线程安全问题产生原因：

1、多个线程操作共享的数据；

2、操作共享数据的线程代码有多条。

 

当一个线程正在执行操作共享数据的多条代码过程中，其它线程也参与了运算，

就会导致线程安全问题的发生。

class Ticket extends Thread{    private int num = 100;    public void run()    {        while(num > 0)        {                   try {                    Thread.currentThread().sleep(10);                } catch (InterruptedException e) {                    // TODO Auto-generated catch block                    e.printStackTrace();                }           System.out.println(Thread.currentThread().getName() + "..." + num-- );        }    }}public class MyDemo {    public static void main(String[] args) {        // TODO Auto-generated method stub                Runnable ticket = new Ticket();                Thread t1 = new Thread(ticket);        Thread t2 = new Thread(ticket);        Thread t3 = new Thread(ticket);        Thread t4 = new Thread(ticket);                t1.start();        t2.start();        t3.start();        t4.start();        }}

Output：

Thread-1...11

Thread-2...10

Thread-4...9

Thread-3...8

Thread-2...7

Thread-1...6

Thread-3...5

Thread-4...4

Thread-1...2

Thread-2...3

Thread-4...1

Thread-3...0

Thread-2...-1

Thread-1...-2

 

解决思路：

就是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，

其他线程时不可以参与运算的。

必须要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 ；

}

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

同步的前提：同步中必须有多个线程并使用同一个锁。

Example:

class Ticket implements Runnable{    private int num = 100;    Object obj = new Object();        public void run()    {        // 如果将同步锁对象obj定义在此处，则会在每一个线程中都创建一个同步锁，                //即无法实现线程的同步。        while(true)        {            synchronized(obj)    //同步代码块            {                if(num > 0)                {                    try {                        Thread.currentThread().sleep(100);                    } catch (InterruptedException e) {                        // TODO Auto-generated catch block                        e.printStackTrace();                    }                    System.out.println(Thread.currentThread().getName() + "..." + num--);                }            }        }            }}public class MyDemo {    public static void main(String[] args) {        // TODO Auto-generated method stub                Runnable ticket = new Ticket();                Thread t1 = new Thread(ticket);        Thread t2 = new Thread(ticket);        Thread t3 = new Thread(ticket);        Thread t4 = new Thread(ticket);            t1.start();        t2.start();        t3.start();        t4.start();    }}

Output:

Thread-2...17

Thread-2...16

Thread-2...15

Thread-2...14

Thread-2...13

Thread-2...12

Thread-3...11

Thread-3...10

Thread-3...9

Thread-3...8

Thread-3...7

Thread-3...6

Thread-3...5

Thread-3...4

Thread-3...3

Thread-3...2

Thread-3...1

 

Example:

class Ticket extends Thread{    private int num = 100;    Object obj = new Object();        public void run()    {                while(true)        {            synchronized(obj)            {                if(num > 0)                {                    try {                        Thread.currentThread().sleep(100);                    } catch (InterruptedException e) {                        // TODO Auto-generated catch block                        e.printStackTrace();                    }                    System.out.println(Thread.currentThread().getName() + "..." + num--);                }            }        }            }}public class MyDemo {    public static void main(String[] args) {        // TODO Auto-generated method stub                Ticket t1 = new Ticket();        Ticket t2 = new Ticket();        Ticket t3 = new Ticket();        Ticket t4 = new Ticket();        t1.start();        t2.start();        t3.start();        t4.start();    }}

Output:

