单例模式 懒汉式

线程不安全

class Singleton {
    private Singleton() {
    }

    private static Singleton instance;

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

线程安全

class Singleton {
    private static Singleton instance;

    private Singleton(){}

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