Java多线程——线程安全问题

一、什么情况下会产生线程安全问题？

同时满足以下两个条件时：

1，多个线程在操作共享的数据。
2，操作共享数据的线程代码有多条。

当一个线程在执行操作共享数据的多条代码过程中，其他线程参与了运算，就会导致线程安全问题的产生。

例1：四个线程卖100张票

public class TicketDemo implements Runnable {
	private int tickets = 100;

	public void run() {
		while (true) {

			if (tickets > 0) {
				try {
					Thread.sleep(10);
				} catch (InterruptedException e) {
				}
				System.out.println(Thread.currentThread().getName() + "....sale:...." + tickets--);
			}
		}
	}

	public static void main(String[] args) {
		TicketDemo ticketDemo = new TicketDemo();
		Thread t1 = new Thread(ticketDemo);
		Thread t2 = new Thread(ticketDemo);
		Thread t3 = new Thread(ticketDemo);
		Thread t4 = new Thread(ticketDemo);
		t1.start();
		t2.start();
		t3.start();
		t4.start();
	}
}

Thread-3....sale....100
Thread-2....sale....99
Thread-0....sale....97
Thread-1....sale....98
Thread-3....sale....96
Thread-1....sale....94
Thread-0....sale....94
Thread-2....sale....95
Thread-1....sale....93
Thread-0....sale....92
Thread-2....sale....92
Thread-3....sale....92
Thread-0....sale....91
Thread-2....sale....89
Thread-3....sale....90
Thread-1....sale....91
Thread-1....sale....88
Thread-3....sale....86
Thread-0....sale....88
Thread-2....sale....87
Thread-2....sale....84
Thread-3....sale....84
Thread-1....sale....85
Thread-0....sale....83
Thread-1....sale....82
Thread-0....sale....80
Thread-3....sale....79
Thread-2....sale....81
Thread-3....sale....78
Thread-2....sale....75
Thread-1....sale....76
Thread-0....sale....77
Thread-2....sale....74
Thread-1....sale....71
Thread-0....sale....73
Thread-3....sale....72
Thread-1....sale....70
Thread-0....sale....68
Thread-3....sale....69
Thread-2....sale....67
Thread-2....sale....66
Thread-3....sale....64
Thread-0....sale....63
Thread-1....sale....65
Thread-2....sale....62
Thread-0....sale....62
Thread-1....sale....60
Thread-3....sale....61
Thread-2....sale....59
Thread-0....sale....57
Thread-3....sale....58
Thread-1....sale....59
Thread-0....sale....56
Thread-1....sale....56
Thread-3....sale....55
Thread-2....sale....56
Thread-1....sale....54
Thread-2....sale....54
Thread-0....sale....54
Thread-3....sale....53
Thread-0....sale....52
Thread-3....sale....52
Thread-2....sale....50
Thread-1....sale....51
Thread-2....sale....49
Thread-0....sale....49
Thread-3....sale....48
Thread-1....sale....48
Thread-2....sale....46
Thread-0....sale....44
Thread-3....sale....45
Thread-1....sale....47
Thread-1....sale....43
Thread-0....sale....42
Thread-2....sale....42
Thread-3....sale....41
Thread-1....sale....40
Thread-0....sale....39
Thread-3....sale....39
Thread-2....sale....40
Thread-2....sale....38
Thread-1....sale....37
Thread-3....sale....35
Thread-0....sale....36
Thread-3....sale....34
Thread-1....sale....33
Thread-0....sale....32
Thread-2....sale....31
Thread-3....sale....30
Thread-1....sale....29
Thread-0....sale....29
Thread-2....sale....28
Thread-3....sale....27
Thread-0....sale....25
Thread-1....sale....26
Thread-2....sale....24
Thread-1....sale....23
Thread-0....sale....23
Thread-3....sale....22
Thread-2....sale....21
Thread-1....sale....20
Thread-3....sale....20
Thread-0....sale....20
Thread-2....sale....19
Thread-3....sale....16
Thread-0....sale....17
Thread-1....sale....18
Thread-2....sale....15
Thread-0....sale....13
Thread-1....sale....12
Thread-3....sale....14
Thread-2....sale....11
Thread-3....sale....10
Thread-0....sale....8
Thread-1....sale....9
Thread-2....sale....7
Thread-3....sale....6
Thread-0....sale....5
Thread-1....sale....4
Thread-2....sale....3
Thread-1....sale....2
Thread-3....sale....2
Thread-2....sale....1
Thread-0....sale....2

