ReentrantLock กับ synchronized

คีย์เวิร์ดการล็อกอัตโนมัติ

class Counter {
    private int count = 0;
    
    synchronized void increment() {
        count++; // only one thread at a time
    }
    
    synchronized int get() { return count; }
}

Counter c = new Counter();
// Multiple threads can safely call increment()

พื้นฐานของ ReentrantLock

import java.util.concurrent.locks.*;

class SafeCounter {
    private int count = 0;
    private final ReentrantLock lock = new ReentrantLock();
    
    void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock(); // always release in finally!
        }
    }
    
    int get() {
        lock.lock();
        try { return count; }
        finally { lock.unlock(); }
    }
}

tryLock: การได้มาซึ่งล็อกโดยไม่บล็อก

ReentrantLock lock = new ReentrantLock();

if (lock.tryLock()) {
    try {
        System.out.println("Got the lock, doing work");
    } finally {
        lock.unlock();
    }
} else {
    System.out.println("Lock busy, skipping or retrying");
}

tryLock พร้อมการหมดเวลา

try {
    if (lock.tryLock(500, TimeUnit.MILLISECONDS)) {
        try {
            // work with protected resource
            System.out.println("Acquired within 500ms");
        } finally {
            lock.unlock();
        }
    } else {
        System.out.println("Timed out waiting for lock");
    }
} catch (InterruptedException e) {
    Thread.currentThread().interrupt();
}

lockInterruptibly

try {
    lock.lockInterruptibly(); // throws InterruptedException if interrupted
    try {
        // do work
    } finally {
        lock.unlock();
    }
} catch (InterruptedException e) {
    System.out.println("Interrupted while waiting for lock");
    Thread.currentThread().interrupt();
}

การเข้าล็อกซ้ำ

synchronized void outer() {
    System.out.println("outer");
    inner(); // same thread re-enters — OK
}

synchronized void inner() {
    System.out.println("inner"); // same lock, same thread
}

// ReentrantLock:
ReentrantLock lock = new ReentrantLock();
lock.lock();
lock.lock(); // acquire again — hold count = 2
lock.unlock(); // hold count = 1
lock.unlock(); // hold count = 0, lock released

นโยบายความเป็นธรรม

// Unfair (default): no ordering guarantee, better throughput
ReentrantLock unfair = new ReentrantLock();

// Fair: threads acquire in arrival order
ReentrantLock fair = new ReentrantLock(true);

System.out.println(fair.isFair()); // true

ตัวแปรเงื่อนไข

ReentrantLock lock2 = new ReentrantLock();
Condition notEmpty = lock2.newCondition();
Queue<String> queue = new LinkedList<>();

// Producer
lock2.lock();
try {
    queue.offer("item");
    notEmpty.signal(); // wake one waiting thread
} finally { lock2.unlock(); }

// Consumer
lock2.lock();
try {
    while (queue.isEmpty()) notEmpty.await(); // wait & release lock
    System.out.println(queue.poll());
} finally { lock2.unlock(); }

ควรใช้ ReentrantLock หรือการล็อกอัตโนมัติเมื่อใด

จำนวนครั้งที่ล็อกและการตรวจติดตาม

ReentrantLock lock3 = new ReentrantLock();
lock3.lock();
lock3.lock(); // reentrant — hold count 2

System.out.println(lock3.getHoldCount());    // 2
System.out.println(lock3.isHeldByCurrentThread()); // true
System.out.println(lock3.isLocked());        // true
System.out.println(lock3.getQueueLength());  // 0 (no waiting threads)

lock3.unlock();
lock3.unlock(); // fully released

ข้อผิดพลาดที่พบบ่อย: ปลดล็อกโดยไม่ได้ล็อก

ReentrantLock lock4 = new ReentrantLock();
try {
    // If lock() is never called (e.g., skipped by exception before this line)
    lock4.unlock(); // throws IllegalMonitorStateException
} catch (IllegalMonitorStateException e) {
    System.out.println("Must lock before unlock!");
}

ตรวจสอบอย่างรวดเร็ว

สรุป: ReentrantLock เทียบกับการล็อกอัตโนมัติ