初步认识-单例模式

初步认识-单例模式

两种方式

懒汉式

// NOTE: This is not thread safe!

public class Singleton {
	private static Singleton uniqueInstance;
 
	private Singleton() {}
 
	public static Singleton getInstance() {
		if (uniqueInstance == null) {
			uniqueInstance = new Singleton();
		}
		return uniqueInstance;
	}
 
	// other useful methods here
	public String getDescription() {
		return "I'm a classic Singleton!";
	}
}

饿汉式

public class Singleton {
    private static Singleton uniqueInstance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return uniqueInstance;
    }

    public String getDescription() {
        return "I'm a statically initialized Singleton!";
    }
}

线程不安全的原因：

多线程解决

加锁

public class Singleton {
	private static Singleton uniqueInstance;
 
	// other useful instance variables here
 
	private Singleton() {}
 
	public static synchronized Singleton getInstance() {
		if (uniqueInstance == null) {
			uniqueInstance = new Singleton();
		}
		return uniqueInstance;
	}
 
	// other useful methods here
	public String getDescription() {
		return "I'm a thread safe Singleton!";
	}
}

缺点：执行效率下降

DCL （Double Check Lock）双重检查加锁

public class Singleton {
    /**
     * 多线程可见性
     */
    private volatile static Singleton uniqueInstance;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (uniqueInstance == null) {
            // 只有第一次才彻底执行这块代码
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}

只有第一次才会加锁同步，提高了效率，并保证了线程安全。