1. 单线程可用

public class Singleton {

    private static Singleton singleton;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (null == singleton) {
            singleton = new Singleton();
        }
        return singleton;
    }
}

2. 使用final常量

public class Singleton {

    private static final Singleton singleton = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return singleton;
    }
}

3. 加锁构造

public class Singleton {

    private static Singleton singleton;

    private static final Lock LOCK = new ReentrantLock();

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (null == singleton) {
            LOCK.lock();
            if (null == singleton) {
                singleton = new Singleton();
            }
            LOCK.unlock();
        }
        return singleton;
    }
}

4. 方法级加锁

public class Singleton {

    private static Singleton singleton;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (null == singleton) {
            init();
        }
        return singleton;
    }

    private static synchronized void init(){
        if(null == singleton){
            singleton = new Singleton();
        }
    }
}

5. 静态内部类实现，多线程可用

public class Singleton {

    private Singleton() {
    }

    public static Singleton getInstance() {
        return Container.singleton;
    }

    private static class Container {
        private static Singleton singleton = new Singleton();
    }
}

6. 拒绝强行反射创建我

public class Singleton {

    private Singleton() {
        if (null != Container.singleton) {
            throw new UnsupportedOperationException(); // 报错啦
        }
    }

    public static Singleton getInstance() {
        return Container.singleton;
    }

    private static class Container {
        private static Singleton singleton = new Singleton();
    }
}