Java多线程技术学习笔记（一）

概述
多线程的好处与弊端
JVM中的多线程解析
多线程的创建方式之一：继承Thread类
线程的状态
多线程创建的方式之二：实现Runnable接口
使用方式二创建多线程的好处
多线程示例
线程安全问题现象
线程安全问题产生的原因
同步代码块
同步的好处与弊端
同步的前提
同步函数
验证同步函数的锁
单例模式的线程安全问题的解决方案
死锁示例

一、概述目录

首先得了解进程，打开我们电脑的windows资源管理器，可以直观看到进程的样子：

进程直观上理解就是正在进行的程序。而每个进程包含一个或者多个线程。也就是说一个进程是由若干线程组成的，在程序执行期间，真正执行的是线程，而进程只是负责给该进程中的线程分配执行路径，

所以，线程就是进程中负责程序执行的控制单元（执行路径），一个进程可以有多个执行路径，称为多线程。就像我们再使用QQ给多个好友聊天一样，每一个聊天过程都是一个线程，这些线程都属于QQ这个进程。

而开启多线程就是为了同时运行多部分代码。每一线程都有自己运行的内容，这个内容可以称为线程要执行的任务。

二、多线程的好处与弊端目录

上一部分说到多线程是为了同时运行多部分代码，但是对于一个cpu而言，在每一个时刻只能执行一个线程，它会在不同线程之间快速切换，由于切换速度很快，所以感觉上去像是多个线程在"同时"执行，现在虽然出现多核技术核数是几乎不可能多过线程数的，所以仍然需要cpu不断在多个线程之间切换，以提高cpu的利用效率。然而，但是每一个线程都需要一定的内存空间去执行，线程一多，内存空间不足，就会使得电脑显得特别卡，这就是多线程的弊端。注意到cpu在线程之间的切换是随机的。

三、JVM中的多线程解析目录

JVM启动时就启动了多个线程，至少有两个线程可以分析出来：一个是执行main函数的线程（也称为主线程），另一个是负责垃圾回收的线程。

在JVM垃圾回收方法是finalize方法，该方法由垃圾回收器来调用，而gc() 方法是用来运行垃圾回收器的：

下面展示主线程和垃圾回收线程的运行：

 1 package thread.demo;
 2 
 3 class Demo extends Object{
 4     public void finalize(){
 5         System.out.println("demo ok");
 6     }
 7 }
 8 public class ThreadDemo_1 {
 9 
10     /**
11      * @param args
12      */
13     public static void main(String[] args) {
14         // TODO Auto-generated method stub
15         new Demo();
16         new Demo();
17         System.gc();
18         new Demo();
19         System.out.println("Hello World!"); 
20     }
21 
22 }

运行结果：

Hello World!
demo ok
demo ok

可以发现，虽然第19行在第17行代码（垃圾回收）之前，但是第19行代码却先执行，怎么回事呢？因为垃圾回收（第17行）是由垃圾回收线程执行，而第19行代码主线程的部分，cpu从主线程开始执行，然后在主线程和垃圾回收线程之间切换，创建完两个Demo()对象之后，虽然我们调用垃圾回收器，但是垃圾回收程序还没来得及执行，cpu切换到了主线程，于是先打印出了“Hello World!”

于是虽然第19行代码，执行完了，看似整个程序都执行完了，但是JVM（Java 虚拟机）还没有结束，即虽然主线程结束了，但是JVM还要执行垃圾回收线程。

四、多线程创建方式之一：继承Thread类目录

首先看一看一个简单的打印程序：

 1 package thread.demo;
 2 class Demo_2{
 3     private String name;
 4     Demo_2(String name){
 5         this.name = name;
 6     }
 7     public void show(){
 8         for (int x = -99999999; x < 99999999; x++){};
 9         for (int i = 0; i < 10; i++){
10             System.out.println(name + "...i" + i);
11         }
12     }
13 }
14 public class ThreadDemo_2 {
15 
16     /**
17      * @param args
18      */
19     public static void main(String[] args) {
20         // TODO Auto-generated method stub
21         Demo_2 d1 = new Demo_2("旺财");
22         Demo_2 d2 = new Demo_2("xiaoqiang");
23         d1.show();
24         d2.show();
25     }
26 
27 }

View Code

运行结果：

旺财...i0
旺财...i1
旺财...i2
旺财...i3
旺财...i4
旺财...i5
旺财...i6
旺财...i7
旺财...i8
旺财...i9
xiaoqiang...i0
xiaoqiang...i1
xiaoqiang...i2
xiaoqiang...i3
xiaoqiang...i4
xiaoqiang...i5
xiaoqiang...i6
xiaoqiang...i7
xiaoqiang...i8
xiaoqiang...i9

