Java基础面试题--单利模式及在多线程情况下的运用

/**
 * 懒汉单利模式 线程不安全
 * @author 15735400536
 */
public class Singleton {
	
	/**
	 * 实例对象
	 */
	private static Singleton singleton;
	
	/**
	 * 私有化构造函数，使外部不能创建对象
	 */
	private Singleton() {
		
	}
	
	/**
	 * 获取实例方法
	 * @return
	 */
	public static Singleton getInstance() {
		if(null == singleton) {
			singleton = new Singleton();
		}
		return singleton;
	}
	
}

　　解决方案：

/**
  *     懒汉单例   线程安全
 * @author 15735400536
 *
 */
public class IdlerSingleton {

	/**
	 * 实例对象
	 */
	private static IdlerSingleton idlerSingleton;
	
	/**
	 * 私有化构造函数
	 */
	private IdlerSingleton() {
		
	}
	
	/**
	 * 加了synchronized关键字
	 * @return
	 */
	public static synchronized IdlerSingleton getInstance() {
		if(null == idlerSingleton) {
			idlerSingleton = new IdlerSingleton();
		}
		return idlerSingleton;
	}
	
	//或者添加同步代码块：
//	public static IdlerSingleton getNewInstance() {
//		if(null == idlerSingleton) {
//			synchronized (IdlerSingleton.class) {
//				idlerSingleton = new IdlerSingleton();
//			}
//		}
//		return idlerSingleton;
//	}
}

　　

/**
 * 双重检查单利模式  解决懒汉单利在多线程情况下发生不安全问题解决方案
 * @author 15735400536
 *
 */
public class DoubleCheckSingleton {
    
    //volatile 禁止重排序
    private static volatile DoubleCheckSingleton doubleCheckSingleton;
    
    private DoubleCheckSingleton() {}
    
    public static DoubleCheckSingleton getInstance() {
        if(null == doubleCheckSingleton) {
            synchronized (DoubleCheckSingleton.class) {
                if(null == doubleCheckSingleton) {
                    doubleCheckSingleton = new DoubleCheckSingleton();
                }
            }
        }
        return doubleCheckSingleton;
    }
    
}

/**
 * 饿汉单例 线程安全
 * @author 15735400536
 *
 */
public class HungrySingleton {

	/**
	 * 静态代码块
	 */
	static {
		hungrySingleton = new HungrySingleton();
	}
	
	private static HungrySingleton hungrySingleton;
	
	private HungrySingleton() {}
	
	public static HungrySingleton getInstance() {
		return hungrySingleton;
	}
}

　　

/**
 * 饿汉单利  线程安全
 * @author 15735400536
 *
 */
public class NewHungrySingleton {
	
	/**
	 * 静态常量实例对象
	 */
	private final static NewHungrySingleton newHungrySingleton = new NewHungrySingleton();
	
	private NewHungrySingleton() {}
	
	/**
	 * 饿汉 NewHungrySingleton类加载时，实例对象就已经创建好了
	 * @return
	 */
	public static NewHungrySingleton getInstance() {
		return newHungrySingleton;
	}
	
}

　　

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