设计模式之单例模式的线程安全

1.线程不安全的单例模式

1.1 创建单例模式类 Singleton

没有加锁，线程不安全，多个线程同时访问，会执行多次创建类对象。

java 复制代码

public class Singleton {
	private static Singleton instance;
	
	private int mCountNumber;
	
	private Singleton() {
		System.out.println(Thread.currentThread().getName() + " Singleton is Instantiated");
	}
	
	public static Singleton getInstance() {
		if (instance == null) {
			instance = new Singleton();
		}
		return instance;
	}
	
	public void printCountNumber() {
		System.out.println(Thread.currentThread().getName() + " Singleton current count number is " + mCountNumber++);
	}

}

1.2 多线程访问不安全

getInstance 多次执行了 new Singleton的处理，创建类多个实例，导致 Singleton 的成员变量 mCountNumber 的值也不唯一。

java 复制代码

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("---------------------- main begin ----------------------");
		
		
		for (int i = 0; i < 10; i++) {
			Thread thread = new Thread(() -> {
			    System.out.println(Thread.currentThread().getName() + " 线程运行开始");

			    Singleton.getInstance().printCountNumber();
			    
			    System.out.println(Thread.currentThread().getName() + " 线程运行结束");
			});
			
			thread.start();
		}
		
		System.out.println("---------------------- main end ----------------------");
	}

log输出：

java 复制代码

---------------------- main begin ----------------------
---------------------- main end ----------------------
Thread-4 线程运行开始
Thread-9 线程运行开始
Thread-7 线程运行开始
Thread-8 线程运行开始
Thread-0 线程运行开始
Thread-5 线程运行开始
Thread-3 线程运行开始
Thread-1 线程运行开始
Thread-2 线程运行开始
Thread-6 线程运行开始
Thread-6 Singleton is Instantiated
Thread-5 Singleton is Instantiated
Thread-3 Singleton is Instantiated
Thread-7 Singleton is Instantiated
Thread-9 Singleton is Instantiated
Thread-2 Singleton is Instantiated
Thread-4 Singleton is Instantiated
Thread-0 Singleton is Instantiated
Thread-8 Singleton is Instantiated
Thread-1 Singleton is Instantiated
Thread-4 Singleton current count number is 0
Thread-1 Singleton current count number is 0
Thread-5 Singleton current count number is 0
Thread-5 线程运行结束
Thread-8 Singleton current count number is 0
Thread-6 Singleton current count number is 0
Thread-6 线程运行结束
Thread-9 Singleton current count number is 0
Thread-2 Singleton current count number is 0
Thread-2 线程运行结束
Thread-3 Singleton current count number is 0
Thread-3 线程运行结束
Thread-0 Singleton current count number is 0
Thread-4 线程运行结束
Thread-1 线程运行结束
Thread-7 Singleton current count number is 0
Thread-7 线程运行结束
Thread-8 线程运行结束
Thread-9 线程运行结束
Thread-0 线程运行结束

2.无锁的线程安全单例模式

2.1 创建单例模式类 LockFreeSingleton

通过在声明时直接实例化静态成员的方式，来保证一个类只有一个实例。这种实现方式避免了使用同步锁机制和额外检查判断实例是否被创建。

java 复制代码

public class LockFreeSingleton {
	private static final LockFreeSingleton instance = new LockFreeSingleton();
	
	private int mCountNumber;
	
	private LockFreeSingleton() {
		System.out.println(Thread.currentThread().getName() + " LockFreeSingleton is Instantiated");
	}
	
	public static synchronized LockFreeSingleton getInstance() {
		return instance;
	}
	
	public void printCountNumber() {
		System.out.println(Thread.currentThread().getName() + " LockFreeSingleton current count number is " + mCountNumber++);
	}

}

2.2 多线程访问安全

getInstance 只创建了一个对象，成员变量 mCountNumber 的值也是递增的，每次都不一样，保证类唯一性。

java 复制代码

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("---------------------- main begin ----------------------");
		
		
		for (int i = 0; i < 10; i++) {
			Thread thread = new Thread(() -> {
			    System.out.println(Thread.currentThread().getName() + " 线程运行开始");

			    //Singleton.getInstance().printCountNumber();
			    LockFreeSingleton.getInstance().printCountNumber();
			    