View Code

这时显示的只是主线程运行的结果，很容易理解！下面通过一种方式，让“旺财”和“xiaoqiang”的打印能够分别运行在不同的线程中，首先查看java的API文档：

翻译：线程是程序中的执行线程。Java 虚拟机允许应用程序并发地执行多个执行线程。

接着文档中给出了创建线程的方法：

总结出来就是继承Thread类然后重写Thread类中的run()方法，run方法中的代码就是线程要执行的代码，然后调用start方法开启一个线程。

 1 package thread.demo;
 2 class Demo_2 extends Thread {
 3     private String name;
 4     Demo_2(String name){
 5         this.name = name;
 6     }
 7     public void run() {
 8         show();
 9     }
10     public void show(){
11         for (int x = -99999999; x < 99999999; x++){};
12         for (int i = 0; i < 10; i++){
13             System.out.println(name + "...i" + i);
14         }
15     }
16 }
17 public class ThreadDemo_2 extends Thread {
18 
19     /**
20      * @param args
21      */
22     public static void main(String[] args) {
23         /* 创建线程的目的是为了开启一条执行路径，去运行指定代码和
24         其他代码实现同时运行，而运行的指定代码就是这个执行路径的
25         任务。 JVM创建的主线程的任务都定义在主函数中。
26         而自定义的线程它的任务：
27         Thread类用于描述线程，线程需要任务，所以Thread类也有对
28         任务的描述，这个任务就通过Thread类中的run方法来体现的。
29         也就是说，run方法就是封装自定义线程任务的函数。
30         run方法中定义的就是线程要运行的任务代码！！！
31         
32         开启线程是为了运行指定代码，所以只有继承Thread类，并重写run方法。
33         并将运行代码定义在run方法中即可
34         */
35         Demo_2 d1 = new Demo_2("旺财");
36         Demo_2 d2 = new Demo_2("xiaoqiang");
37         d1.start();
38         d2.start();
39     }
40 
41 }

View Code

运行结果：

xiaoqiang...i0
旺财...i0
xiaoqiang...i1
旺财...i1
旺财...i2
旺财...i3
xiaoqiang...i2
xiaoqiang...i3
xiaoqiang...i4
xiaoqiang...i5
xiaoqiang...i6
xiaoqiang...i7
旺财...i4
xiaoqiang...i8
xiaoqiang...i9
旺财...i5
旺财...i6
旺财...i7
旺财...i8
旺财...i9

View Code

可以看出，运行结果正如上面分析的一样，cpu在多个线程之间随机切换，于是打印出的结果与上面只有主线程时结果差别很大。

下面我们让主线程参与进来，即同时看看线程d1,d2和主线程的运行结果：

 1 package thread.demo;
 2 class Demo_2 extends Thread {
 3     private String name;
 4     Demo_2(String name){
 5         this.name = name;
 6     }
 7     public void run() {
 8         show();
 9     }
10     public void show(){
11         for (int x = -99999999; x < 99999999; x++){};
12         for (int i = 0; i < 10; i++){
13             System.out.println(name + "...i" + i);
14         }
15     }
16 }
17 public class ThreadDemo_2 extends Thread {
18 
19     /**
20      * @param args
21      */
22     public static void main(String[] args) {
23         /* 创建线程的目的是为了开启一条执行路径，去运行指定代码和
24         其他代码实现同时运行，而运行的指定代码就是这个执行路径的
25         任务。 JVM创建的主线程的任务都定义在主函数中。
26         而自定义的线程它的任务：
27         Thread类用于描述线程，线程需要任务，所以Thread类也有对
28         任务的描述，这个任务就通过Thread类中的run方法来体现的。
29         也就是说，run方法就是封装自定义线程任务的函数。
30         run方法中定义的就是线程要运行的任务代码！！！
31         
32         开启线程是为了运行指定代码，所以只有继承Thread类，并重写run方法。
33         并将运行代码定义在run方法中即可
34         */
35         Demo_2 d1 = new Demo_2("旺财");
36         Demo_2 d2 = new Demo_2("xiaoqiang");
37         d1.start();
38         d2.start();
39         System.out.println("over");
40     }
41 
42 }

运行结果：

over
xiaoqiang...i0
旺财...i0
xiaoqiang...i1
旺财...i1
xiaoqiang...i2
旺财...i2
xiaoqiang...i3
xiaoqiang...i4
旺财...i3
xiaoqiang...i5
旺财...i4
xiaoqiang...i6
旺财...i5
xiaoqiang...i7
旺财...i6
xiaoqiang...i8
旺财...i7
xiaoqiang...i9
旺财...i8
旺财...i9

多次执行，会发现显示结果一直变化，这就是多个线程随机占用cpu的结果。当然，如果想看到到底是哪个线程正在执行，可以调用Thread中的currentThread().getName（）方法,其中currentThread()是用来返回当前的线程对象，然后用线程对象继续调用getName()就是返回当前线程的名字。程序如下：

