阿里面试官：你连个java多线程都说不清楚，我招你进来干什么

创建线程的方法

• 继承Thread类

• 继承Thread类，重写run方法，通过线程类实例.start()方法开启线程。

public class TestThread1 extends Thread{
    @override
    public void run(){
        System.out.println("线程run方法！");
    }
    
    public static void main(String){
        new TestThread1().start();
    }
}

• 实现Runnable接口

• 实现Runnable接口，重写run方法，通过new Thread(线程类实例).start()开启线程

• 推荐使用该方式，避免java的单继承局限性

public class TestThread2 implements Runnable{
	@Override
	public void run() {
		System.out.println("线程run方法！");
	}
	
	public static void main(String[] args) {
		new Thread(new TestThread2()).start();
	}
}

• 实现Callable接口

• 实现Callable接口，重写call方法，call方法有返回值

• 启动方式：

*   创建执行服务

*   提交执行

*   获取结果

*   关闭服务

public class TestThread2 implements Callable{
	@Override
	public Boolean call() {
		System.out.println("线程call方法！");
		return true;
	}
	
	public static void main(String[] args) throws InterruptedException, ExecutionException {
		
		TestThread2 t1 = new TestThread2();
		TestThread2 t2 = new TestThread2();
		TestThread2 t3 = new TestThread2();
		//创建执行服务
		ExecutorService ser = Executors.newFixedThreadPool(3);
		//提交执行
		Future<Boolean> r1 = ser.submit(t1);
		Future<Boolean> r2 = ser.submit(t2);
		Future<Boolean> r3 = ser.submit(t3);
		//获取结果
		boolean rs1 = r1.get();
		boolean rs2 = r2.get();
		boolean rs3 = r3.get();
		//关闭服务
		ser.shutdownNow();
	}

线程同步

• 多个线程同时操作同一资源，线程不安全，变量值紊乱

• 加锁

• 队列+锁(synchronized)

• synchronized默认锁this，可以显示指定锁的对象来修改

1. synchronized修饰方法，线程安全方法

public class TestThreadSafe {

    public static void main(String[] args) {
        BuyTicket bt1 = new BuyTicket();

        Thread thread1 = new Thread(bt1,"张三");
        Thread thread2 = new Thread(bt1,"李四");
        Thread thread3 = new Thread(bt1,"黄牛");

        thread1.start();
        thread2.start();
        thread3.start();
    }

}

class BuyTicket implements Runnable{

    private int ticketNumber = 10;

    private boolean flag = true;

    @Override
    public void run() {
        while(flag) {
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public synchronized void buy() throws InterruptedException {
        //买票
        if(ticketNumber <= 0){
            System.out.println("票卖完了！");
            flag = false;
            return;
        }
        Thread.sleep(100);
        //Thread.yield();
        System.out.println(Thread.currentThread().getName() + "买到了一张票，还剩下"+(--ticketNumber) + "张票！");
    }
}

2. synchronized修饰代码块，线程安全代码块

public class TestThreadSafe {

    public static void main(String[] args) {
        BuyTicket bt1 = new BuyTicket();

        Thread thread1 = new Thread(bt1,"张三");
        Thread thread2 = new Thread(bt1,"李四");
        Thread thread3 = new Thread(bt1,"黄牛");

        thread1.start();
        thread2.start();
        thread3.start();
    }

}

class BuyTicket implements Runnable{

    private int ticketNumber = 10;

    private boolean flag = true;

    @Override
    public void run() {
        while(flag) {
            System.out.println(Thread.currentThread().getName() + "准备买票" + flag);
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void buy() throws InterruptedException {
        synchronized(this){
            //买票
            if(ticketNumber <= 0){
                flag = false;
                System.out.println("票卖完了！");
                return;
            }
            Thread.sleep(100);
            //Thread.yield();
            System.out.println(Thread.currentThread().getName() + "买到了一张票，还剩下"+(--ticketNumber) + "张票！");
        }
    }
}

3. 使用可重复锁ReentrantLock

import java.util.concurrent.locks.ReentrantLock;

public class TestLock {

    public static void main(String[] args) {
        BuyTicket bt1 = new BuyTicket();

        Thread thread1 = new Thread(bt1,"张三");
        Thread thread2 = new Thread(bt1,"李四");
        Thread thread3 = new Thread(bt1,"黄牛");

        thread1.start();
        thread2.start();
        thread3.start();
    }

}

class BuyTicket implements Runnable{

    private int ticketNumber = 1000;

    private boolean flag = true;

  //定义可重复锁
    private	final ReentrantLock lock = new ReentrantLock();
    
    @Override
    public void run() {
        while(flag) {
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void buy() throws InterruptedException {
        	lock.lock();
            //买票
            if(ticketNumber <= 0){
                System.out.println("票卖完了！");
                flag = false;
            }else {
            	Thread.sleep(100);
                //Thread.yield();
                System.out.println(Thread.currentThread().getName() + "买到了一张票，还剩下"+(--ticketNumber) + "张票！");
            }
            lock.unlock();
    }
}

• 死锁

• 两个以上的对象锁，每个线程互相占有对方需要的资源。形成死锁。

线程状态

• 新生状态（new）
• 就绪状态（start）
• 阻塞状态（sleep、wait、同步锁定）
• 运行状态
• 死亡状态 线程对象进入死亡状态后，将不能再次调用start()方法再次启动

线程（Thread类）方法

• setPriority(int newPriority) 更改线程优先级 newPriority从1到10

• static void sleep(long millis) 使当前正在执行的线程休眠指定毫秒，不会释放线程锁对象

• void join() 线程合并，等待该线程终止

• static void yield() 暂停当前正在执行的线程，执行其它线程

• void interrupt() 中断线程

• boolean isAlive() 该线程是否活跃

守护线程

• setDaemon(true) : 设置为守护线程

• 线程分为用户线程和守护线程

• JVM虚拟机确保用户线程执行完毕

• JVM虚拟机不用等待守护线程执行完毕

         eg：后台记录操作日志，监控内存，垃圾回收等待

最后

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