目录：
一、概述
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
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
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 args12 */13 public static void main(String[] args) {14 // TODO Auto-generated method stub15 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 okdemo 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
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 args18 */19 public static void main(String[] args) {20 // TODO Auto-generated method stub21 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 }
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...i9xiaoqiang...i0xiaoqiang...i1xiaoqiang...i2xiaoqiang...i3xiaoqiang...i4xiaoqiang...i5xiaoqiang...i6xiaoqiang...i7xiaoqiang...i8xiaoqiang...i9
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 args21 */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 }
运行结果：
xiaoqiang...i0旺财...i0xiaoqiang...i1旺财...i1旺财...i2旺财...i3xiaoqiang...i2xiaoqiang...i3xiaoqiang...i4xiaoqiang...i5xiaoqiang...i6xiaoqiang...i7旺财...i4xiaoqiang...i8xiaoqiang...i9旺财...i5旺财...i6旺财...i7旺财...i8旺财...i9
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 args21 */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：
overxiaoqiang...i0旺财...i0xiaoqiang...i1旺财...i1xiaoqiang...i2旺财...i2xiaoqiang...i3xiaoqiang...i4旺财...i3xiaoqiang...i5旺财...i4xiaoqiang...i6旺财...i5xiaoqiang...i7旺财...i6xiaoqiang...i8旺财...i7xiaoqiang...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 args21 */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-0xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...i...name: Thread-1xiaoqiang...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
六、多线程创建的方式之二：实现Runnable接口
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 args19 */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-1Thread-0Thread-1Thread-0Thread-1Thread-0Thread-0Thread-1Thread-0Thread-1Thread-0Thread-1Thread-0Thread-1Thread-0Thread-1Thread-1Thread-1Thread-0Thread-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
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
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...100Thread-3...sale...97Thread-2...sale...98Thread-1...sale...99Thread-2...sale...94Thread-3...sale...95Thread-0...sale...96Thread-3...sale...91Thread-2...sale...92Thread-2...sale...88Thread-2...sale...87Thread-2...sale...86Thread-1...sale...93Thread-1...sale...84Thread-1...sale...83Thread-2...sale...85Thread-3...sale...89Thread-0...sale...90Thread-3...sale...80Thread-2...sale...81Thread-1...sale...82Thread-1...sale...76Thread-1...sale...75Thread-2...sale...77Thread-3...sale...78Thread-0...sale...79Thread-3...sale...72Thread-2...sale...73Thread-1...sale...74Thread-2...sale...69Thread-3...sale...70Thread-0...sale...71Thread-3...sale...66Thread-2...sale...67Thread-1...sale...68Thread-2...sale...63Thread-2...sale...61Thread-3...sale...64Thread-0...sale...65Thread-3...sale...59Thread-2...sale...60Thread-1...sale...62Thread-2...sale...56Thread-3...sale...57Thread-0...sale...58Thread-3...sale...53Thread-2...sale...54Thread-1...sale...55Thread-1...sale...49Thread-2...sale...50Thread-3...sale...51Thread-0...sale...52Thread-0...sale...45Thread-3...sale...46Thread-2...sale...47Thread-1...sale...48Thread-2...sale...42Thread-3...sale...43Thread-0...sale...44Thread-3...sale...39Thread-2...sale...40Thread-1...sale...41Thread-2...sale...36Thread-3...sale...37Thread-0...sale...38Thread-3...sale...33Thread-2...sale...34Thread-1...sale...35Thread-2...sale...30Thread-3...sale...31Thread-0...sale...32Thread-3...sale...27Thread-2...sale...28Thread-1...sale...29Thread-2...sale...24Thread-3...sale...25Thread-0...sale...26Thread-3...sale...21Thread-2...sale...22Thread-1...sale...23Thread-2...sale...18Thread-3...sale...19Thread-0...sale...20Thread-3...sale...15Thread-2...sale...16Thread-1...sale...17Thread-2...sale...12Thread-3...sale...13Thread-0...sale...14Thread-3...sale...9Thread-2...sale...10Thread-1...sale...11Thread-2...sale...6Thread-3...sale...7Thread-0...sale...8Thread-3...sale...3Thread-2...sale...4Thread-1...sale...5Thread-3...sale...1Thread-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
