java中加锁的方式有哪些，怎么个写法？

class LockExamples {
    private final Object monitor = new Object();
    private final java.util.concurrent.locks.ReentrantLock lock =
            new java.util.concurrent.locks.ReentrantLock();
    private final java.util.concurrent.locks.ReadWriteLock rw =
            new java.util.concurrent.locks.ReentrantReadWriteLock();

    public synchronized void syncMethod() {
        // lock: this
    }

    public static synchronized void staticSyncMethod() {
        // lock: LockExamples.class
    }

    public void syncBlock() {
        synchronized (monitor) {
            // lock: private final monitor
        }
    }

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

    public void readWriteStyle() {
        rw.readLock().lock();
        try {
            // read shared state
        } finally {
            rw.readLock().unlock();
        }

        rw.writeLock().lock();
        try {
            // mutate shared state
        } finally {
            rw.writeLock().unlock();
        }
    }
}

• synchronized 方法和代码块由 JVM 自动释放锁；显式锁必须把 unlock 放进 finally。
• 示例使用 private final 锁对象，避免把 this 或公共对象暴露给外部代码参与竞争。