The Lock interface provides more flexible lock control than synchronized, supporting attempt acquisition, interruption, timeout acquisition and fair lock; 2. Condition allows accurate inter-thread communication through multiple condition variables to avoid false wake-up; 3. Common Synchronizers include CountDownLatch for waiting for multiple tasks to complete, CyclicBarrier for multi-thread synchronization to reach barrier points, Semaphore for controlling the number of concurrent threads, and Phaser for phased synchronization of dynamic threads; when using it, the simplicity of synchronized must be given priority. Lock must combine try-finally to prevent deadlocks, Condition should be used with while, and appropriate synchronization tools should be selected according to the scene to avoid over-design.

In Java concurrent programming, although synchronized keywords are simple and easy to use, they are not flexible enough in more complex scenarios. To do this, Java provides the java.util.concurrent.locks package, which contains more powerful concurrency control tools: Locks, Conditions, and Synchronizers . They allow developers to control thread synchronization behavior more granularly.

1. Lock interface: a more flexible lock mechanism than synchronized

Lock is an enhanced alternative to synchronized , with the main advantages being:

• You can try to acquire the lock ( tryLock() ) to avoid blocking
• Can interrupt thread waiting for lock
• Can timeout to acquire the lock
• More finer lock control (such as read and write separation)

Common implementation classes: ReentrantLock

 Lock lock = new ReentrantLock();

lock.lock();
try {
    // Critical area System.out.println("Thread enters critical area");
} finally {
    lock.unlock(); // Must be placed in finally to prevent deadlock}

?? Note: The lock must be released manually, otherwise it will cause deadlock or resource leakage.

Comparison with synchronized

2. Condition: Precise control of inter-thread communication

Condition is similar to Object 's wait() and notify() , but is more flexible. A Lock can create multiple Condition instances to implement multiple waiting/notification .

Use scenario: Producer-Consumer Model

 Lock lock = new ReentrantLock();
Condition notFull = lock.newCondition();
Condition notEmpty = lock.newCondition();

// Producer lock.lock();
try {
    while (queue.size() == CAPACITY) {
        notFull.await(); // Wait queue is dissatisfied}
    queue.add(item);
    notEmpty.signal(); // Notify the consumer} finally {
    lock.unlock();
}

// Consumer lock.lock();
try {
    while (queue.isEmpty()) {
        notEmpty.await(); // Wait queue not empty}
    String item = queue.poll();
    notFull.signal(); // Notify the producer} finally {
    lock.unlock();
}

? Advantages: Multiple condition variables can be controlled independently to avoid false wake-ups or notification disorders.

3. Common Synchronizers: Advanced synchronization tools provided by JUC

Java provides several out-of-the-box synchronization helper classes, located in java.util.concurrent package:

① CountDownLatch: Countdown latch

Used to make one or more threads wait for other threads to complete operations.

 CountDownLatch latch = new CountDownLatch(3);

// Worker thread latch.countDown();

// Wait for thread latch.await(); // Block until the count returns to zero

? Applicable scenario: The main thread waits for multiple background tasks to complete.

② CyclicBarrier: Circular Fence

Let a group of threads wait for each other until they all reach a certain barrier point, and then continue to execute together.

 CyclicBarrier barrier = new CyclicBarrier(3, () -> {
    System.out.println("All threads are in place, start action!");
});

barrier.await(); // Wait after each thread is called

? Applicable scenarios: Multi-threaded parallel computing is synchronized in stages, such as parallel test data preparation.

③ Semaphore: semaphore, control the number of concurrent

Limits the number of threads that access a resource at the same time (such as database connection pool).

 Semaphore semaphore = new Semaphore(3); // Up to 3 threads access semaphore.acquire(); // Obtain permission try {
    // Perform restricted operations} finally {
    semaphore.release(); // Release the license}

? Applicable scenarios: current limiting, resource pool control.

④ Phaser: More flexible synchronization barrier (advanced)

It supports dynamic registration of participating threads, suitable for scenarios where phased tasks and the number of threads is not fixed.

Suggestions and precautions for use

• Priority to use synchronized : If the scene is simple, synchronized is safer and simpler.
• Lock must be equipped with try-finally : prevent exceptions from causing the lock to be unreleased.
• Avoid nested locks : It is easy to cause deadlocks. If you must use them, pay attention to the locking order.
• Condition must be used in conjunction with while judgment : prevent false awakening.
• Synchronizers are not all-purpose : choose the right tools and avoid over-designing.

Basically that's it. Java's concurrency tools provide a wide range of options from the underlying Lock to the high-level Synchronizers . The key is to understand the applicable scenarios of each mechanism and avoid "willing to use but wrongly".

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