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 block14 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...100Thread-2...sale...98Thread-1...sale...100Thread-0...sale...99Thread-0...sale...95Thread-3...sale...97Thread-2...sale...97Thread-1...sale...96Thread-2...sale...94Thread-3...sale...94Thread-1...sale...94Thread-0...sale...94Thread-0...sale...93Thread-3...sale...91Thread-1...sale...92Thread-2...sale...93Thread-3...sale...88Thread-0...sale...89Thread-2...sale...89Thread-1...sale...90Thread-0...sale...87Thread-2...sale...84Thread-3...sale...85Thread-1...sale...86Thread-2...sale...83Thread-3...sale...82Thread-1...sale...82Thread-0...sale...83Thread-0...sale...81Thread-3...sale...80Thread-1...sale...80Thread-2...sale...81Thread-3...sale...78Thread-0...sale...79Thread-1...sale...79Thread-2...sale...77Thread-2...sale...76Thread-1...sale...75Thread-3...sale...75Thread-0...sale...76Thread-2...sale...74Thread-1...sale...73Thread-3...sale...73Thread-0...sale...74Thread-0...sale...72Thread-1...sale...70Thread-3...sale...71Thread-2...sale...69Thread-0...sale...68Thread-2...sale...65Thread-1...sale...67Thread-3...sale...66Thread-1...sale...64Thread-2...sale...61Thread-0...sale...62Thread-3...sale...63Thread-2...sale...59Thread-1...sale...57Thread-3...sale...60Thread-0...sale...58Thread-2...sale...56Thread-1...sale...55Thread-3...sale...54Thread-0...sale...53Thread-1...sale...52Thread-2...sale...51Thread-0...sale...50Thread-3...sale...49Thread-2...sale...48Thread-1...sale...47Thread-3...sale...46Thread-0...sale...45Thread-1...sale...44Thread-3...sale...42Thread-0...sale...43Thread-2...sale...41Thread-1...sale...39Thread-0...sale...38Thread-3...sale...40Thread-2...sale...37Thread-2...sale...36Thread-0...sale...34Thread-3...sale...35Thread-1...sale...33Thread-1...sale...32Thread-2...sale...31Thread-3...sale...30Thread-0...sale...29Thread-0...sale...28Thread-3...sale...27Thread-1...sale...26Thread-2...sale...25Thread-1...sale...24Thread-2...sale...23Thread-0...sale...22Thread-3...sale...21Thread-3...sale...20Thread-1...sale...18Thread-2...sale...19Thread-0...sale...20Thread-2...sale...17Thread-3...sale...15Thread-0...sale...16Thread-1...sale...14Thread-3...sale...13Thread-0...sale...12Thread-2...sale...11Thread-1...sale...10Thread-0...sale...9Thread-3...sale...8Thread-1...sale...7Thread-2...sale...6Thread-3...sale...5Thread-0...sale...4Thread-2...sale...3Thread-1...sale...2Thread-3...sale...1Thread-0...sale...0Thread-2...sale...-1Thread-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
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
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 block16 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...100Thread-0...sale...99Thread-0...sale...98Thread-0...sale...97Thread-0...sale...96Thread-0...sale...95Thread-0...sale...94Thread-0...sale...93Thread-0...sale...92Thread-0...sale...91Thread-0...sale...90Thread-0...sale...89Thread-0...sale...88Thread-2...sale...87Thread-2...sale...86Thread-2...sale...85Thread-3...sale...84Thread-3...sale...83Thread-3...sale...82Thread-3...sale...81Thread-1...sale...80Thread-1...sale...79Thread-1...sale...78Thread-1...sale...77Thread-1...sale...76Thread-1...sale...75Thread-3...sale...74Thread-2...sale...73Thread-2...sale...72Thread-2...sale...71Thread-2...sale...70Thread-2...sale...69Thread-2...sale...68Thread-0...sale...67Thread-0...sale...66Thread-0...sale...65Thread-0...sale...64Thread-0...sale...63Thread-0...sale...62Thread-2...sale...61Thread-2...sale...60Thread-3...sale...59Thread-3...sale...58Thread-3...sale...57Thread-3...sale...56Thread-3...sale...55Thread-3...sale...54Thread-3...sale...53Thread-3...sale...52Thread-3...sale...51Thread-3...sale...50Thread-3...sale...49Thread-3...sale...48Thread-3...sale...47Thread-3...sale...46Thread-3...sale...45Thread-3...sale...44Thread-3...sale...43Thread-3...sale...42Thread-3...sale...41Thread-1...sale...40Thread-1...sale...39Thread-1...sale...38Thread-1...sale...37Thread-1...sale...36Thread-1...sale...35Thread-1...sale...34Thread-1...sale...33Thread-1...sale...32Thread-1...sale...31Thread-1...sale...30Thread-1...sale...29Thread-1...sale...28Thread-1...sale...27Thread-1...sale...26Thread-1...sale...25Thread-3...sale...24Thread-2...sale...23Thread-2...sale...22Thread-2...sale...21Thread-0...sale...20Thread-0...sale...19Thread-0...sale...18Thread-0...sale...17Thread-2...sale...16Thread-2...sale...15Thread-2...sale...14Thread-2...sale...13Thread-3...sale...12Thread-3...sale...11Thread-3...sale...10Thread-3...sale...9Thread-3...sale...8Thread-3...sale...7Thread-1...sale...6Thread-1...sale...5Thread-1...sale...4Thread-1...sale...3Thread-3...sale...2Thread-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
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
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
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 args26 */27 public static void main(String[] args) {28 // TODO Auto-generated method stub29 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 ＝ 100sum ＝ 200sum ＝ 300sum ＝ 500sum ＝ 400sum ＝ 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 ＝ 200sum ＝ 200sum ＝ 400sum ＝ 400sum ＝ 600sum ＝ 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 block11 e.printStackTrace();12 }13 System.out.println("sum ＝ " + sum);14 }15 }16 }
运行结果：
sum ＝ 100sum ＝ 200sum ＝ 300sum ＝ 400sum ＝ 500sum ＝ 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 block11 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
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 block22 e.printStackTrace();23 }24 System.out.println(Thread.currentThread().getName() + "...bloc k..." + num--);25 }26 }27 }28 29 }//end if30 else31 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 block44 e.printStackTrace();45 }46 System.out.println(Thread.currentThread().getName() + "...function..." + num--);47 }48 } 49 }50 51 public class SynFunctionLockDemo52 {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...100Thread-0...function...99Thread-0...function...98Thread-0...function...97Thread-0...function...96Thread-0...function...95Thread-0...function...94Thread-0...function...93Thread-0...function...92Thread-0...function...91Thread-0...function...90Thread-0...function...89Thread-0...function...88Thread-0...function...87Thread-0...function...86Thread-0...function...85Thread-1...function...84Thread-1...function...83Thread-1...function...82Thread-1...function...81Thread-0...function...80Thread-0...function...79Thread-0...function...78Thread-0...function...77Thread-0...function...76Thread-0...function...75Thread-0...function...74Thread-1...function...73Thread-1...function...72Thread-0...function...71Thread-0...function...70Thread-0...function...69Thread-1...function...68Thread-1...function...67Thread-1...function...66Thread-0...function...65Thread-0...function...64Thread-0...function...63Thread-0...function...62Thread-1...function...61Thread-1...function...60Thread-1...function...59Thread-1...function...58Thread-0...function...57Thread-0...function...56Thread-0...function...55Thread-1...function...54Thread-1...function...53Thread-1...function...52Thread-1...function...51Thread-1...function...50Thread-1...function...49Thread-1...function...48Thread-1...function...47Thread-0...function...46Thread-0...function...45Thread-0...function...44Thread-0...function...43Thread