【学习笔记】Java多线程 - Lock & Condition & Synchronized

Core Java里面说:

A lock object can have one or more associated condition objects. 下面，我们看一个具体的例子，在main thread创建三个子线程，每一个线程各自负责一个操作，但第一个操作完成后，第二个操作才能开始，第二个操作完成后，第三个操作才能开始。
Leetcode上面就有一道类似的题目。
这个例子，可以用两种方式来实现，第一种是使用Lock和Condition，第二种是使用synchronized。两种方式，代码感觉没有多大区别。

Lock & Condition

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

class ShareResource {
    private int num = 1;
    private Lock lock = new ReentrantLock();
    private Condition conditionA = lock.newCondition();
    private Condition conditionB = lock.newCondition();
    private Condition conditionC = lock.newCondition();
 
    public void print5() {
        lock.lock();
        try {
            while (num != 1) {
                conditionA.await();
            }
            for (int i = 0; i < 5; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + (i+1));
            }
            num = 2;
            conditionB.signal();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
 
    public void print10() {
        lock.lock();
        try {
            while (num != 2) {
                conditionB.await();
            }
            for (int i = 0; i < 10; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + (i+1));
            }
            num = 3;
            conditionC.signal();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
 
    public void print15() {
        lock.lock();
        try {
            while (num != 3) {
                conditionC.await();
            }

            for (int i = 0; i < 15; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + (i+1));
            }

            num = 1;
            conditionA.signal();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}


public class LockCondition {
    public static void main(String[] args) {
        ShareResource shareResource = new ShareResource();
        
        new Thread(() -> {
        	shareResource.print5();
        }, "A Thread").start();
 
        new Thread(() -> {
        	shareResource.print10();
        }, "B Thread").start();
 
        new Thread(() -> {
        	shareResource.print15();
        }, "C Thread").start();
    }
}

synchronized

class ShareResource {
    
    private int num = 1;
     
    // 也可以把synchronized拿出来，放在void前面
    public void print5() {
        synchronized(this) {
            try {
                while (num != 1) wait(); 
                for (int i = 0; i < 5; i++) {
                    System.out.println(Thread.currentThread().getName() + "\t" + (i+1));
                }
                num = 2; 
                notifyAll();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
 
    public void print10() {
        synchronized(this) {
            try {
                while (num != 2) wait(); 
                for (int i = 0; i < 10; i++) {
                    System.out.println(Thread.currentThread().getName() + "\t" + (i+1));
                }
                num = 3; 
                notifyAll(); 
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
 
    public void print15() {
        synchronized(this) {
            try {
                while (num != 3) wait(); 
                for (int i = 0; i < 15; i++) {
                    System.out.println(Thread.currentThread().getName() + "\t" + (i+1));
                }
                num = 1;
                notifyAll(); 
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class LockCondition {
    public static void main(String[] args) {
        ShareResource shareResource = new ShareResource();
        
        new Thread(() -> {
        	shareResource.print5();
        	}, "A Thread").start();
 
        new Thread(() -> {
        	shareResource.print10();
        }, "B Thread").start();
 
        new Thread(() -> {
        	shareResource.print15();
        }, "C Thread").start();
    }
}