面试常用代码

死锁

public class DeadLockDemo {
    private static Object resource1 = new Object();//资源 1
    private static Object resource2 = new Object();//资源 2

    public static void main(String[] args) {
        new Thread(() -> {
            synchronized (resource1) {
                System.out.println(Thread.currentThread() + "get resource1");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread() + "waiting get resource2");
                synchronized (resource2) {
                    System.out.println(Thread.currentThread() + "get resource2");
                }
            }
        }, "线程 1").start();

        new Thread(() -> {
            synchronized (resource2) {
                System.out.println(Thread.currentThread() + "get resource2");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread() + "waiting get resource1");
                synchronized (resource1) {
                    System.out.println(Thread.currentThread() + "get resource1");
                }
            }
        }, "线程 2").start();
    }
}

双重校验锁单例

public class Singleton {

    private volatile static Singleton uniqueInstance;

    private Singleton() {
    }

    public  static Singleton getUniqueInstance() {
       //先判断对象是否已经实例过，没有实例化过才进入加锁代码
        if (uniqueInstance == null) {
            //类对象加锁
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}