-1...function...42Thread-1...function...41Thread-1...function...40Thread-1...function...39Thread-1...function...38Thread-1...function...37Thread-1...function...36Thread-1...function...35Thread-1...function...34Thread-1...function...33Thread-1...function...32Thread-1...function...31Thread-1...function...30Thread-1...function...29Thread-1...function...28Thread-1...function...27Thread-0...function...26Thread-0...function...25Thread-0...function...24Thread-1...function...23Thread-1...function...22Thread-1...function...21Thread-1...function...20Thread-1...function...19Thread-1...function...18Thread-1...function...17Thread-1...function...16Thread-0...function...15Thread-0...function...14Thread-0...function...13Thread-1...function...12Thread-1...function...11Thread-1...function...10Thread-1...function...9Thread-1...function...8Thread-1...function...7Thread-0...function...6Thread-0...function...5Thread-1...function...4Thread-1...function...3Thread-1...function...2Thread-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 block17 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...100Thread-0...bloc k...99Thread-1...function...98Thread-0...bloc k...96Thread-1...function...97Thread-0...bloc k...94Thread-1...function...95Thread-0...bloc k...93Thread-1...function...92Thread-0...bloc k...91Thread-1...function...90Thread-1...function...89Thread-0...bloc k...89Thread-0...bloc k...88Thread-1...function...88Thread-0...bloc k...87Thread-1...function...87Thread-1...function...86Thread-0...bloc k...86Thread-1...function...84Thread-0...bloc k...85Thread-1...function...82Thread-0...bloc k...83Thread-0...bloc k...80Thread-1...function...81Thread-0...bloc k...79Thread-1...function...78Thread-0...bloc k...77Thread-1...function...76Thread-1...function...75Thread-0...bloc k...74Thread-1...function...73Thread-0...bloc k...72Thread-1...function...71Thread-0...bloc k...70Thread-1...function...69Thread-0...bloc k...68Thread-1...function...67Thread-0...bloc k...66Thread-1...function...65Thread-0...bloc k...64Thread-1...function...63Thread-0...bloc k...62Thread-1...function...61Thread-0...bloc k...60Thread-1...function...59Thread-0...bloc k...58Thread-1...function...57Thread-0...bloc k...56Thread-1...function...55Thread-0...bloc k...54Thread-1...function...53Thread-0...bloc k...52Thread-1...function...51Thread-0...bloc k...50Thread-1...function...49Thread-0...bloc k...48Thread-1...function...47Thread-0...bloc k...46Thread-1...function...45Thread-0...bloc k...44Thread-1...function...43Thread-0...bloc k...42Thread-1...function...41Thread-0...bloc k...40Thread-1...function...39Thread-0...bloc k...38Thread-1...function...37Thread-0...bloc k...36Thread-1...function...35Thread-0...bloc k...34Thread-1...function...33Thread-0...bloc k...32Thread-1...function...31Thread-0...bloc k...30Thread-1...function...29Thread-0...bloc k...28Thread-1...function...27Thread-0...bloc k...26Thread-1...function...25Thread-0...bloc k...24Thread-1...function...23Thread-0...bloc k...22Thread-1...function...21Thread-0...bloc k...20Thread-1...function...19Thread-0...bloc k...18Thread-0...bloc k...17Thread-1...function...16Thread-1...function...15Thread-0...bloc k...14Thread-1...function...13Thread-0...bloc k...12Thread-1...function...11Thread-0...bloc k...10Thread-1...function...9Thread-0...bloc k...8Thread-1...function...7Thread-0...bloc k...6Thread-1...function...5Thread-0...bloc k...4Thread-1...function...3Thread-0...bloc k...2Thread-1...function...1Thread-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...100Thread-0...bloc k...99Thread-0...bloc k...98Thread-0...bloc k...97Thread-0...bloc k...96Thread-0...bloc k...95Thread-0...bloc k...94Thread-0...bloc k...93Thread-0...bloc k...92Thread-0...bloc k...91Thread-0...bloc k...90Thread-0...bloc k...89Thread-0...bloc k...88Thread-0...bloc k...87Thread-0...bloc