ReentrantLock

1. ReentrantLock 是重入鎖的意思，實現了Lock介面
2. ReentrantLock 可以實現執行緒間的同步，用Condition實現執行緒間的通訊， synchronized 與 wait() 方法和 notify() 方式結合實現執行緒間通訊的時候，notify/notifyAll 的通知等待的執行緒時是隨機的，而condition可以實現選擇性通知

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import
 
 java.util.concurrent.locks.ReentrantLock;


public class LockConditionDemo {

    private Lock lock = new ReentrantLock();
    private Condition conditionA = lock.newCondition();
    private Condition conditionB = lock.newCondition();

    public static void main(String[] args) throws InterruptedException {

        LockConditionDemo demo = new
 
 LockConditionDemo();

        new Thread(() -> demo.await(demo.conditionA), "thread1_conditionA").start();
        new Thread(() -> demo.await(demo.conditionB), "thread2_conditionB").start();
        new Thread(() -> demo.signal(demo.conditionB), "thread3_conditionB").start();
        System
 
.out.println("稍等5秒再通知其他的執行緒！");
        Thread.sleep(5000);
        new Thread(() -> demo.signal(demo.conditionA), "thread4_conditionA").start();

    }

    private void await(Condition condition) {
        try {
            lock.lock();
            System.out.println("開始等待await！ ThreadName：" + Thread.currentThread().getName());
            condition.await();
            System.out.println("等待await結束！ ThreadName：" + Thread.currentThread().getName());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    private void signal(Condition condition) {
        lock.lock();
        System.out.println("傳送通知signal！ ThreadName：" + Thread.currentThread().getName());
        condition.signal();
        lock.unlock();
    }
}

如上的例子，先通知conditionB，再通知conditionA

3.公平鎖和非公平鎖：公平鎖：先進先出。非公平鎖：隨機獲得鎖。
ReentrantLock 提供了一個構造方法，可以很簡單的實現公平鎖或非公平鎖
4.ReentrantReadWriteLock
ReentrantReadWriteLock 有兩個鎖：一個是與讀相關的鎖，稱為“共享鎖”；另一個是與寫相關的鎖，稱為“排它鎖”
讀讀共享；
寫寫互斥；
讀寫互斥；
寫讀互斥