 1 package thread.demo;
 2 class Demo_2 extends Thread {
 3     private String name;
 4     Demo_2(String name){
 5         this.name = name;
 6     }
 7     public void run() {
 8         show();
 9     }
10     public void show(){
11         for (int x = -99999999; x < 99999999; x++){};
12         for (int i = 0; i < 10; i++){
13             System.out.println(name + "...i" + "...name: " + getName());
14         }
15     }
16 }
17 public class ThreadDemo_2 extends Thread {
18 
19     /**
20      * @param args
21      */
22     public static void main(String[] args) {
23         /* 创建线程的目的是为了开启一条执行路径，去运行指定代码和
24         其他代码实现同时运行，而运行的指定代码就是这个执行路径的
25         任务。 JVM创建的主线程的任务都定义在主函数中。
26         而自定义的线程它的任务：
27         Thread类用于描述线程，线程需要任务，所以Thread类也有对
28         任务的描述，这个任务就通过Thread类中的run方法来体现的。
29         也就是说，run方法就是封装自定义线程任务的函数。
30         run方法中定义的就是线程要运行的任务代码！！！
31         
32         开启线程是为了运行指定代码，所以只有继承Thread类，并重写run方法。
33         并将运行代码定义在run方法中即可
34         */
35         Demo_2 d1 = new Demo_2("旺财");
36         Demo_2 d2 = new Demo_2("xiaoqiang");
37         d1.start();
38         d2.start();
39         System.out.println("over" + "..." + Thread.currentThread().getName());
40     }
41 
42 }

运行结果：

over...main
旺财...i...name: Thread-0
旺财...i...name: Thread-0
旺财...i...name: Thread-0
旺财...i...name: Thread-0
旺财...i...name: Thread-0
旺财...i...name: Thread-0
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
xiaoqiang...i...name: Thread-1
旺财...i...name: Thread-0
旺财...i...name: Thread-0
旺财...i...name: Thread-0
旺财...i...name: Thread-0

注意主线程的名字是固定的就是main,然后自定义的线程从0开始编号。

五、线程的状态目录

六、多线程创建的方式之二：实现Runnable接口目录

首先来读一下API文档关于Runnable的描述：

划红线部分：Runnable接口由那些打算通过某一线程执行其实例的类来实现。类必须定义一个称为run的无参数方法。于是采用实现接口的方式来实现多线程：

定义Runnable接口
覆盖Runnable接口中的run方法，将线程任务代码封装到run方法中。
通过Thread类创建对线，并将Runnable接口中的子类对象作为参数传递给Thread类的构造函数。因为线程任务都封装在Runnable接口子类对象的run方法中，所以在创建时必须要明确要运行的任务，如果不传递这个对象，Thread类会运行它自己的run方法，而不是我们定义在接口中的方法。
调用线程的start方法开启线程。

代码如下：

 1 package thread.demo;
 2 //通过实现接口的方式实现多线程创建
 3 class Demo_3 implements Runnable {
 4     public void run() {
 5         show();
 6     }
 7     public void show(){
 8         for (int x = -99999999; x < 99999999; x++){};
 9         for (int i = 0; i < 10; i++){
10             System.out.println(Thread.currentThread().getName());
11         }
12     }
13 }
14 
15     public class ThreadDemo_3 extends Thread {
16 
17         /**
18          * @param args
19          */
20         public static void main(String[] args) {
21             //创建一个Runnable接口的子类对象
22             Demo_3 d = new Demo_3(); 
23             //将上述Runnable接口的子类对象传入Thread构造函数，
24             //创建线程
25             Thread t1 = new Thread(d);
26             Thread t2 = new Thread(d);
27             t1.start();
28             t2.start();
29     }
30 
31 }

运行结果：

Thread-1
Thread-0
Thread-1
Thread-0
Thread-1
Thread-0
Thread-0
Thread-1
Thread-0
Thread-1
Thread-0
Thread-1
Thread-0
Thread-1
Thread-0
Thread-1
Thread-1
Thread-1
Thread-0
Thread-0

同样可以看见两个线程在随机切换执行。

细节：通过阅读API文档，可以发现，Thread类里面定义了自己的run方法，而Runnable也有run方法，而Thread的构造方法包含着下面两种（当然不止两种）：

第一种构造方法不包含任何参数，那么在使用Thread创建的线程对象在运行时，就调用Thread类自己的run方法，如果传入一个Runnable子类对象，那么在使用Thread类创建对象时，运行的任务就是Runnable接口中定义run方法。原理用代码简单解释如下：

 1 package thread.demo;
 2 
 3 class Thread {
 4     private Runnable r;
 5     Thread() {}
 6     Thread(Runnable) {
 7         this.r = r;
 8     }
 9     
10     public void run() {
11         r.run();
12     }
13     
14     public void start() {
15         run();
16     }
17 }
18 
19 public class SubThread extends Thread {
20 
21     public void run() {
22         System.out.println("dsa");
23     }
24 
25 }

在Thread内部有一个私有的Runable子类对象，可以看出，当我们把Runable子类 r 对象传递给Thread类构造函数的时候，启动start()就会调用run()，而run()接着调用 r 的run方法；

但是当我们直接通过上面介绍的方式一，即直接继承Thread类创建线程的时候，如19-23行所示，我们需要覆盖Thread类中的run方法，那么SubThread类的对象就在通过start方法启动线程的时候调用的run方法时就会调用我们在Thread子类中自己定义的run方法（21-22行）！

