Variables: Monitors allow threads to wait for specific conditions to become true using wait(), and to signal waiting threads when a condition changes using notify() or notifyAll().

Real-Life Use Case

Consider a banking application where multiple threads need to access and modify a shared bank account. Without proper synchronization, concurrent transactions could lead to incorrect balances. Locks ensure that only one thread can update the balance at a time, preventing race conditions and maintaining data integrity.

Another example is a resource pool where multiple threads compete for a limited number of resources. Locks can be used to manage the allocation and deallocation of resources, ensuring that they are not accessed concurrently by multiple threads.

Best Practices

• Minimize Lock Holding Time: Keep the critical section (the code executed while holding the lock) as short as possible to reduce contention and improve performance.
• Use Try-Finally Blocks: Always release locks in a try-finally block to ensure they are released even if exceptions occur.
• Avoid Deadlocks: Be careful about the order in which locks are acquired to prevent deadlocks. If multiple locks are required, acquire them in a consistent order.
• Use Higher-Level Abstractions: Consider using higher-level concurrency abstractions like java.util.concurrent classes (e.g., BlockingQueue, ExecutorService) when appropriate, as they often provide safer and more efficient solutions.

Interview Tip

When discussing locks and monitors in an interview, be prepared to explain the differences between intrinsic locks (synchronized) and explicit locks (ReentrantLock), as well as the advantages and disadvantages of each. Also, be ready to discuss common concurrency problems like race conditions and deadlocks, and how locks and monitors can be used to solve them. Know about fairness and its impact on performance and thread starvation.

When to Use Them

• Intrinsic Locks (synchronized): Use when simplicity and ease of use are paramount, and performance is not a critical concern. Suitable for simple synchronization scenarios within a single object.
• ReentrantLock: Use when more flexibility and control are required, such as when needing to specify fairness, attempt lock acquisition with a timeout, or use condition variables. Ideal for complex synchronization scenarios and high-performance applications.

Memory Footprint

Intrinsic Locks: The memory footprint is implicit and tied to each object. Each object has a header that contains information about its lock state. This overhead is minimal unless the object is frequently contended.

ReentrantLock: Has a slightly larger memory footprint because it is an explicit object. Each instance of ReentrantLock consumes memory for its internal state. The difference is generally negligible unless you are creating a very large number of locks.

Alternatives

• Atomic Variables: For simple atomic operations (e.g., incrementing a counter), consider using java.util.concurrent.atomic classes like AtomicInteger or AtomicLong. These provide lock-free alternatives that can be more efficient in some cases.
• Concurrent Collections: For concurrent data structures, use classes from java.util.concurrent like ConcurrentHashMap or ConcurrentLinkedQueue. These collections provide thread-safe operations without the need for explicit locking in many common scenarios.
• StampedLock: A more advanced lock introduced in Java 8, which provides optimistic locking and read-write locking capabilities with improved performance in certain situations.

Pros of Locks and Monitors

• Data Integrity: Prevents race conditions and ensures that shared resources are accessed in a controlled manner, maintaining data consistency.
• Synchronization: Provides a mechanism for coordinating the execution of multiple threads, allowing them to work together safely.
• Flexibility: Offers various locking options, from simple intrinsic locks to more advanced explicit locks with fine-grained control.

Cons of Locks and Monitors

• Complexity: Can be challenging to use correctly, especially in complex concurrent applications. Requires careful consideration of locking strategies and potential pitfalls like deadlocks.
• Performance Overhead: Locking introduces overhead, as threads may be blocked waiting to acquire a lock. Excessive locking can degrade performance.
• Potential for Errors: Incorrectly used locks can lead to subtle and hard-to-debug concurrency errors, such as race conditions or deadlocks.