java中的线程

如何创建一个控制单元或线程？
第一种方式：
1、继承Thread类。覆盖run方法，将多线程要运行的代码放到run方法中。
2、调用start方法，开启线程，并执行run方法。

<wbr><wbr> ※特性：随机性(每次执行的结果不一样，多个线程抢资源的结果)<br><wbr><wbr> ※创建一个Thread类或其子类，才能够产生一个控制单元.</wbr></wbr></wbr></wbr>

第二种方式：
1、实现Runnable接口，覆盖run方法.
2、调用Thread类建立线程对象，并将实现Runnable接口的子类对象作为参数传递给Thread的构造函数.
3、调用Thread类对象的start方法，开启并运行run方法.

为什么要将Runnable接口的子类对象作为参数传递给Thread类的构造方法呢？为了让多线程去执行自定义的run方法，该
run方法有自己所属的对象，所以将该对象传入.

<wbr>线程的安全问题，产生的原因：<br><wbr>1、多个线程访问出现延迟；<br><wbr>2、线程的随机性；<br><wbr>同步：<br><wbr>1、解决线程的安全问题，同时会消耗资源，降低效率（每个线程都需要去判断同步上的锁）. 例子：火车上的卫生间（锁）;<br><wbr>2、需要同步的两个前提：①至少两个以上的线程；②多线程使用的是同一把锁；<br><wbr>3、注意同步的死锁，尽量避免同步嵌套的情况.<br><wbr>4、同步的两种表现形式：<br><wbr>①同步代码块，传递一个对象作为锁.<br><wbr>②同步函数，使用this作为锁。使用同步函数时，一定要注意分析哪些需要同步，在Tickets中，如将synchronized放到run方法上，<br><wbr>那么线程进入while无限循环，无法退出，锁无法被其他线程使用.所以，while循环不需要同步，而其中的内容才是需要同步的内容.<br><wbr><br><wbr>注意：1、线程在同步时，还是出现了安全隐患，结果很大程序出现在线程同步问题上，检查是否多线程使用的同一把锁.<br><wbr><wbr><wbr> 2、静态方法的同步函数，锁肯定不是this，因为静态方法不能使用this，这是，锁为方法所在的类的class对象，例如，Tickets.class.<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

class MyThread extends Thread
{
<wbr>private String name;<br><wbr>MyThread(String name)<br><wbr>{<br><wbr><wbr>this.name = name;<br><wbr>}<br><wbr>public void run() //覆盖run方法<br><wbr>{<br><wbr><wbr>for (int x = 0; x &lt; 10 ; x++)<br><wbr><wbr>{<br><wbr><wbr><wbr>System.out.println(Thread.currentThread().getName() + "..name:" + name + "-" + x);<br><wbr><wbr>}<br><wbr>}<br> }</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

//铁路售票例子，四个窗口同时售票
class Tickets implements Runnable
{
<wbr>private static int ticketsCnt = 100; //总共100张票<wbr><br><wbr>Object obj = new Object();<br><wbr>boolean b = true;<br><wbr>public void run()<br><wbr>{<br><wbr><wbr>//for (int x = ticketsCnt; x &gt; 0; x--)<wbr> //为什么for循环这样写不行呢？<br><wbr><wbr>if (b)<br><wbr><wbr>{<wbr><wbr><br><wbr><wbr><wbr>while (true)<br><wbr><wbr><wbr>{<wbr><wbr><wbr><wbr><br><wbr><wbr><wbr><wbr>show();<wbr><wbr><wbr><br><wbr><wbr><wbr>}<br><wbr><wbr>}<br><wbr><wbr>else<br><wbr><wbr>{<br><wbr><wbr><wbr>while (true)<br><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr>synchronized(Tickets.class) //(this) //obj<br><wbr><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr><wbr>if (ticketsCnt &gt; 0)<br><wbr><wbr><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr><wbr><wbr>try<br><wbr><wbr><wbr><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>Thread.sleep(10);<wbr>//模拟线程的安全问题。<br><wbr><wbr><wbr><wbr><wbr><wbr>}<br><wbr><wbr><wbr><wbr><wbr><wbr>catch (Exception e)<br><wbr><wbr><wbr><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr><wbr><wbr>//<br><wbr><wbr><wbr><wbr><wbr><wbr>}<br><wbr><wbr><wbr><wbr><wbr><wbr>System.out.println(Thread.currentThread().getName() + " -&gt;code.... " +ticketsCnt--);<wbr><wbr><wbr><br><wbr><wbr><wbr><wbr><wbr>}<br><wbr><wbr><wbr><wbr>}<br><wbr><wbr><wbr>}<wbr><wbr><wbr><br><wbr><wbr>}<br><wbr>}</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr>public synchronized static void show()<wbr> //同步函数，添加关键字synchronized，锁为this,如果声明为static，则锁为Tickets.class<br><wbr>{<br><wbr><wbr>if (ticketsCnt &gt; 0)<br><wbr><wbr>{<br><wbr><wbr><wbr>try<br><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr>Thread.sleep(10);<wbr>//模拟线程的安全问题。<br><wbr><wbr><wbr>}<br><wbr><wbr><wbr>catch (Exception e)<br><wbr><wbr><wbr>{<br><wbr><wbr><wbr><wbr>//<br><wbr><wbr><wbr>}<br><wbr><wbr><wbr>System.out.println(Thread.currentThread().getName() + " -&gt;func.... " +ticketsCnt--);<wbr><wbr><wbr><br><wbr><wbr>}<br><wbr>}<br> }</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

class ThreadDemo
{
<wbr>public static void main(String[] args) // 当前程序有三个线程，包括主线程！<br><wbr>{<br><wbr><wbr><br><wbr><wbr>//MyThread d1 = new MyThread("one");<wbr> //线程1<br><wbr><wbr>//MyThread d2 = new MyThread("two -");//线程2<br><wbr><wbr>//d1.start();<br><wbr><wbr>//d2.start();<br><wbr><wbr><br><wbr><wbr>//主线程<br><wbr><wbr>//for (int n = 0; n &lt; 10; n++)<br><wbr><wbr>//{<br><wbr><wbr>//<wbr>System.out.println(Thread.currentThread().getName() + ".." + n);<br><wbr><wbr>//}<br><wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr><wbr>Tickets t = new Tickets();<br><wbr><wbr>Thread t1 = new Thread(t);<br><wbr><wbr>Thread t2 = new Thread(t);<br><wbr><wbr>//Thread t3 = new Thread(t);<br><wbr><wbr>//Thread t4 = new Thread(t);<br><wbr><wbr><br><wbr><wbr>t1.start();<br><wbr><wbr>try<br><wbr><wbr>{<br><wbr><wbr><wbr>Thread.sleep(10);<wbr>//让主线程停下来，不让程序首先执行 t.b = false；<br><wbr><wbr>}<br><wbr><wbr>catch (Exception e)<br><wbr><wbr>{<br><wbr><wbr><wbr>//<br><wbr><wbr>}<br><wbr><wbr>t.b = false;</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr><wbr>t2.start();<br><wbr><wbr>//t3.start();<br><wbr><wbr>//t4.start();<br><wbr><wbr><br><wbr>}<br> }<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>