七、使用方式二创建多线程的好处目录

创建线程的两种方式：

方式一：继承Thread
方式二：实现Runnable接口

好处：

1）将线程的任务从线程的子类中分离出来，进行了单独封装，按照面向对象的思想将任务封装成对象。

2）避免了java单继承的局限性

所以，创建线程第二种方式较为常用！！！

八、多线程示例目录

下面实现四个售票员（四个线程）一起卖100张票的示例。

 1 package thread.demo;
 2 //买票：四个售票员一起卖100张票
 3 class Ticket implements Runnable {
 4     private int num = 100;
 5     public void run() {
 6         while(true) {
 7             if (num > 0) {
 8                 System.out.println(Thread.currentThread().getName() + "...sale..." + num--);
 9             }
10         }
11     }
12 }
13 
14 public class TicketDemo {
15 
16 
17     public static void main(String[] args) {
18 
19         Ticket t = new Ticket();
20         /*
21         Ticket t1 = new Ticket();
22         Ticket t2 = new Ticket();
23         Ticket t3 = new Ticket();
24         Ticket t4 = new Ticket();
25         */
26         Thread seller1 = new Thread(t);
27         Thread seller2 = new Thread(t);
28         Thread seller3 = new Thread(t);
29         Thread seller4 = new Thread(t);
30         
31         seller1.start();
32         seller2.start();
33         seller3.start();
34         seller4.start();
35     }
36 
37 }

运行结果：

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

看到四个线程一起把100张票卖完了.

九、线程安全问题现象目录

分析上面的示例，由于线程之间随机切换执行，假如售票员0（线程0），卖到了1号票，此时 num = 1 ;

            if (num > 0) {
                System.out.println(Thread.currentThread().getName() + "...sale..." + num--);
            }

经过判断，满足if 条件，进入后面的代码块，但是存在一种情况就是，线程0还没有来得及执行打印语句，就切换到了线程1，此时线程0处于等待状态（等待继续执行...）, 此时num仍然为1，然后线程1判断

满足条件，顺利执行完，之后num--, 于是num = 0; 恰好随机切换到线程0，然后线程0执行打印语句（Thread-0...sale...0），就是说售票员0 把0号票卖出去了，显然不可以！这种情况就导致了线程不安全！

为了使得程序出现我们分析的这种不安全的情况，需要在示例代码在第7行之后稍微停顿一下，然后执行后面的打印语句，如果不停顿，现在cpu的计算速度很快，判断 if (num > 0)为真之后往往很快就会执行到打印语句，上面分析的情况很难观测到，于是为了说明问题，做如下添加：

 1 package thread.demo;
 2 //买票：四个售票员一起卖100张票
 3 class Ticket implements Runnable {
 4     private int num = 100;
 5     public void run() {
 6         while(true) {
 7             if (num > 0) {
 8                 // 让线程sleep一会，好让打印语句还没来得及执行，其他线程
 9                 // 就切换进来，这样方便我们观测线程安全隐患
10                 try {
11                     Thread.sleep(20);
12                 } catch (InterruptedException e) {
13                     // TODO Auto-generated catch block
14                     e.printStackTrace();
15                 }
16                 
17                 System.out.println(Thread.currentThread().getName() + "...sale..." + num--);
18             }
19         }
20     }
21 }
22 
23 public class TicketDemo {
24 
25 
26     public static void main(String[] args) {
27 
28         Ticket t = new Ticket();
29         /*
30         Ticket t1 = new Ticket();
31         Ticket t2 = new Ticket();
32         Ticket t3 = new Ticket();
33         Ticket t4 = new Ticket();
34         */
35         Thread seller1 = new Thread(t);
36         Thread seller2 = new Thread(t);
37         Thread seller3 = new Thread(t);
38         Thread seller4 = new Thread(t);
39         
40         seller1.start();
41         seller2.start();
42         seller3.start();
43         seller4.start();
44     }
45 
46 }

运行结果：

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

可以看到售票员竟然卖出了0，-1，-2号票！！这就是线程安全问题的简单演示，每次运行出现的结果不一样，也有可能恰好正常，没有出现安全问题，以为线程切换的不确定性，但是这种问题一旦出现，通常很致命，所以必须注意！

十、线程安全问题产生的原因目录

由上面可以知道，线程安全问题必须要解决，但是解决问题关键是分析原因！根据上面示例总结出以下原因：

多个线程操作共享的数据
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。

十一、同步代码块目录

既然知道产生线程安全问题的原因，就开始着手解决。

解决思路：

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

必须要当前线程把这些代码都执行完毕以后，其他线程才可以参与运算.

