JAVA学习第二十五课（多线程（四））- 单例设计模式涉及的多线程问题

一、多线程下的单例设计模式

利用双重判断的形式解决懒汉式的安全问题和效率问题

//饿汉式
/*class Single
{
	private static final Single t = new Single();
	private Single(){}
	public static Single getInstance()
	{
		return t;
	}
}
*/

//懒汉式
class Single
{
	private static Single t = null;
	private Single(){}
	public static Single getInstance()
	{
		if(t==null)//解决效率问题
		{
				synchronized(Single.class)//解决安全问题
				{
					if(t==null)
						t = new Single();
				}

		}
		return t;
	}
}

所以说开发中还是应用饿汉式，但是在面试里考察懒汉式，因为其技术含量高

二、死锁示例

死锁：常见就是，同步的嵌套

面试时，会让写死锁程序，只要写的出来，就说明死锁已经理解

class Deadlock implements Runnable
{
	private boolean flag;
	public Deadlock(boolean flag) {
		// TODO Auto-generated constructor stub
	this.flag = flag;
	}
	public void run()
	{
		if(flag)
		{
			while(true)
			synchronized (lock.A_LOCK) {
				System.out.println("if...alock");
				synchronized (lock.B_LOCK) {
					System.out.println("if...block");
				}
			}
		}
		else {
			while(true)
			synchronized (lock.B_LOCK) {
				System.out.println("else...block");
				synchronized (lock.A_LOCK) {
					System.out.println("else...alock");

				}
			}
		}
	}
}

class lock
{
	public static final Object A_LOCK = new Object();
	public static final Object B_LOCK = new Object();

}
class Main
{
	public static void main(String[] args)
	{
		Deadlock t1 = new Deadlock(true);
		Deadlock t2 = new Deadlock(false);
		Thread j1 = new Thread(t1);
		Thread j2 = new Thread(t2);
		j1.start(); j2.start();

	}
}

if...alock

else...block

锁上了，Thread-0拿a锁，Thread-1拿b锁