Thread-3...2

Thread-1...7

Thread-2...10

Thread-0...4

Thread-3...1

Thread-2...9

Thread-1...6

Thread-0...3

Thread-1...5

Thread-0...2

Thread-2...8

Thread-2...7

Thread-0...1

Thread-1...4

Thread-1...3

Thread-2...6

Thread-2...5

Thread-1...2

Thread-2...4

Thread-1...1

Thread-2...3

Thread-2...2

Thread-2...1

/*同步函数的使用的锁是this；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  int num = 100;    boolean flag = true;    public void run()    {        if(flag)            while(true)            {                synchronized(this)                {                    if(num>0)                    {                        try{Thread.sleep(10);}catch (InterruptedException e){}                                                System.out.println(Thread.currentThread().getName()+".....obj...."+num--);                    }                }            }        else            while(true)                this.show();    }    public synchronized void show()    {        if(num>0)        {            try{Thread.sleep(10);}catch (InterruptedException e){}                        System.out.println(Thread.currentThread().getName()+".....function...."+num--);        }    }}class SynFunctionLockDemo {    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();    }}

Output:

Thread-0.....obj....100

Thread-0.....obj....99

Thread-0.....obj....98

Thread-1.....function....97

Thread-1.....function....96

Thread-1.....function....95

Thread-1.....function....94

Thread-1.....function....93

Thread-1.....function....92

Thread-1.....function....91

Thread-1.....function....90

Thread-1.....function....89

Thread-1.....function....88

Thread-1.....function....87

Thread-1.....function....86

Thread-1.....function....85

Thread-1.....function....84

Thread-1.....function....83

Thread-1.....function....82

Thread-1.....function....81

Thread-1.....function....80

Thread-1.....function....79

Thread-1.....function....78

Thread-1.....function....77

Thread-1.....function....76

Thread-1.....function....75

Thread-1.....function....74

Thread-1.....function....73

Thread-1.....function....72

Thread-1.....function....71

Thread-1.....function....70

Thread-1.....function....69

Thread-1.....function....68

Thread-1.....function....67

Thread-1.....function....66

Thread-1.....function....65

Thread-1.....function....64

Thread-1.....function....63

Thread-1.....function....62

Thread-1.....function....61

Thread-1.....function....60

Thread-1.....function....59

Thread-1.....function....58

Thread-1.....function....57

Thread-1.....function....56

Thread-1.....function....55

Thread-1.....function....54

Thread-1.....function....53

Thread-1.....function....52

Thread-1.....function....51

Thread-1.....function....50

Thread-1.....function....49

Thread-1.....function....48

Thread-1.....function....47

Thread-1.....function....46

Thread-1.....function....45

Thread-1.....function....44

Thread-1.....function....43

Thread-1.....function....42

Thread-1.....function....41

Thread-1.....function....40

Thread-1.....function....39

Thread-1.....function....38

Thread-1.....function....37

Thread-1.....function....36

Thread-1.....function....35

Thread-1.....function....34

Thread-1.....function....33

Thread-1.....function....32

Thread-1.....function....31

Thread-1.....function....30

Thread-1.....function....29

Thread-1.....function....28

Thread-1.....function....27

Thread-1.....function....26

Thread-1.....function....25

Thread-1.....function....24

Thread-1.....function....23

Thread-1.....function....22

Thread-1.....function....21

Thread-1.....function....20

Thread-1.....function....19

Thread-1.....function....18

Thread-1.....function....17

Thread-1.....function....16

Thread-1.....function....15

Thread-1.....function....14

Thread-1.....function....13

Thread-1.....function....12

Thread-1.....function....11

Thread-1.....function....10

Thread-1.....function....9

Thread-1.....function....8

Thread-1.....function....7

Thread-1.....function....6

Thread-1.....function....5

Thread-1.....function....4

Thread-1.....function....3

Thread-1.....function....2

Thread-1.....function....1

 

