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Java多线程技术学习笔记（一）

时间：2015-02-27 21:29:55      阅读：538      评论：0      收藏：0      [点我收藏+]

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目录：

  1. 概述
  2. 多线程的好处与弊端
  3. JVM中的多线程解析
  4. 多线程的创建方式之一：继承Thread类
  5. 线程的状态
  6. 多线程创建的方式之二：实现Runnable接口
  7. 使用方式二创建多线程的好处
  8. 多线程示例
  9. 线程安全问题现象
  10. 线程安全问题产生的原因
  11. 同步代码块
  12. 同步的好处与弊端
  13. 同步的前提
  14. 同步函数
  15. 验证同步函数的锁
  16. 单例模式的线程安全问题的解决方案
  17. 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：

技术分享

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：

技术分享

技术分享

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：

技术分享 
 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：

技术分享 
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：

技术分享

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：

技术分享

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。

技术分享 
 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：

技术分享 
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

 技术分享

 

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：

技术分享

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）：

技术分享

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：

 技术分享

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：

技术分享

 

 

 

 

 

 

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》

 

Java多线程技术学习笔记（一）

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原文地址：http://www.cnblogs.com/90zeng/p/java_multithread_1.html
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