Java多线程synchronized关键字

synchronized关键字代表着同步的意思，在Java中被synchronized修饰的有三种情况

1.同步代码块

//锁为obj
synchronized(obj){
	while(true){
		if(product > 0){
			System.out.println(Thread.currentThread().getName()+"消费："+product--);
		}
	}
}

 

2.同步函数

//锁为this
public synchronized void  consume() {
		while(true){
			if(product > 0){
				System.out.println(Thread.currentThread().getName()+"消费："+product--);
			}
		}
}

　　

3.静态同步函数

//锁为this.getClass()
static public synchronized void  consume() {
		while(true){
			if(product > 0){
				System.out.println(Thread.currentThread().getName()+"消费："+product--);
			}
		}
}

死锁案例

1.同步嵌套

package threaddemo;

public class DeathLock {
	public static void main(String[] args) {
		Callable c1 = new Callable(true);
		Callable c2 = new Callable(false);
		
		Thread t1 = new Thread(c1);
		Thread t2 = new Thread(c2);
		
		t1.start();
		/*try {
			Thread.sleep(10);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}*/
		t2.start();
	}
}

class MyLock {
	static final public Object locka = new Object();
	static final public Object lockb = new Object();
}

class Callable implements Runnable {
	private boolean flag;

	public Callable(boolean flag) {
		this.flag = flag;
	}

	@Override
	public void run() {
		String name  = Thread.currentThread().getName();
		if (flag) {
			synchronized (MyLock.locka) {
				System.out.println(name+":获取到了locka");
				synchronized (MyLock.lockb) {
					System.out.println(name+":获取到了lockb");
				}
				System.out.println(name+":释放了lockb");
			}
			System.out.println(name+":释放了locka");
			
		} else {
			synchronized (MyLock.lockb) {
				System.out.println(name+":获取到了lockb");
				synchronized (MyLock.locka) {
					System.out.println(name+":获取到了locka");
				}
				System.out.println(name+":释放了locka");
			}
			System.out.println(name+":释放了lockb");
		}
	}
}

2.多生产者多消费者时只使用notify()，线程通讯死锁

　　解决办法采用notifyAll代替notify,唤醒所有线程就不会导致死锁

package threaddemo;

/**
 * 多生产者消费者线程通讯死锁
 * @author wpy
 *
 */
public class ProducerConsumerDeathLock {
	public static void main(String[] args) {
		Resource resource = new Resource();

		Producer producer0 = new Producer(resource);
		Producer producer1 = new Producer(resource);

		Consumer consumer2 = new Consumer(resource);
		Consumer consumer3 = new Consumer(resource);

		Thread t0 = new Thread(producer0);
		Thread t1 = new Thread(producer1);
		Thread t2 = new Thread(consumer2);
		Thread t3 = new Thread(consumer3);

		t0.start();
		t1.start();

		t2.start();
		t3.start();

	}

}

class Producer implements Runnable {
	private Resource resource;

	public Producer(Resource resource) {
		this.resource = resource;
	}

	@Override
	public void run() {
		while (true) {
			resource.set("资源");
		}
	}
}

class Consumer implements Runnable {
	private Resource resource;

	public Consumer(Resource resource) {
		this.resource = resource;
	}

	@Override
	public void run() {
		while (true) {
			resource.out();
		}
	}
}

class Resource {
	private String name;
	private int count = 1;
	// 是否生成完毕
	private boolean flag = false;

	public synchronized void set(String name) {
		String threadName = Thread.currentThread().getName();
		while (flag) {
			try {
				System.out.println(threadName+"进入等待状态");
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println(threadName+"取得执行权");

		this.name = name + count;
		count++;
		System.out.println("生产者：" + this.name);
		flag = true;
		this.notify();
//		this.notifyAll();
	}

	public synchronized void out() {
		String threadName = Thread.currentThread().getName();
		while (!flag) {
			try {
				System.out.println(threadName+"进入等待状态");
				this.wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
		System.out.println(threadName+"取得执行权");
		
		System.out.println("============消费者：" + name);
		flag = false;
		this.notify();
//		this.notifyAll();
	}
}

　　代码执行结果：

Thread-0取得执行权
生产者：资源1
Thread-0进入等待状态
Thread-1进入等待状态
Thread-3取得执行权
============消费者：资源1
Thread-3进入等待状态
Thread-2进入等待状态
Thread-0取得执行权
生产者：资源2
Thread-0进入等待状态
Thread-1进入等待状态