Detailed explanation of Java multi-threaded concurrency lock

Java concurrency lock mechanism ensures that in a multi-threaded environment, shared resources are only accessed by one thread. Its types include pessimistic locking (acquire the lock and then access) and optimistic locking (check for conflicts after accessing). Java provides built-in concurrency lock classes such as ReentrantLock (mutex lock), Semaphore (semaphore) and ReadWriteLock (read-write lock). Using these locks can ensure thread-safe access to shared resources, such as ensuring that when multiple threads access the shared variable counter at the same time, only one thread updates its value.

Java multi-threaded concurrency lock detailed explanation

Introduction

In a multi-threaded environment , multiple threads may access shared resources simultaneously, resulting in data inconsistencies or program errors. To prevent this, Java provides a concurrency lock mechanism, which ensures that only one thread accesses shared resources at a time.

Concurrency lock types

There are two main types of concurrency locks in Java:

• Pessimistic Lock (Pessimistic Lock) : It is assumed that all threads will access the shared resource, so the lock is acquired before accessing the shared resource. This results in more frequent context switches, but at the cost of lower concurrency.
• Optimistic Lock: It is assumed that most threads will not access shared resources, so conflicts are checked only after accessing shared resources. If a conflict occurs, the operation is rolled back. This results in fewer context switches, but at the cost of more possible conflicts.

Concurrency Locks in Java

The following built-in concurrency lock classes are provided in Java:

• ReentrantLock: A reentrant mutex, that is, a thread can acquire the same lock multiple times.
• Semaphore: A counting semaphore used to control the maximum number of concurrent threads accessing shared resources.
• ReadWriteLock: A read-write lock that allows multiple threads to read shared resources at the same time, but only allows one thread to write to shared resources.

Practical case

Suppose we have two threads accessing a shared variable counter at the same time, and we want to ensure that there is only one thread at a time Update the value of counter. We can use ReentrantLock to achieve this:

public class Counter {

    private int counter;
    private ReentrantLock lock = new ReentrantLock();

    public int getCounter() {
        lock.lock();
        try {
            return counter;
        } finally {
            lock.unlock();
        }
    }

    public void incrementCounter() {
        lock.lock();
        try {
            counter++;
        } finally {
            lock.unlock();
        }
    }
}

In this example, both the getCounter() and incrementCounter() methods use lock to ensure that access to counter is thread-safe.

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