Java：创建线程

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);
        }
    }