Java Threads 创建线程

Process and threads

Process

A process runs isolated from other processes
A process cannot directly access shared data in other processes
The resources of the process, e.g. memory and CPU time, are allocated to it via the operating system
Example: Java Virtual Machine is a process

Thread

A thread is a lightwight process
- Each has its own stack
- Can access shared data of other threads in the same process
- Every thread has its own memory cache
- If a thread reads shared data, it stores this data in its own memory cache.A thread can re-read the shared data

Key Concepts in concurrent programming

Atomicity

Cannot be interrupted
Once it starts is always completes
One example assignment a=5

Visibility

什么是线程间的可见性？
一个线程对共享变量值的修改，能够及时的被其他线程看到。
线程间的可见性

Order of execution

The order in concurrent programming is not guaranteed!

Critical code

A part of code that must be executed by a single thread at one time

Two ways of creating a thread

Creating a thread by implements


public class MyFirstRunnable implements Runnable
{
    
    
    public void run(){
    
    //implement the abstract run() method of the runnable interface
        System.out.println("Thread");
    }
}

MyFirstRunnable xyz = new MyFirstRunnable();
Thread thread = new Thread(xyz);
thread.start();

You can use the same runnable several times

Runnable runnable = new MyFirstRunnable();

for(int i =0;i<25;i++>){
    
    
    new Thread(runnable).start();
}

//How many threads will be running?
//26 together with the main thread

Creating a thread using extends

public class MyThread extends Thread{
    
    
    public void run(){
    
    //override the run() method
        System.out.println("MyThread running");
        System.out.println("do some interesting stuff");
		System.out.println("Leaving thread using MyThread extends Thread");
    }

    public static void main(String[] args) {
    
    
		// This approach uses a class the extends Thread
		MyFirstThreadExtend myFirstThreadExtend = new MyFirstThreadExtend();
		myFirstThreadExtend.start();
	}
}

Give names to a Thread

public class Order implements Runnable {
    
    
    public void run() {
    
    
    String threadName = Thread.currentThread().getName();
    System.out.println("I'm running in thread‐" + threadName);
    }

    public static void main(String[] args) {
    
    
    Order order = new Order();
        for (int i = 0; i < 25; i++) {
    
    
        Thread thread = new Thread(order);
        thread.setName("Thread " + i);
        thread.start();
        }
    }
}

One of the results:

I'm running in thread-Thread 0
I'm running in thread-Thread 2
I'm running in thread-Thread 4
I'm running in thread-Thread 3
I'm running in thread-Thread 6
I'm running in thread-Thread 1
I'm running in thread-Thread 7
I'm running in thread-Thread 9
I'm running in thread-Thread 10
I'm running in thread-Thread 11
I'm running in thread-Thread 5
I'm running in thread-Thread 12
I'm running in thread-Thread 16
I'm running in thread-Thread 18
I'm running in thread-Thread 14
I'm running in thread-Thread 15
I'm running in thread-Thread 19
I'm running in thread-Thread 8
I'm running in thread-Thread 13
I'm running in thread-Thread 17
I'm running in thread-Thread 22
I'm running in thread-Thread 24
I'm running in thread-Thread 21
I'm running in thread-Thread 20
I'm running in thread-Thread 23

当java虚拟机（也就是经常说的 JVM）执行main方法的时候，他会找操作系统（也就是 os）开辟一条main方法通向cpu的路径，这个路径就是main线程，也就是所说的主线程。

接着cpu通过这个线程，也就是这个路径，就可以执行main方法了。

当我们new了一个thread对象，这个时候又开辟了一条通向cpu的路径，而这条路径是用来执行run方法的。

而现在对于cpu而言，他就有了两条执行的路径，cpu就有了选择的权利，cpu他喜欢谁，就会执行哪条路径，我们是控制不了cpu的，所有就有了出现随机打印的结果。

反过来说，两个线程，一个main线程，一个新线程，他们一起抢夺cpu的执行权（即 cpu的执行时间），谁抢到了，谁先执行。

Inline definition

inline:

Thread t = new Thread(new Runnable()
{
    
     public void run() {
    
     /*stuff here*/ } });
t.start();

Difference between mythread.run() and mythread.start()