			    System.out.println(Thread.currentThread().getName() + " 线程运行结束");
			});
			
			thread.start();
		}
		
		System.out.println("---------------------- main end ----------------------");
	}

log输出：

java 复制代码

---------------------- main begin ----------------------
---------------------- main end ----------------------
Thread-4 线程运行开始
Thread-7 线程运行开始
Thread-9 线程运行开始
Thread-3 线程运行开始
Thread-0 线程运行开始
Thread-5 线程运行开始
Thread-6 线程运行开始
Thread-2 线程运行开始
Thread-8 线程运行开始
Thread-4 LockFreeSingleton is Instantiated
Thread-1 线程运行开始
Thread-8 LockFreeSingleton current count number is 2
Thread-9 LockFreeSingleton current count number is 1
Thread-6 LockFreeSingleton current count number is 4
Thread-9 线程运行结束
Thread-4 LockFreeSingleton current count number is 0
Thread-2 LockFreeSingleton current count number is 3
Thread-2 线程运行结束
Thread-0 LockFreeSingleton current count number is 6
Thread-7 LockFreeSingleton current count number is 8
Thread-5 LockFreeSingleton current count number is 5
Thread-3 LockFreeSingleton current count number is 7
Thread-3 线程运行结束
Thread-1 LockFreeSingleton current count number is 9
Thread-8 线程运行结束
Thread-6 线程运行结束
Thread-4 线程运行结束
Thread-0 线程运行结束
Thread-7 线程运行结束
Thread-5 线程运行结束
Thread-1 线程运行结束

3.双重校验锁机制的同步锁单例模式

3.1 创建双重校验同步锁单例模式类 DoubleCheckSingleton

在syncchronized 代码块中也需要进行一次检查。

java 复制代码

/**
 * 单例模式
 *
 * 双检锁/双重校验锁（DCL，即 double-checked locking）
 * JDK 版本：JDK1.5 起
 * 是否 Lazy 初始化：是
 * 是否多线程安全：是
 * 实现难度：较复杂
 * 描述：这种方式采用双锁机制，安全且在多线程情况下能保持高性能。
 * getInstance() 的性能对应用程序很关键。
 */
public class DoubleCheckSingleton {
	private volatile static DoubleCheckSingleton instance;
	
	private int mCountNumber;
	
	private DoubleCheckSingleton() {
		System.out.println(Thread.currentThread().getName() + " DoubleCheckSingleton is Instantiated");
	}
	
	public static DoubleCheckSingleton getInstance() {
		if (instance == null) {
			synchronized (DoubleCheckSingleton.class) {
				if (instance == null) {
					instance = new DoubleCheckSingleton();
				}
			}
		}
		return instance;
	}
	
	public void printCountNumber() {
		System.out.println(Thread.currentThread().getName() + " DoubleCheckSingleton current count number is " + mCountNumber++);
	}

}

3.2 多线程访问安全

java 复制代码

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("---------------------- main begin ----------------------");
		
		
		for (int i = 0; i < 10; i++) {
			Thread thread = new Thread(() -> {
			    System.out.println(Thread.currentThread().getName() + " 线程运行开始");

			    DoubleCheckSingleton.getInstance().printCountNumber();
			    
			    System.out.println(Thread.currentThread().getName() + " 线程运行结束");
			});
			
			thread.start();
		}
		
		System.out.println("---------------------- main end ----------------------");
	}

log输出：

java 复制代码

---------------------- main begin ----------------------
Thread-0 线程运行开始
Thread-4 线程运行开始
Thread-1 线程运行开始
Thread-2 线程运行开始
Thread-3 线程运行开始
Thread-0 DoubleCheckSingleton is Instantiated
Thread-5 线程运行开始
Thread-6 线程运行开始
Thread-7 线程运行开始
Thread-8 线程运行开始
---------------------- main end ----------------------
Thread-9 线程运行开始
Thread-4 DoubleCheckSingleton current count number is 1
Thread-9 DoubleCheckSingleton current count number is 9
Thread-4 线程运行结束
Thread-0 DoubleCheckSingleton current count number is 0
Thread-3 DoubleCheckSingleton current count number is 4
Thread-6 DoubleCheckSingleton current count number is 6
Thread-2 DoubleCheckSingleton current count number is 3
Thread-1 DoubleCheckSingleton current count number is 2
Thread-8 DoubleCheckSingleton current count number is 8
Thread-8 线程运行结束
Thread-5 DoubleCheckSingleton current count number is 5
Thread-5 线程运行结束
Thread-7 DoubleCheckSingleton current count number is 7
Thread-9 线程运行结束
Thread-0 线程运行结束
Thread-3 线程运行结束
Thread-6 线程运行结束
Thread-2 线程运行结束
Thread-1 线程运行结束
Thread-7 线程运行结束

参考：

https://www.runoob.com/design-pattern/singleton-pattern.html