ReentrantLock在Java中提供比synchronized更靈活的線程控制。 1.它支持非阻塞獲取鎖（tryLock()）、帶超時的鎖獲取（tryLock(long timeout, TimeUnit unit)）和可中斷等待鎖；2.允許設(shè)置公平鎖，避免線程飢餓；3.支持多個條件變量，實現(xiàn)更精細的等待/通知機制；4.需手動釋放鎖，必須在finally塊中調(diào)用unlock()以避免資源洩漏；5.適用於需要高級同步控制的場景，如自定義同步工具或複雜並發(fā)結(jié)構(gòu)，但對簡單互斥需求仍推薦使用synchronized。

A ReentrantLock in Java is a synchronization tool that gives you more control over how threads access shared resources compared to the standard synchronized keyword. It's part of the java.util.concurrent.locks package and supports advanced features like trying for a lock without blocking, attempting with a timeout, or even interrupting a thread that's waiting for a lock.

What Does "Reentrant" Mean?

The term reentrant refers to the fact that if a thread already holds a lock, it can re-enter (acquire it again) without causing a deadlock. This behavior mirrors how synchronized blocks work in Java but gives you explicit control when using ReentrantLock .

For example:

• If one method locks the object and calls another method on the same object that also tries to acquire the same lock, it doesn't block because the thread already owns the lock.

This feature avoids certain types of deadlocks and makes recursive locking possible.

How Is It Different from synchronized ?

While both synchronized and ReentrantLock provide mutual exclusion, there are key differences:

• Flexibility : With synchronized , you don't have much control — once you enter a synchronized block, the thread waits until the lock becomes available. ReentrantLock lets you try to acquire the lock without blocking ( tryLock() ), wait only up to a certain time ( tryLock(long timeout, TimeUnit unit) ), or interrupt a waiting thread.

• Fairness Option : You can create a ReentrantLock with a fairness policy. A fair lock tends to offer the lock to the longest-waiting thread, which can help avoid starvation, though it might come at a performance cost.

• Condition Variables : ReentrantLock allows multiple condition objects via its newCondition() method, giving finer control over signaling and waiting logic than the single condition associated with Object.wait() and notify() used with synchronized .

When Should You Use It?

You might want to use ReentrantLock instead of synchronized in these cases:

• You need a non-blocking attempt to acquire a lock.
• You want to support timeouts or interrupts during lock acquisition.
• Your application benefits from a fair locking mechanism.
• You're building complex synchronization constructs (like custom semaphores or barriers).

However, if your needs are simple — just mutual exclusion without special features — stick with synchronized . It's less error-prone because the JVM automatically releases the lock when exiting a block, whereas with ReentrantLock , you must manually call unlock() inside a finally block to avoid leaks.

Here's a quick usage example:

 ReentrantLock lock = new ReentrantLock();

lock.lock();
try {
    // critical section
} finally {
    lock.unlock();
}

This pattern ensures the lock gets released even if an exception occurs inside the critical section.

One Gotcha: Always Unlock in a Finally Block

Unlike synchronized , where releasing the lock is automatic, forgetting to call unlock() can lead to deadlocks or resource leaks. That's why it's crucial to always wrap the critical code inside a try block and call unlock() in a finally block.

Also, be aware that unlike synchronized , ReentrantLock does not implicitly associate with the object it protects — so it's easier to misuse if not handled carefully.

Using ReentrantLock adds flexibility but also responsibility. It's a powerful tool when you need more nuanced control over concurrency, but it's easy to shoot yourself in the foot if you're not careful with lock management. For most straightforward cases, synchronized still works fine.

基本上就這些。

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