Java中Thread.start和Thread.run

start() : 它的作用是启动一个新线程，新线程会执行相应的run()方法。start()不能被重复调用。
run() : run()就和普通的成员方法一样，可以被重复调用。单独调用run()的话，会在当前线程中执行run()，而并不会启动新线程。

// Demo.java 的源码
class MyThread extends Thread{
    
    
    public MyThread(String name) {
    
    
        super(name);
    }

    public void run(){
    
    
        System.out.println(Thread.currentThread().getName()+" is running");
    }
};

public class Demo {
    
    
    public static void main(String[] args) {
    
    
        Thread mythread=new MyThread("mythread");

        System.out.println(Thread.currentThread().getName()+" call mythread.run()");
        mythread.run();

        System.out.println(Thread.currentThread().getName()+" call mythread.start()");
        mythread.start();
    }
}

运行结果：

main call mythread.run()
main is running
main call mythread.start()
mythread is running

结果说明：
(01) Thread.currentThread().getName()是用于获取“当前线程”的名字。当前线程是指正在cpu中调度执行的线程。
(02) mythread.run()是在“主线程main”中调用的，该run()方法直接运行在“主线程main”上。
(03) mythread.start()会启动“线程mythread”，“线程mythread”启动之后，会调用run()方法；此时的run()方法是运行在“线程mythread”上。

sleep method

Only make current thread sleep
Sleep for a certain milliseconds
Syntax: Thread.sleep(996)
The thread will pause(suspend)
Free up CPU time for other threads

public void run(){
    
    
    try{
    
    
    Thread.sleep(1000);
    }
    catch(InterruptedException e){
    
    
    e.printStackTrace();
    }
}

InterruptedException

This exception is thrown when a thread is interrupted
Enables you to deal with the interrupt elegantly
Another thread might needs resources

How to interrupt a thread

thread.interrupt sets the interrupted flag
Blocking methods, such Thread.sleep(), try to detect when a thread has been interruptd and return early
They respond to interruption by clearing the setting the flag to false and throwing InterruptedException.
Interrupting a thread that is not live need not have any effect.

If any thread is in sleeping or waiting state (i.e. sleep() or wait() is
invoked), calling the interrupt() method on the thread, breaks out the
sleeping or waiting state throwing InterruptedException.
If the thread is not in the sleeping or waiting state, calling the
interrupt() method performs normal behaviour and doesn’t interrupt
the thread but sets the interrupt flag to true.
当一个线程处于等待，睡眠，或者占用，也就是说阻塞状态，而这时线程被中断就会抛出这类错误。Java6之后结束某个线程A的方法是A.interrupt()。如果这个线程正处于非阻塞状态，比如说线程正在执行某些代码的时候，被interrupt，那么该线程的interrupt变量会被置为true，告诉别人说这个线程被中断了,这时不会有interruptedException。但如果这时线程被阻塞了，比如说正在睡眠，那么就会抛出这个错误。请注意，这个时候变量interrupt置为false。在写代码的时候最好考虑一下这个标志位。

public void interrupt()
public static boolean interrupted()
public boolean isInterrupted()

Yield

The executing thread is suspended
The CPU is given to some other runnable thread
This thread will wait until the CPU becomes available again
Technically, in process scheduler’s terminology,
- the executing thread is returned to the ready queue of the processor and
- waits for its next turn.
  yield方法
  Java线程中的Thread.yield( )方法，译为线程让步。顾名思义，就是说当一个线程使用了这个方法之后，它就会把自己CPU执行的时间让掉，

让自己或者其它的线程运行，注意是让自己或者其他线程运行，并不是单纯的让给其他线程。

yield()的作用是让步。它能让当前线程由“运行状态”进入到“就绪状态”，从而让其它具有相同优先级的等待线程获取执行权；但是，并不能保

证在当前线程调用yield()之后，其它具有相同优先级的线程就一定能获得执行权；也有可能是当前线程又进入到“运行状态”继续运行！

举个例子：一帮朋友在排队上公交车，轮到Yield的时候，他突然说：我不想先上去了，咱们大家来竞赛上公交车。然后所有人就一块冲向公交车，

有可能是其他人先上车了，也有可能是Yield先上车了。

但是线程是有优先级的，优先级越高的人，就一定能第一个上车吗？这是不一定的，优先级高的人仅仅只是第一个上车的概率大了一点而已，

最终第一个上车的，也有可能是优先级最低的人。并且所谓的优先级执行，是在大量执行次数中才能体现出来的。

Interrupt

How to get a reference to yourself as a thread…

public class MyThread implements Runnable{
    
    
    public void run(){
    
    
        Thread.currentThread().interrupt();
        //If implements Runnable eed to use 
        //Thread.currentThread()
    }
}

