1、synchronized

单例模式下synchronized实现同步

public class Single {
    
    private Single() {}

    private static Single instance;

    public static Single getInstance() {

        if (instance == null) {
            synchronized (Single.class) {
                if (instance == null) {
                    instance = new Single();
                }
            }
        }
        return instance;
    }
}

2、lock

单例模式下lock实现同步

public class Single {

    private Single() {}

    private static Single instance;

    private static final ReentrantLock lock = new ReentrantLock();

    public static Single getInstance() {

        if (instance == null) {
            lock.lock();
            if (instance == null) {
                instance = new Single();
            }
            lock.unlock();

        }
        return instance;
    }

}

3、condition

指定条件来唤醒线程

public class Demo {

    private int signal;
    Lock lock = new ReentrantLock();
    Condition a = lock.newCondition();
    Condition b = lock.newCondition();
    Condition c = lock.newCondition();
    public void a() {
        lock.lock();
        while(signal != 0 ) {
            try {
                a.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("a");
        signal ++;
        b.signal();
        lock.unlock();
    }

    public  void b() {
        lock.lock();
        while(signal != 1) {
            try {
                b.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("b");
        signal ++;
        c.signal();
        lock.unlock();
    }

    public  void c () {
        lock.lock();
        while(signal != 2) {
            try {
                c.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("c");
        signal = 0;
        a.signal();
        lock.unlock();
    }

    public static void main(String[] args) {
        Demo d = new Demo();
        A a = new A(d);
        B b = new B(d);
        C c = new C(d);
        new Thread(a).start();
        new Thread(b).start();
        new Thread(c).start();
    }
}

class A implements Runnable {

    private Demo demo;
    public A(Demo demo) {
        this.demo = demo;
    }
    @Override
    public void run() {
        while(true) {
            demo.a();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

}
class B implements Runnable {

    private Demo demo;
    public B(Demo demo) {
        this.demo = demo;
    }

    @Override
    public void run() {
        while(true) {
            demo.b();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
class C implements Runnable {

    private Demo demo;
    public C(Demo demo) {
        this.demo = demo;
    }
    @Override
    public void run() {
        while(true) {
            demo.c();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}