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；

}

其中的“对象”可以看做一个标记，可以使用任何类型的对象，包括自定义的对象。当然，为了方便，使用Object类的对象即可。

 1 package thread.demo;
 2 //买票：四个售票员一起卖100张票
 3 class Ticket implements Runnable {
 4     private int num = 100;
 5     Object obj = new Object();
 6     public void run() {
 7         while(true) {
 8             synchronized(obj) {
 9                 if (num > 0) {
10                     // 让线程sleep一会，好让打印语句还没来得及执行后面的打印语句，其他线程
11                     // 就切换进来，这样方便我们观测线程安全隐患
12                     try {
13                         Thread.sleep(20);
14                     } catch (InterruptedException e) {
15                         // TODO Auto-generated catch block
16                         e.printStackTrace();
17                     }
18                     
19                     System.out.println(Thread.currentThread().getName() + "...sale..." + num--);
20                 }
21             }
22         }
23     }
24 }
25 
26 public class TicketDemo {
27 
28 
29     public static void main(String[] args) {
30 
31         Ticket t = new Ticket();
32         /*
33         Ticket t1 = new Ticket();
34         Ticket t2 = new Ticket();
35         Ticket t3 = new Ticket();
36         Ticket t4 = new Ticket();
37         */
38         Thread seller1 = new Thread(t);
39         Thread seller2 = new Thread(t);
40         Thread seller3 = new Thread(t);
41         Thread seller4 = new Thread(t);
42         
43         seller1.start();
44         seller2.start();
45         seller3.start();
46         seller4.start();
47     }
48 
49 }

运行结果：

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

可以看出，问题得到了很好地解决！

十二、同步的好处与弊端目录

首先讨论下，同步到底是如何实现的。

假如0线程执行到run方法的同步代码块，那么 0 线程就持有了 obj, 即obj 被加载到了0线程里面，当其他线程过来时，它们也需要加载obj才能进入同步代码块，但是在线程没有执行完同步代码块之前，obj一直被0线程占有，所以其他线程无法进入同步代码块，知道0线程执行完同步代码块释放 obj，其他线程才有机会加载obj, 然后进入同步代码块。所以obj就像一把锁一样，里面的不出来，外面的就进不去。所以常常称为同步锁！谁拿到了锁，谁就进得去，出来之后释放锁，以便其他线程有机会使用。

同步的好处：解决了线程安全问题。

同步的弊端：相对降低了效率，因为同步外的线程都会去判断同步锁。

十三、同步的前提目录

同步中，必须有多个线程并使用同一个锁，因为一个线程没必要同步，数据全被这一个线程使用，就不存在所谓的线程安全问题。而如果不使用同一个锁，即使一个线程在执行加了锁的代码块，其他线程同样可以通过拿到其他锁进来参与运算。

十四、同步函数目录

一个稍微简单点的多线程程序示例：

 1 package thread.demo;
 2 /*
 3  需求：两个储户，每个都到银行存钱，每次存100，共存3次 
 4  */
 5 class Bank {
 6     private int sum;
 7     public void add(int num) {
 8         sum += num;
 9         System.out.println("sum　＝　"  + sum);
10     }
11 }
12 
13 class Customer implements Runnable {
14     Bank bank = new Bank();
15     public void run() {
16         for (int x = 0; x < 3; x++) {
17             bank.add(100);
18         }
19     }
20 }
21 
22 public class BankDemo {
23 
24     /**
25      * @param args
26      */
27     public static void main(String[] args) {
28         // TODO Auto-generated method stub
29         Customer customer = new Customer();
30         Thread t1 = new Thread(customer);
31         Thread t2 = new Thread(customer);
32 
33         t1.start();
34         t2.start();
35 
36     }
37 
38 }

运行结果：

sum　＝　100
sum　＝　200
sum　＝　300
sum　＝　500
sum　＝　400
sum　＝　600

当然，由于线程的随机切换，显示结果有点乱，最终两人共向银行存入了600.

分析一下这个程序：run方法里面的代码调用了add方法，sum 是两个线程的共享数据，对于共享数据的操作不止一条：

        sum += num;
        System.out.println("sum　＝　"  + sum);

假如第一个用户（线程0）存入100，执行第一条语句，sum 就变为100. 正常接下来就应该输出 sum = 100, 但是这时存在一种情况：线程0 还没来得及输出，第二个用户（线程1）存入100，于是sum = 100 + 100 = 200, 线程1 然后正常输出 sum = 200,刚输出完成，线程0又切入了，接着执行打印语句，本来他存入的只是100，但是由于sum = 200, 线程0 也打印出sum = 200, 对应这实际情况就是，用户1存入了100，系统却显示存入了200，显然存在着线程安全问题。同样，为了展示这个安全问题，在上面两条语句之间假如一定的停顿：

        sum += num;
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("sum　＝　"  + sum);

运行结果：

sum　＝　200
sum　＝　200
sum　＝　400
sum　＝　400
sum　＝　600
sum　＝　600

可见，200显示了两次，肯定是有一个用户存入100时，却打印出了200. 显然不合理！

首先想到的就是同步：

 1 class Bank {
 2     private int sum;
 3     Object obj = new Object();
 4     public void add(int num) {
 5         synchronized(obj) {
 6             sum += num;
 7             try {
 8                 Thread.sleep(20);
 9             } catch (InterruptedException e) {
10                 // TODO Auto-generated catch block
11                 e.printStackTrace();
12             }
13             System.out.println("sum　＝　"  + sum);
14         }
15     }
16 }