wait(), notify(), notifyAll()

sleep()和wait()的区别
 sleep和wait的区别，wait需要抛出异常吗？

sleep(long millis)是Thread的静态方法，用于为线程指定睡眠时间，期间并不释放同步锁，时间到达后恢复线程执行；wait()是Object的final方法，会把调用者线程挂起，同时释放锁，直到被notify/notifyAll等方法唤醒或者超时，之后和其它线程共同竞争CPU。
最主要是sleep()不释放同步锁，而wait()释放同步锁，使得其他线程可以使用同步控制块或者方法。
wait，notify和notifyAll只能在同步控制方法或者同步控制块里面使用，而sleep可以在任何地方使用。
Methods wait, notify and notifyAll may only be called when the current thread has a lock on the object (i.e., from within a synchronized method or from within a method that has been called by a synchronized method).
sleep,wait必须捕获异常，而notify和notifyAll不需要捕获异常。

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Since there is no way of specifying which thread is to be woken, this is only really appropriate if there is only one waiting thread.
If all threads waiting for a lock on a given object are to be woken, then we use notifyAll.
This wait() method should only be called by a thread that is the owner of this
object’s monitor.

public final void wait()
• Exception
• IllegalMonitorStateException − if the current thread is not the owner
of the object’s monitor.
• InterruptedException − if another thread has interrupted the current
thread. The interrupted status of the current thread is cleared when
this exception is thrown.

Homework for lesson1

Write a Java Thread using both methods that:
Loops 100 times printing “Hello World”

   public class HelloWorld implements Runnable{
    
    
	public void run() {
    
    
		System.out.println("hello");
	}
	
	public static void main(String[] args) {
    
    
		HelloWorld helloWorld = new HelloWorld();
		for(int i =0;i<100;i++) {
    
    
		Thread mythread = new Thread(helloWorld);
		mythread.start();
		}
	}
}

public class HelloWorld extends Thread{
    
    
	public void run() {
    
    
		int i=0;
		while(i<100) {
    
    
			//print hello
			System.out.println("hello");	
			//counter++
			i++;	
		}	
	}
	
	public static void main(String[] args) throws InterruptedException {
    
    
		
		//prepare the thread
		HelloWorld helloWorld = new HelloWorld();
		
		//Start the thread
		helloWorld.start();
		
	}
	
}

Make the thread sleep for 1 second between each iteration

public class HelloWorld implements Runnable{
    
    
	public void run() {
    
    
		int i=0;
		while(i<100) {
    
    
			
			//sleep 1s
			try {
    
    
				Thread.sleep(1000);
			} catch (InterruptedException e) {
    
    
				e.printStackTrace();
			}
			
			//print hello
			System.out.println("hello");
			
			//counter++
			i++;	
		}	
	}
	
	public static void main(String[] args) throws InterruptedException {
    
    
		
		//prepare the thread
		HelloWorld helloWorld = new HelloWorld();
		Thread mythread = new Thread(helloWorld);
		
		//Start the thread
		mythread.start();
		
	}
	
}

Write code to interrupt the thread so that it finishes early

public class HelloWorld implements Runnable{
    
    
	public void run() {
    
    
		while(true) {
    
    	
			System.out.println("hello");	
			if(Thread.currentThread().isInterrupted()==true) {
    
    
				System.out.println("Terminated!");
				break;
			}
			
		}	
	}
	
	public static void main(String[] args) throws InterruptedException {
    
    
		
		//prepare the thread
		HelloWorld helloWorld = new HelloWorld();
		Thread mythread = new Thread(helloWorld);
		
		//Start the thread
		mythread.start();
		
		//Run some rubbish code
		for(int i=0;i<1000000;i++) {
    
    
		int ii=1000;
		}
		
		//Interrupt
		mythread.interrupt();
		
	}
	
}