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
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 }
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。