Difference Between Lock and Monitor in Java Concurrency

Lock

Locks in Java are synchronization primitives, used to control access to shared resources or critical sections of code to ensure that only one thread can access them at a time.

A lock is a simple synchronization construct that allows one thread to acquire a lock on a resource. Once a thread has acquired a lock on a resource, the first thread can acquire the lock once the first thread releases it.

Locks ensure that only one thread can access a protected resource at a time, preventing race conditions and data corruption.

Java provides several mechanisms for implementing locks, with the most commonly used one being the 'ReentrantLock' class from the java.util.concurrent.locks package. Here's an overview of locks in Java concurrency:

1. ReentranLock

ReentrantLock in Java is a synchronization mechanism that allows a thread to acquire the same lock multiple times without blocking itself. It is useful for situations where a thread needs to access a shared resource that is protected by a lock.

ReentrantLock provides several methods for locking and unlocking, including lock(), unlock(), tryLock(), and newCondition().

It allows for finer-grained control over synchronization compared to synchronized blocks or methods.

ReentrantLockExample.java

Output:

Thread 1 is performing a task. Thread 1 has completed the task. Thread 2 is performing a task. Thread 2 has completed the task. 

In this example, the doSomething() method acquires the lock before accessing the shared resource. When the method is finished with the resource, it releases the lock.

2. Lock Interfaces

Java provides various lock interfaces, such as Lock, ReadWriteLock, and Condition, allowing us to choose the right type of lock for our specific concurrency needs.

3. Exception Handling

Locks should be used within try-finally blocks to ensure proper lock release, even in the presence of exceptions.

It is important to prevent resource leaks and ensure that other threads can access the locked resource once the current thread is done.

4. Concurrent Collection

Many classes in the java.util.concurrent package uses locks internally to provide thread-safe collections, such as ConcurrentHashMap and CopyOnWriteArrayList.

Monitor

In Java concurrency, a monitor is a synchronization concept that is associated with objects and methods. It provides fundamental requirements of multithreading.

A monitor is a synchronization mechanism that allows threads to have:

Mutual exclusion- Only one thread can execute the method at a certain point in time, using locks.

Cooperation - The ability to make threads wait for certain conditions to be met using wait-set.

Monitors basically 'monitor' the access control of shared resources and objects among threads.

1. Synchronized Methods

We can declare a method as synchronized in Java to make it a synchronized method. When a thread enters a synchronized method, it acquires the monitor associated with the object instance.

2. Synchronized Blocks

In addition to synchronized methods, we can use synchronized blocks to protect specific sections of code. It allows for more fine-grained control over synchronization.

SynchronisedBlockExample.java

Output:

Final Count: 0 

3. Intrinsic Lock

In Java, each object has an intrinsic lock, also known as a monitor lock. It allows a thread to acquire exclusive Access to an object. It means that only one thread can be executing code that is synchronized on the object at a time. Intrinsic locks are used to prevent race conditions.

To acquire an intrinsic lock, a thread calls the synchronized() method. The synchronized() method takes an object as its argument. The thread then acquires the intrinsic lock for the object. Once the thread has acquired the lock, it can execute the synchronized code without worrying about other threads interfering.

When the thread is finished executing the synchronized code, it must release the intrinsic lock. The thread calls the synchronized() method again, this time with the false argument. The false argument tells the synchronized() method to release the lock.

IntrinsicLockExample.java

Output:

Thread 1 is performing a synchronized task. Thread 1 has completed the task. Thread 2 is performing a synchronized task. Thread 2 has completed the task. 

4. Ensuring Thread Safety

Monitors are used to ensure thread safety when multiple threads access shared data or resources concurrently.

By synchronizing methods or blocks, you can prevent race conditions, data corruption, and other concurrency issues.

5. Deadlock

Deadlocks occur when two or more threads are waiting for each other to release locks, causing the program to come to a standstill.

Proper design and careful use of synchronized blocks can help avoid deadlocks.

DeadlockExample.java

Output:

Thread 1: Holding resource1... Thread 2: Holding resource2... Thread 1: Waiting for resource2... Thread 2: Waiting for resource1... 

While monitors help prevent race conditions, it can also lead to deadlocks if not used carefully.

Key Differences Between Locks and Monitors