k...86Thread-1...function...85Thread-1...function...84Thread-1...function...83Thread-0...bloc k...82Thread-0...bloc k...81Thread-0...bloc k...80Thread-0...bloc k...79Thread-1...function...78Thread-1...function...77Thread-1...function...76Thread-0...bloc k...75Thread-0...bloc k...74Thread-0...bloc k...73Thread-0...bloc k...72Thread-0...bloc k...71Thread-1...function...70Thread-1...function...69Thread-0...bloc k...68Thread-0...bloc k...67Thread-0...bloc k...66Thread-1...function...65Thread-1...function...64Thread-1...function...63Thread-1...function...62Thread-1...function...61Thread-0...bloc k...60Thread-0...bloc k...59Thread-1...function...58Thread-1...function...57Thread-1...function...56Thread-1...function...55Thread-0...bloc k...54Thread-0...bloc k...53Thread-0...bloc k...52Thread-0...bloc k...51Thread-1...function...50Thread-1...function...49Thread-1...function...48Thread-0...bloc k...47Thread-0...bloc k...46Thread-0...bloc k...45Thread-1...function...44Thread-1...function...43Thread-1...function...42Thread-0...bloc k...41Thread-0...bloc k...40Thread-0...bloc k...39Thread-0...bloc k...38Thread-0...bloc k...37Thread-0...bloc k...36Thread-0...bloc k...35Thread-0...bloc k...34Thread-1...function...33Thread-1...function...32Thread-1...function...31Thread-1...function...30Thread-1...function...29Thread-1...function...28Thread-1...function...27Thread-0...bloc k...26Thread-0...bloc k...25Thread-0...bloc k...24Thread-0...bloc k...23Thread-0...bloc k...22Thread-1...function...21Thread-0...bloc k...20Thread-0...bloc k...19Thread-0...bloc k...18Thread-0...bloc k...17Thread-1...function...16Thread-1...function...15Thread-1...function...14Thread-0...bloc k...13Thread-0...bloc k...12Thread-0...bloc k...11Thread-0...bloc k...10Thread-1...function...9Thread-1...function...8Thread-1...function...7Thread-0...bloc k...6Thread-0...bloc k...5Thread-0...bloc k...4Thread-0...bloc k...3Thread-0...bloc k...2Thread-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...100Thread-0...block...99Thread-0...block...98Thread-1...function...97Thread-1...function...96Thread-0...block...95Thread-0...block...94Thread-1...function...93Thread-1...function...92Thread-0...block...91Thread-0...block...89Thread-1...function...90Thread-1...function...88Thread-0...block...87Thread-1...function...86Thread-0...block...86Thread-1...function...85Thread-0...block...85Thread-1...function...84Thread-0...block...83Thread-1...function...82Thread-0...block...81Thread-1...function...80Thread-0...block...80Thread-0...block...79Thread-1...function...78Thread-1...function...77Thread-0...block...77Thread-0...block...76Thread-1...function...75Thread-0...block...74Thread-1...function...73Thread-1...function...72Thread-0...block...72Thread-0...block...71Thread-1...function...71Thread-1...function...70Thread-0...block...69Thread-1...function...68Thread-0...block...67Thread-1...function...66Thread-0...block...65Thread-0...block...64Thread-1...function...63Thread-0...block...62Thread-1...function...61Thread-1...function...60Thread-0...block...59Thread-1...function...58Thread-0...block...57Thread-1...function...56Thread-0...block...55Thread-1...function...53Thread-0...block...54Thread-0...block...52Thread-1...function...52Thread-0...block...51Thread-1...function...50Thread-1...function...49Thread-0...block...48Thread-1...function...47Thread-0...block...46Thread-1...function...45Thread-0...block...44Thread-0...block...43Thread-1...function...42Thread-0...block...41Thread-1...function...40Thread-1...function...39Thread-0...block...38Thread-0...block...37Thread-1...function...36Thread-0...block...35Thread-1...function...34Thread-1...function...33Thread-0...block...32Thread-0...block...31Thread-1...function...30Thread-0...block...29Thread-1...function...28Thread-0...block...27Thread-1...function...26Thread-1...function...25Thread-0...block...