运行结果：

sum　＝　100
sum　＝　200
sum　＝　300
sum　＝　400
sum　＝　500
sum　＝　600

问题解决！

但是，发现，要创建对象，写synchronized代码块是不是有点麻烦呢？？？我们发现add函数里面的内容都需要同步，就是说add函数里面的代码就是我们要同步的代码，于是直接同步这个函数就可以了，即同步函数：

 1 class Bank {
 2     private int sum;
 3     //Object obj = new Object();
 4     public synchronized void add(int num) {//同步函数
 5         //synchronized(obj) {
 6             sum += num;
 7             try {
 8                 Thread.sleep(20);
 9             } catch (InterruptedException e) {
10                 // TODO Auto-generated catch block
11                 e.printStackTrace();
12             }
13             System.out.println("sum　＝　"  + sum);
14         //}
15     }
16 }

运行结果同上！

十五、同步函数锁的验证目录

由前面的分析知道，同步是需要一把“锁”，而锁可以是任意类型的对象，那么同步函数到底使用的是什么锁呢？？下面来验证。

回到前面售票的程序，改成两个售票员，一个售票员卖票线程放在同步代码块里面，另一个售票员的的线程放在同步函数里面，如果这两个线程用的是同一个锁，那么就不会出现安全隐患。

于是在程序改为下面：

 1 package thread.demo;
 2 //买票：四个售票员一起卖100张票
 3 class Ticket_2 implements Runnable {
 4     private int num = 100;
 5     Object obj = new Object();
 6     boolean flag = true;
 7     public void run() { 
 8         if (flag) 
 9         {
10             while (true) 
11             {
12                 synchronized(obj)
13                 {
14                     if (num > 0) 
15                     {
16                         // 让线程sleep一会，好让打印语句还没来得及执行后面的打印语句，其他线程
17                         // 就切换进来，这样方便我们观测线程安全隐患
18                         try { 
19                             Thread.sleep(20);
20                         } catch (InterruptedException e) {
21                             // TODO Auto-generated catch block
22                             e.printStackTrace();
23                         }
24                         System.out.println(Thread.currentThread().getName() + "...bloc k..." + num--);
25                     }
26                 }
27             }
28                 
29         }//end if
30         else
31             while (true) show();
32     }
33     
34     public synchronized void show() 
35     {
36         if (num > 0) 
37         {
38             // 让线程sleep一会，好让打印语句还没来得及执行，其他线程
39             // 就切换进来，这样方便我们观测线程安全隐患
40             try { 
41                 Thread.sleep(20);
42             } catch (InterruptedException e) {
43                 // TODO Auto-generated catch block
44                 e.printStackTrace();
45             }
46             System.out.println(Thread.currentThread().getName() + "...function..." + num--);
47         }
48     } 
49 }
50 
51 public class SynFunctionLockDemo
52 {
53 
54 
55     public static void main(String[] args) 
56     {
57 
58         Ticket_2 t = new Ticket_2();
59 
60         Thread seller1 = new Thread(t);
61         Thread seller2 = new Thread(t);
62 
63         
64         seller1.start(); //在同步代码块执行
65         t.flag = false; // 标志变为false，使得下一个线程在同步函数执行
66         seller2.start();
67 
68     }
69 }

如果是在同步函数里面执行的代码块会打印出带有“function”的字符串。如果是在同步代码块里面执行的会打印出带有"bloc k"的字符串，执行如下：

Thread-0...function...100
Thread-0...function...99
Thread-0...function...98
Thread-0...function...97
Thread-0...function...96
Thread-0...function...95
Thread-0...function...94
Thread-0...function...93
Thread-0...function...92
Thread-0...function...91
Thread-0...function...90
Thread-0...function...89
Thread-0...function...88
Thread-0...function...87
Thread-0...function...86
Thread-0...function...85
Thread-1...function...84
Thread-1...function...83
Thread-1...function...82
Thread-1...function...81
Thread-0...function...80
Thread-0...function...79
Thread-0...function...78
Thread-0...function...77
Thread-0...function...76
Thread-0...function...75
Thread-0...function...74
Thread-1...function...73
Thread-1...function...72
Thread-0...function...71
Thread-0...function...70
Thread-0...function...69
Thread-1...function...68
Thread-1...function...67
Thread-1...function...66
Thread-0...function...65
Thread-0...function...64
Thread-0...function...63
Thread-0...function...62
Thread-1...function...61
Thread-1...function...60
Thread-1...function...59
Thread-1...function...58
Thread-0...function...57
Thread-0...function...56
Thread-0...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-0...function...46
Thread-0...function...45
Thread-0...function...44
Thread-0...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-0...function...26
Thread-0...function...25
Thread-0...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-0...function...15
Thread-0...function...14
Thread-0...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-0...function...6
Thread-0...function...5
Thread-1...function...4
Thread-1...function...3
Thread-1...function...2
Thread-1...function...1