运行结果

　　观察结果，我们发现会有多个线程卖到同一张票和卖到0号票的情况，这就是线程安全问题。

---

解决思路：

将多条操作共享数据的线程代码封装起来，当有线程在执行这些代码的时候，其他线程不可以参与运算。

当前线程把这些代码都执行完毕后，其他线程才可以参与运算。

在java中，用同步代码块就可以解决这个问题。

同步代码块的格式：
synchronized(对象)
{
需要被同步的代码 ；
}

这个对象一般称为同步锁。

同步的前提：同步中必须有多 个线程并使用同一个锁。

同步的好处：解决了线程的安全问题。

同步的弊端：相对降低了效率，因为同步外的线程的都会判断同步锁。

解决例1的线程安全问题代码：

public class TicketDemo implements Runnable {
	private int tickets = 100;
	Object obj = new Object();

	public void run() {
		while (true) {
			synchronized (obj) {
				if (tickets > 0) {
					try {
						Thread.sleep(10);
					} catch (InterruptedException e) {
					}
					System.out.println(Thread.currentThread().getName() + "....sale...." + tickets--);
				}
			}
		}
	}

	public static void main(String[] args) {
		TicketDemo ticketDemo = new TicketDemo();
		Thread t1 = new Thread(ticketDemo);
		Thread t2 = new Thread(ticketDemo);
		Thread t3 = new Thread(ticketDemo);
		Thread t4 = new Thread(ticketDemo);
		t1.start();
		t2.start();
		t3.start();
		t4.start();
	}
}

　　

---

二、同步锁是什么：

同步函数使用的锁是 this。

静态的同步函数使用的锁是该函数所属 字节码文件对象 ，可以用 getClass()方法获取，也可以用 当前类名.class  表示。

同步函数和同步代码块的区别：

同步函数的锁是固定的this。

同步代码块的锁是任意的对象。

建议使用同步代码块。

 class Ticket implements Runnable {
     private static int num = 100;
     boolean flag = true;

     public void run() {
         if (flag)
             while (true) {
                 synchronized (Ticket.class)//(this.getClass())同步代码块
                 {
                     if (num > 0) {
                         try {
                             Thread.sleep(10);
                         } catch (InterruptedException e) {
                         }
                         System.out.println(Thread.currentThread().getName() + ".....obj...." + num--);
                     }
                 }
             }
         else
             while (true)
                 this.show();
     }

     public static synchronized void show()//同步函数
     {
         if (num > 0) {
             try {
                 Thread.sleep(10);
             } catch (InterruptedException e) {
             }
             System.out.println(Thread.currentThread().getName() + ".....function...." + num--);
         }
     }
 }

 class StaticSynFunctionLockDemo {
     public static void main(String[] args) {
         Ticket t = new Ticket();

         Thread t1 = new Thread(t);
         Thread t2 = new Thread(t);

         t1.start();
         try {
             Thread.sleep(10);
         } catch (InterruptedException e) {
         }
         t.flag = false;
         t2.start();
     }
 }

---

三、死锁常见情况：

同步嵌套时，两个线程你拿了我的锁，我拿了你的锁，都不释放，造成死锁。

可以记一套死锁情况代码，面试可能用得到。

死锁情况：

class Testa implements Runnable {
	private boolean flag;
	Testa(boolean flag) {
		this.flag = flag;
	}
	public void run() {
		if (flag) {
			while (true)
				synchronized (MyLock.locka) {
					System.out.println(Thread.currentThread().getName() + "..if   locka....");
					synchronized (MyLock.lockb) {
						System.out.println(Thread.currentThread().getName() + "..if   lockb....");
					}
				}
		} else {
			while (true)
				synchronized (MyLock.lockb) {
					System.out.println(Thread.currentThread().getName() + "..else  lockb....");
					synchronized (MyLock.locka) {
						System.out.println(Thread.currentThread().getName() + "..else   locka....");
					}
				}
		}
	}
}
class MyLock {
	public static final Object locka = new Object();
	public static final Object lockb = new Object();
}
class DeadLockTest {
	public static void main(String[] args) {
		Testa a = new Testa(true);
		Testa b = new Testa(false);
		Thread t1 = new Thread(a);
		Thread t2 = new Thread(b);
		t1.start();
		t2.start();
	}
}

　　

---

四、单例设计模式中的线程安全问题

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

//懒汉式
/*
*加入同步是为了解决多线程安全问题。
*
*加入双重判断不用每次都判断是否上锁，是为了解决效率问题。
**/

class Single
{
	private static Single s = null;
	private Single(){}
	public static Single getInstance()
	{
		if(s==null)
		{
			synchronized(Single.class)
			{
				if(s==null)
		//				-->0 -->1
					s = new Single();
			}
		}
		return s;
	}
}

开发用饿汉式，没有线程安全问题。——饿汉式在类创建的同时就已经创建好一个静态的对象供系统使用，以后不再改变，所以天生是线程安全的。

面试懒汉式，记住如何解决线程安全问题。