24Thread-0...block...23Thread-1...function...22Thread-0...block...21Thread-1...function...20Thread-1...function...19Thread-0...block...18Thread-0...block...17Thread-1...function...16Thread-0...block...15Thread-1...function...14Thread-1...function...13Thread-0...block...12Thread-0...block...11Thread-1...function...10Thread-0...block...9Thread-1...function...8Thread-1...function...7Thread-0...block...6Thread-1...function...5Thread-0...block...4Thread-0...block...3Thread-1...function...2Thread-1...function...1Thread-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...100Thread-0...block...99Thread-0...block...98Thread-0...block...97Thread-1...function...96Thread-1...function...95Thread-1...function...94Thread-1...function...93Thread-1...function...92Thread-1...function...91Thread-1...function...90Thread-1...function...89Thread-1...function...88Thread-1...function...87Thread-1...function...86Thread-1...function...85Thread-1...function...84Thread-1...function...83Thread-1...function...82Thread-1...function...81Thread-1...function...80Thread-1...function...79Thread-1...function...78Thread-1...function...77Thread-1...function...76Thread-1...function...75Thread-1...function...74Thread-1...function...73Thread-1...function...72Thread-1...function...71Thread-1...function...70Thread-1...function...69Thread-0...block...68Thread-0...block...67Thread-0...block...66Thread-0...block...65Thread-0...block...64Thread-1...function...63Thread-1...function...62Thread-1...function...61Thread-1...function...60Thread-1...function...59Thread-1...function...58Thread-1...function...57Thread-1...function...56Thread-1...function...55Thread-1...function...54Thread-1...function...53Thread-1...function...52Thread-1...function...51Thread-1...function...50Thread-1...function...49Thread-1...function...48Thread-1...function...47Thread-1...function...46Thread-1...function...45Thread-1...function...44Thread-1...function...43Thread-1...function...42Thread-1...function...41Thread-1...function...40Thread-1...function...39Thread-1...function...38Thread-1...function...37Thread-1...function...36Thread-1...function...35Thread-1...function...34Thread-1...function...33Thread-1...function...32Thread-1...function...31Thread-1...function...30Thread-1...function...29Thread-1...function...28Thread-1...function...27Thread-1...function...26Thread-1...function...25Thread-1...function...24Thread-1...function...23Thread-1...function...22Thread-1...function...21Thread-1...function...20Thread-1...function...19Thread-1...function...18Thread-1...function...17Thread-1...function...16Thread-1...function...15Thread-1...function...14Thread-1...function...13Thread-1...function...12Thread-1...function...11Thread-1...function...10Thread-1...function...9Thread-1...function...8Thread-1...function...7Thread-1...function...6Thread-1...function...5Thread-1...function...4Thread-1...function...3Thread-1...function...2Thread-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
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_l19 {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 args41 */42 public static void main(String[] args) {43 // TODO Auto-generated method stub44 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
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 if20 else21 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 block36 e.printStackTrace();37 }38 System.out.println(Thread.currentThread().getName() + "...function..." + num--);39 } //end if 40 }//end synchronized41 } 42 }43 44 public class DeadLock45 {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 block62 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 @Override16 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 else30 {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 args47 */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》