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。
1 class Ticket implements Runnable {
2 private static int num = 100;
3 boolean flag = true;
4
5 public void run() {
6 if (flag)
7 while (true) {
8 synchronized (Ticket.class)//(this.getClass())同步代码块
9 {
10 if (num > 0) {
11 try {
12 Thread.sleep(10);
13 } catch (InterruptedException e) {
14 }
15 System.out.println(Thread.currentThread().getName() + ".....obj...." + num--);
16 }
17 }
18 }
19 else
20 while (true)
21 this.show();
22 }
23
24 public static synchronized void show()//同步函数
25 {
26 if (num > 0) {
27 try {
28 Thread.sleep(10);
29 } catch (InterruptedException e) {
30 }
31 System.out.println(Thread.currentThread().getName() + ".....function...." + num--);
32 }
33 }
34 }
35
36 class StaticSynFunctionLockDemo {
37 public static void main(String[] args) {
38 Ticket t = new Ticket();
39
40 Thread t1 = new Thread(t);
41 Thread t2 = new Thread(t);
42
43 t1.start();
44 try {
45 Thread.sleep(10);
46 } catch (InterruptedException e) {
47 }
48 t.flag = false;
49 t2.start();
50 }
51 }View Code
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。
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