/*

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。

*/

/* synchronized作用于静态方法和非静态方法的区别:  *  非静态方法:  *           给对象加锁(可以理解为给这个对象的内存上锁,注意 只是这块内存,其他同类对象都会有各自的内存锁),这时候  *          在其他一个以上线程中执行该对象的这个同步方法(注意:是该对象)就会产生互斥  *  静态方法:   *          相当于在类上加锁(*.class 位于代码区,静态方法位于静态区域,这个类产生的对象公用这个静态方法,所以这块  *          内存，N个对象来竞争), 这时候,只要是这个类产生的对象,在调用这个静态方法时都会产生互斥 */

class Ticket implements Runnable{    private static int num = 100;    boolean flag = true;    public void run()    {        if(flag)            while(true)            {                synchronized(this.getClass())   //  Ticket.class                {                    if(num>0)                    {                        try{Thread.sleep(10);}catch (InterruptedException e){}                                                System.out.println(Thread.currentThread().getName()+".....obj...."+num--);                    }                }            }        else            while(true)                show();    }    public static synchronized void show()    {        if(num>0)        {            try{Thread.sleep(10);}catch (InterruptedException e){}                        System.out.println(Thread.currentThread().getName()+".....function...."+num--);        }    }}class SynFunctionLockDemo {    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();    }}

Output:

Thread-0.....obj....100

Thread-1.....function....99

Thread-1.....function....98

Thread-0.....obj....97

Thread-0.....obj....96

Thread-0.....obj....95

Thread-0.....obj....94

Thread-0.....obj....93

Thread-0.....obj....92

Thread-0.....obj....91

Thread-0.....obj....90

Thread-0.....obj....89

Thread-0.....obj....88

Thread-0.....obj....87

Thread-0.....obj....86

Thread-0.....obj....85

Thread-0.....obj....84

Thread-0.....obj....83

Thread-0.....obj....82

Thread-0.....obj....81

Thread-0.....obj....80

Thread-0.....obj....79

Thread-0.....obj....78

Thread-0.....obj....77

Thread-0.....obj....76

Thread-0.....obj....75

Thread-0.....obj....74

Thread-0.....obj....73

Thread-0.....obj....72

Thread-0.....obj....71

Thread-0.....obj....70

Thread-0.....obj....69

Thread-0.....obj....68

Thread-0.....obj....67

Thread-0.....obj....66

Thread-0.....obj....65

Thread-0.....obj....64

Thread-0.....obj....63

Thread-0.....obj....62

Thread-0.....obj....61

Thread-0.....obj....60

Thread-0.....obj....59

Thread-0.....obj....58

Thread-0.....obj....57

Thread-0.....obj....56

Thread-0.....obj....55

Thread-0.....obj....54

Thread-0.....obj....53

Thread-0.....obj....52

Thread-0.....obj....51

Thread-0.....obj....50

Thread-0.....obj....49

Thread-0.....obj....48

Thread-0.....obj....47

Thread-0.....obj....46

Thread-0.....obj....45

Thread-0.....obj....44

Thread-0.....obj....43

Thread-0.....obj....42

Thread-0.....obj....41

Thread-0.....obj....40

Thread-0.....obj....39

Thread-0.....obj....38

Thread-0.....obj....37

Thread-0.....obj....36

Thread-0.....obj....35

Thread-0.....obj....34

Thread-0.....obj....33

Thread-0.....obj....32

Thread-0.....obj....31

Thread-0.....obj....30

Thread-0.....obj....29

Thread-0.....obj....28

Thread-0.....obj....27

Thread-0.....obj....26

Thread-0.....obj....25

Thread-0.....obj....24

Thread-0.....obj....23

Thread-0.....obj....22

Thread-0.....obj....21

Thread-0.....obj....20

Thread-0.....obj....19

Thread-0.....obj....18

Thread-0.....obj....17

Thread-0.....obj....16

Thread-0.....obj....15

Thread-0.....obj....14

Thread-1.....function....13

Thread-1.....function....12

Thread-1.....function....11

Thread-1.....function....10

Thread-1.....function....9

Thread-1.....function....8

Thread-1.....function....7

Thread-1.....function....6

Thread-1.....function....5

Thread-1.....function....4

Thread-1.....function....3

Thread-1.....function....2

Thread-1.....function....1