延續昨天介紹 ReentrantLock 的部份，今天要介紹如何使用 ReentrantLock 所提供的 newCondition() 方法。newCondition 類似於 wait 和 notify 的呼叫，它是使用 await 和 signal 的方式來呼叫。

以下的 Sample code 使用 ReentrantLock 的方式來建立 newCondition，做到等待和通知的效果：

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

public class ReentrantLockThread1 implements Runnable {
    private Lock lock;
    private Condition condition;

    public ReentrantLockThread1(Lock lock, Condition condition) {
        this.lock = lock;
        this.condition = condition;
    }

    @Override
    public void run() {
       try {
           this.lock.lock();
           System.out.println("RUN THREAD 1-1");
           this.condition.await();
           System.out.println("RUN THREAD 1-2");
       } catch(Exception e) {
           throw new RuntimeException(e);
       } finally {
           this.lock.unlock();
       }

    }
}

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

public class ReentrantLockThread2 implements Runnable {
  private Lock lock;
  private Condition condition;
  public ReentrantLockThread2(Lock lock, Condition condition) {
    this.lock = lock;
    this.condition = condition;
  }
  @Override
  public void run() {
    try {
      this.lock.lock();
      System.out.println("RUN THREAD 2");
      this.condition.signal();
    } catch(Exception e) {
      throw new RuntimeException(e);
    } finally {
      this.lock.unlock();
    }
  }
}

以上寫了二支 Thread 的程式，主要目的在於啟動 ReentrantLockThread1 執行緒之後會先印出 "RUN THREAD 1-1 字串之後，就會進行等待的動作，等到 ReentrantLockThread2 執行緒執行完成之後再讓 ReentrantLockThread1 把還沒執行完的部份執行完。

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

public class Test {

  public static void main(String args[]) {
    Lock lock = new ReentrantLock();
    Condition condition = lock.newCondition();
    ReentrantLockThread1 t1 = new ReentrantLockThread1(lock, condition);
    Thread thread1 = new Thread(t1);
    thread1.start();

    ReentrantLockThread2 t2 = new ReentrantLockThread2(lock, condition);
    Thread thread2 = new Thread(t2);
    thread2.start();
  }
}

主程式主要會開啟二個執行緒， ReentrantLockThread1 和 ReentrantLockThread2。執行的結果如下：

RUN THREAD 1-1
RUN THREAD 2
RUN THREAD 1-2

從執行結果可以看出會先執行 ReentrantLockThread1 的 Thread 之後就會呼叫 await 方法等待的狀態，之後等到 ReentrantLockThread2 的 Thread 程式去呼叫 signal 方法，這樣 ReentrantLockThread1 的執行緒就會繼續執行把 RUN THREAD 1-2 的字串印出來。