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
 

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