发现竟然都在同步函数里面执行，什么原因呢？原因在于：主线程开启以后一口气运行到了66行代码，注意这里虽然线程0和线程1被开启，即具有了执行资格，但是还不具备执行权，执行权仍然被主线程占有着，等待主线程完事之后，线程0和线程1在真正抢到执行权的时候，发现flag = false,于是全部走进了同步函数！！！要想使得线程0和线程1分别走进不同的同步方法，需要在开启线程0之后，让主线程停顿一会，让线程0开始执行（此时线程1还没有被开启，是不可能执行的），进入同步代码块，然后等到主线程再执行的时候就会把flag置为false，开启线程1，线程1就可以进入同步函数。

具体改动如下：

 1 public static void main(String[] args) 
 2     {
 3 
 4         Ticket_2 t = new Ticket_2();
 5 
 6         Thread seller1 = new Thread(t);
 7         Thread seller2 = new Thread(t);
 8 
 9         
10         seller1.start(); //在同步代码块执行
11         
12         // 主线程停顿一会，给线程0执行的机会！！
13         try {
14             Thread.sleep(20);
15         } catch (InterruptedException e) {
16             // TODO Auto-generated catch block
17             e.printStackTrace();
18         }
19         
20         t.flag = false; // 标志变为false，使得下一个线程在同步函数执行
21         seller2.start();
22 
23     }

运行结果：

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

可以看到，线程用到了不同的同步方式：同步函数（1线程）和同步代码块（0线程）！但是，也看到89，88,87,86号票被出售了两次，而且还售出了0号票（每次执行情况会不同，但是都会出现类似的线程问题），这说明线程0和线程1没有同步，即二者目前使用的锁不一致，即同步函数使用的不是Object 类型的锁！

我们知道类里面的函数都默认持有this代表着调用该类的对象，同步函数当让持有this啦，试着把同步代码块的锁改为this:

synchronized(this)

然后多次执行，结果如下：

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

发现线程问题被解决了！所以验证了同步函数使用的同步锁就是 this!

虽然同步函数书写较为简单（作为同步代码块的简写形式，二者功能一致），但是建议使用同步代码块，因为同步函数使用的锁唯一，而同步代码块可以使用任意对象作为锁，只在需要的若干条语句自由加锁，直观。

但是，注意到，如果需要同步的函数是静态的呢？因为静态函数属于类而不是具体对象，所以静态函数中是不存在this的，所以如果同步函数是静态的，锁显然就不是this！！！

继续按照上面思路验证,在上面程序的基础上，把同步函数改为静态，同时由于静态函数使用了num, 所以把num也改为静态，改动如下：

private static int num = 100;

....


public static synchronized void show()

运行如下：

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

看到86，85 ，0等处又出现了线程安全问题。这说明，显然静态函数使用的不是锁不是this, 当然静态函数不可能持有this的，与我们预料的一样！

然而每一个类都属于它所在的字节码文件对象，虽然静态函数不属于具体的对象，而是属于一个类，所以静态函数必然持有字节码文件的对象，显然这个对象是静态的！

于是在上面改动的基础上再作如下改动：

synchronized(Ticket_2.class)

... ...

运行结果：

Thread-0...block...100
Thread-0...block...99
Thread-0...block...98
Thread-0...block...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-0...block...68
Thread-0...block...67
Thread-0...block...66
Thread-0...block...65
Thread-0...block...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

所以静态的同步函数使用的锁是该函数所述的字节码文件对象，该对象可以用 getClass方法获取，也可以用当前类名.class 表示，但是此时由于getClass()是非静态方法，所以只能用类名.class。

十六、单例模式的线程问题的解决方案目录

 1 package thread.demo;
 2 //单例模式
 3 //饿汉式
 4 /*
 5 class Single
 6 {
 7     private static final Single s = new Single();
 8     private Single(){}
 9     public static Single getInstance()
10     {
11         return s;
12     }
13 }
14 */
15 
16 //懒汉式
17 
18 class Single_l
19 {
20     private static Single_l s = null;
21     private Single_l(){}
22     public static Single_l getInstance()
23     {
24         if (s == null)
25         {
26             synchronized(Single_l.class)
27             {
28                 if (s == null)
29                     s = new Single_l();
30             }
31         }
32         return s;
33     }    
34 }
35 
36 
37 public class SingleDemo {
38 
39     /**
40      * @param args
41      */
42     public static void main(String[] args) {
43         // TODO Auto-generated method stub
44 
45     }
46 
47 }

