面试题目一：请写出多种单例模式，并说出他们的区别

答案：具体代码如下

//单例模式-饿汉式
public class Singleton {

    public static Singleton singleton = new Singleton();
    
    private Singleton() {
    }
    
    public static Singleton getInstance() {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return singleton;
    }
    
}

//单例模式-懒汉式
public class Singleton2 {

    public static Singleton2 singleton2;
    
    private Singleton2() {
    }
    
    public static Singleton2 getInstance() {
        if (singleton2 == null) {
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            singleton2 = new Singleton2();
        }
        return singleton2;
    }
}

//饿汉式单例模式测试代码
public class SingletonTest {

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new MyThread().start();
        }
    }
}

class MyThread extends Thread {
    @Override
    public void run() {
        Singleton instance = Singleton.getInstance();
        System.out.println(instance);
    }
}

//懒汉式单例模式测试代码
public class SingletonTest2 {

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new MyThread2().start();
        }
    }
}

class MyThread2 extends Thread {
    @Override
    public void run() {
        Singleton2 instance = Singleton2.getInstance();
        System.out.println(instance);
    }
}

总结：上述两种单例模式在单线程的情况下都符合单例的要求，但是懒汉式单例模式在多线程的情况下会出现多例的情况，不符合单例模式的要求，而饿汉式单例模式在多线程的情况下仍只会产生一个实例，符合单例模式的要求。故如果在多线程环境下想要使用单例模式那么应用饿汉式单例模式