多线程Singleton模式笔记

单线程下安全使用的Singleton示例

 

public class Singleton {

	private static Singleton instance = null;

	private Singleton() {
		System.out.println("create instance.");
	}

	public static Singleton getInstance() {

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

	public static void main(String[] agrs) {
		new Singleton();
	}
}

 

单线程代码放到多线程下，将会出现线程不安全情况

 

 

class SingletonTestRunnable implements Runnable {
	public void run() {
		Singleton.getInstance();
	}
}

public class Singleton {

	private static Singleton instance = null;

	private Singleton() {
		System.out.println("create instance.");
	}

	public static Singleton getInstance() {

		if (instance == null) {
			try {
				Thread.sleep(2000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			instance = new Singleton();
		}
		System.out.println("instance hashCode : " + instance.hashCode());
		return instance;
	}

	public static void main(String[] agrs) {
		new Thread(new SingletonTestRunnable()).start();
		new Thread(new SingletonTestRunnable()).start();
	}
}

 

多线程Singleton反模式——使用synchronized修饰方法

 

	public static synchronized Singleton getInstance() {
            ....
        }

   添加synchronized关键字确实可以解决多线程安全问题，但是仍然认为是反模式。原因是执行效率低，getInstance()方法只允许一个线程进入，阻塞会增加程序运行时间。

 

★多线程Singleton最佳实践——eager模式

 

class SingletonTestRunnable implements Runnable {
	public void run() {
		Singleton.getInstance();
	}
}

public class Singleton {

	private static Singleton instance = new Singleton(); 

	private Singleton() {
		System.out.println("create instance.");
	}

	public static Singleton getInstance() {
		return instance;
	}

	public static void main(String[] agrs) {
		new Thread(new SingletonTestRunnable()).start();
		new Thread(new SingletonTestRunnable()).start();
	}
}

 

★多线程Singleton最佳实践——lazy模式

 

class SingletonTestRunnable implements Runnable {
	public void run() {
		Singleton.getInstance();
	}
}

public class Singleton {

	private Singleton() {
		System.out.println("create instance.");
	}

	public static Singleton getInstance() {
		return Container.instance;
	}
	
	private static class Container {
		private static Singleton instance = new Singleton();
	}

	public static void main(String[] agrs) {
		new Thread(new SingletonTestRunnable()).start();
		new Thread(new SingletonTestRunnable()).start();
	}
}

 

--heipark