由于懒汉式单例模式的同步代码块的代码有多条，所以可能会出现线程安全问题，所以需要进行同步。

十七、多线程死锁示例目录

 1 package thread.demo;
 2 //买票：四个售票员一起卖100张票
 3 class Ticket_3 implements Runnable 
 4 {
 5     private int num = 100;
 6     Object obj = new Object();
 7     boolean flag = true;
 8     public void run() 
 9     { 
10         if (flag) 
11         {
12             while (true) 
13             {
14                 synchronized(obj)
15                 {
16                      show();//同步代码块中使用同步函数
17                 }
18             }    
19         }//end if
20         else
21             while (true) show();
22     }
23     
24     public synchronized void show() 
25     {
26         synchronized(obj)//同步函数中使用同步代码块
27         {
28             if (num > 0) 
29             {
30                 // 让线程sleep一会，好让打印语句还没来得及执行，其他线程
31                 // 就切换进来，这样方便我们观测线程安全隐患
32                 try { 
33                     Thread.sleep(10);
34                 } catch (InterruptedException e) {
35                     // TODO Auto-generated catch block
36                     e.printStackTrace();
37                 }
38                 System.out.println(Thread.currentThread().getName() + "...function..." + num--);
39             } //end if 
40         }//end synchronized
41     } 
42 }
43 
44 public class DeadLock
45 {
46     public static void main(String[] args) 
47     {
48 
49         Ticket_3 t = new Ticket_3();
50 
51         Thread seller1 = new Thread(t);
52         Thread seller2 = new Thread(t);
53 
54         
55         seller1.start(); //在同步代码块执行
56         
57         // 主线程停顿一会，给线程0执行的机会！！
58         try {
59             Thread.sleep(20);
60         } catch (InterruptedException e) {
61             // TODO Auto-generated catch block
62             e.printStackTrace();
63         }
64         
65         t.flag = false; // 标志变为false，使得下一个线程在同步函数执行
66         seller2.start();
67 
68     }
69 }

程序解释：这是一个同步锁嵌套的情况，可以看到在第14行中的同步代码块中使用了同步函数，而在第24行的同步函数中使用了同步代码块，即两个同步锁互相嵌套。下面分析程序的运行：

程序从主函数开始，运行到58行之后，主线程停顿，线程0得到cpu执行权，flag = true, 执行到14 行，线程 0 拿到同步锁 obj, 执行同步代码块，而同步代码块的语句就是调用24行的同步函数，而同步函数里面是26行开始的同步代码块，执行同步代码块，就需要判断锁 obj, 线程0确实持有锁 obj，于是顺利进入26行开始的同步代码块执行，
线程0执行一会，由于代码块中有sleep的存在，主线程有可能切入进来占有cpu执行权，线程0就停顿一会，主线程继续由65行开始执行，flag = false, 线程1开启，然后主线程，0线程，1线程随机切换。
假如刚才停顿的线程0又抢到了cpu执行权，线程0继续执行，再次拿到同步代码块锁obj, 进入到第15行，存在一种情况，就在此时线程1抢到cpu执行权，线程0则持有着代码块的锁obj，并瞅准机会去拿到同步函数的锁this从16行出进入同步函数.
线程1获得cpu执行权之后，由于flag == false, 执行到第21行，拿到同步函数锁this进入同步函数show(), 然后在第26行准备拿锁obj的时候，却发现被obj被线程0占有，只能等待，而线程0即使抢到了cpu执行权，但是同步函数锁this却被线程1占有，也没法继续，于是程序就停顿不前，这就是死锁产生的一个过程！

多次执行上面的代码，会出现类似下面的情况：

即程序陷入了死锁，无法继续运行，所有线程0和线程1都处于等待状态！！

上面的程序只展示了死锁的样子，下面给出一个较为简单的死锁示例备用：

 1 package thread.demo;
 2 class MyLock
 3 {
 4     //“两把锁”
 5     public static final Object locka = new Object();
 6     public static final Object lockb = new Object();
 7 }
 8 class Test implements Runnable
 9 {
10     private boolean flag;
11     Test(boolean flag)
12     {
13         this.flag = flag;
14     }
15     @Override
16     public void run() 
17     {
18         if (flag)
19         {
20             synchronized(MyLock.locka)
21             {
22                 System.out.println("if...loacka");
23                 synchronized(MyLock.lockb)
24                 {
25                     System.out.println("if...loackb");
26                 }
27             }
28         }
29         else
30         {
31             synchronized(MyLock.lockb)
32             {
33                 System.out.println("else...loackb");
34                 synchronized(MyLock.locka)
35                 {
36                     System.out.println("else...loacka");
37                 }
38             }
39         }
40     }
41     
42 }
43 public class DeadLockTest {
44 
45     /**
46      * @param args
47      */
48     public static void main(String[] args) {
49         Test a = new Test(true);
50         Test b = new Test(false);
51         Thread t1 = new Thread(a);
52         Thread t2 = new Thread(b);
53         
54         t1.start();
55         t2.start();
56     }
57 
58 }

运行结果：

后续：下一篇博文将会记载多线程间通信的学习笔记。

参考文献：传智播客JAVA SE视频教程，李刚《疯狂JAVA讲义》

posted @ 2015-02-27 20:14 90Zeng 阅读(4005) 评论(6) 编辑收藏举报

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