单例模式的两种实现方式

public class SingletonClass {

    private volatile static SingletonClass instance = null;

    public static SingletonClass getInstance() {
        if (instance == null) {
            synchronized (SingletonClass.class) {
                if (instance == null) {
                    instance = new SingletonClass();
                }
            }
        }
        return instance;
    }

    private SingletonClass() {
    }

}

public class SingletonClass {

    private static class SingletonClassInstance {
        private static final SingletonClass instance = new SingletonClass();
    }

    public static SingletonClass getInstance() {
        return SingletonClassInstance.instance;
    }

    private SingletonClass() {
    }

}