Java定义了两种创建线程的方法：

1.实现Runnable接口

要实现Runnable接口，只需简单地实现run()方法即可，声明如下：public void run()

在run()方法中，可以定义构成新线程的代码。需重点理解的是，run()可以调用其它方法、使用其他类和声明变量，就像主线程能做的那样。唯一的区别是：run()方法是程序的另一个并发的执行线程的进入店，这个线程在run()方法返回时结束。

创建新线程以后，直到调用了它的start()方法后才会执行。本质上，start()执行对run()的调用。

// Create a second thread.public class NewThread implements Runnable{    Thread t;    NewThread(){        t=new Thread(this,"Demo Thread");        System.out.println("Child thread: "+t);        t.start();    }        // This is the entry point of the second thread.    public void run(){        try{            for(int i=5;i>0;i--){                System.out.println("Child thread: "+i);                Thread.sleep(500);            }        }        catch(InterruptedException e){            System.out.println("Child thread interrupted");        }        System.out.println("Exiting child thread");    }}class ThreadDemo{    public static void main(String args[]){        new NewThread();        try{            for(int i=5;i>0;i--){            System.out.println("Main thread: "+i);            Thread.sleep(1000);        }    }    catch(InterruptedException e){        System.out.println("Main thread interrupted");    }    System.out.println("Exiting Main thread");    }}

输出：

Child thread: Thread[Demo Thread,5,main]

Main thread: 5

Child thread: 5

Child thread: 4

Main thread: 4

Child thread: 3

Child thread: 2

Main thread: 3

Child thread: 1

Exiting child thread

Main thread: 2

Main thread: 1

Exiting Main thread

 

2. 扩展Thread类本身

第二种创建线程的方法是创建一个扩展Thread类的新类，然后创建该类的一个实例。这个扩展的类必须重写run()方法，这是新线程的进入点，同时它也必须调用start()方法来执行线程。

// Create a second thread.public class NewThread extends Thread{    NewThread(){        super("Demo thread");        System.out.println("Child thread: "+this);        start();    }        // This is the entry point of the second thread.    public void run(){        try{            for(int i=5;i>0;i--){                System.out.println("Child thread: "+i);                Thread.sleep(500);            }        }        catch(InterruptedException e){            System.out.println("Child thread interrupted");        }        System.out.println("Exiting child thread");    }}class ThreadDemo{    public static void main(String args[]){        new NewThread();        try{            for(int i=5;i>0;i--){            System.out.println("Main thread: "+i);            Thread.sleep(1000);        }    }    catch(InterruptedException e){        System.out.println("Main thread interrupted");    }    System.out.println("Exiting Main thread");    }}

输出：

Child thread: Thread[Demo thread,5,main]

Main thread: 5

Child thread: 5

Child thread: 4

Main thread: 4

Child thread: 3

Child thread: 2

Main thread: 3

Child thread: 1

Exiting child thread

Main thread: 2

Main thread: 1

Exiting Main thread

 

同步：当两个或多个线程需要访问同一共享资源时，需要某种方式来确保资源在某一时刻只被一个线程使用，这个方式称为同步。

不使用同步的例子：

class Callme{     void call(String msg){        System.out.print("["+msg);        try{            Thread.sleep(1000);        }        catch(InterruptedException e){            System.out.println("Interrupted");        }        System.out.println("]");    }}class Caller implements Runnable{    String msg;    Callme target;    Thread t;    public Caller(Callme targ, String s){        target=targ;        msg=s;        t=new Thread(this);        t.start();    }        public void run(){        target.call(msg);    }}public class Synch {    public static void main(String args[]){        Callme target=new Callme();        Caller ob1=new Caller(target,"Hello");        Caller ob2=new Caller(target,"Synchronized");        Caller ob3=new Caller(target,"World");                try{            ob1.t.join();            ob2.t.join();            ob3.t.join();        }        catch(InterruptedException e){            System.out.println("Interrupted");        }    }}

输出：

[Hello[World[Synchronized]

]

]

 

使用同步，只需简单地在call()的定义前面加上关键字synchronized即可

class Callme{     synchronized void call(String msg){

输出：

[Hello]

[Synchronized]

[World]

 

当不能在类相应的方法中添加synchronized关键字时，只需要将对该类方法的访问置于一个synchronized快中

public void run(){        synchronized(target){        target.call(msg);        }    }