单例模式

/**
 * 最优实现：饿汉式-枚举（在Spring环境下）
 */
public enum MySingletonEnum{
    INSTANCE;
    
    private MyService myService;
    
    private MySingletonEnum(){
        myService = ApplicationContextUtil.getContext().getBean(MyService.class);
    }
    
    public String answer(){
        return myService.answer();
    }
    
    //调用：MySingletonEnum.INSTANCE.answer();
}

/**
* 推荐实现：懒汉式-静态内部类
 * 静态内部类的作用是：延迟加载&线程安全
 * 因为Singleton被加载时，静态内部类SingletonHolder不会被加载
 * 而是等到getUniqueInstance()被调用时，才会被加载。
**/
public class Singleton {

    private Singleton() {
    }

    private static class SingletonHolder {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getUniqueInstance() {
        return SingletonHolder.INSTANCE;
    }
}

/**
* 饿汉式-线程安全
**/
public class Singleton {

    private static Singleton uniqueInstance = new Singleton();

    private Singleton() {
    }

    public static Singleton getUniqueInstance() {
        return uniqueInstance;
    }
}


/**
* 懒汉式-线程安全-双重校验锁
**/
public class Singleton {

    private volatile static Singleton uniqueInstance;

    private Singleton() {
    }

    public static Singleton getUniqueInstance() {
        if (uniqueInstance == null) {
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}


/**
* 懒汉式-线程不安全
**/
public class Singleton {

    private static Singleton uniqueInstance;

    private Singleton() {
    }

    public static Singleton getUniqueInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }
}