目录：
- 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：
View Code运行结果：
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。
View Code运行结果：
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：
按 Ctrl+C 复制代码
按 Ctrl+C 复制代码
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》
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