java线程系列一:Thread类中的start()方法与run方法

项目github地址：bitcarmanlee easy-algorithm-interview-and-practice
欢迎大家star，留言，一起学习进步

1.java中的线程状态

在java中，线程通常有五种状态：创建，就绪，运行，阻塞与死亡。
1：创建(NEW)
在生成线程对象的时候，并没有调用start方法，这是线程的创建状态。
2：就绪(RUNABLE)
当调用线程对象的start方法以后，线程就进入了就绪状态。但是此时线程调度程序还没有把该线程设置为当前线程，此时处于就绪状态。在线程运行之后，从等待或者睡眠中回来之后，也会处于就绪状态。
3：运行(RUNNING)
线程调度程序将处于就绪状态的线程设置为当前线程，此时线程就进入了运行状态，开始运行run函数当中的代码。
4：阻塞(BLOCKED)
线程正在运行的时候，被暂停，通常是为了等待某个时间的发生(比如说某项资源就绪)之后再继续运行。sleep,suspend，wait等方法都可以导致线程阻塞。
5：死亡(TERMINATED)
如果一个线程的run方法执行结束或者调用stop方法后，该线程就会死亡。对于已经死亡的线程，无法再使用start方法令其进入就绪。

2.Thread中的start()方法与run()方法

java中的线程是通过java.lang.Thread类来实现的。JVM启动时会有一个由主方法所定义的线程，同时可以通过创建Thread的实例来创建新的线程。每个线程都是通过某个特定Thread对象所对应的方法run()来完成其操作的，方法run()称为线程体。通过调用Thread类的start()方法来启动一个线程。

看下jdk中的源码

   /*** If this thread was constructed using a separate* <code>Runnable</code> run object, then that* <code>Runnable</code> object's <code>run</code> method is called;* otherwise, this method does nothing and returns.* <p>* Subclasses of <code>Thread</code> should override this method.** @see     #start()* @see     #stop()* @see     #Thread(ThreadGroup, Runnable, String)*/@Overridepublic void run() {if (target != null) {target.run();}}

再参考一下start方法

   /*** Causes this thread to begin execution; the Java Virtual Machine* calls the <code>run</code> method of this thread.* <p>* The result is that two threads are running concurrently: the* current thread (which returns from the call to the* <code>start</code> method) and the other thread (which executes its* <code>run</code> method).* <p>* It is never legal to start a thread more than once.* In particular, a thread may not be restarted once it has completed* execution.** @exception  IllegalThreadStateException  if the thread was already*               started.* @see        #run()* @see        #stop()*/public synchronized void start() {/*** This method is not invoked for the main method thread or "system"* group threads created/set up by the VM. Any new functionality added* to this method in the future may have to also be added to the VM.** A zero status value corresponds to state "NEW".*/if (threadStatus != 0)throw new IllegalThreadStateException();/* Notify the group that this thread is about to be started* so that it can be added to the group's list of threads* and the group's unstarted count can be decremented. */group.add(this);boolean started = false;try {start0();started = true;} finally {try {if (!started) {group.threadStartFailed(this);}} catch (Throwable ignore) {/* do nothing. If start0 threw a Throwable thenit will be passed up the call stack */}}}

从源码以及当中的注释可以看出，run()其实是个普通方法，只不过当线程调用了start( )方法后，一旦线程被CPU调度，处于运行状态，那么线程才会去调用这个run()方法；
同时，run方法就是一个普通的方法，并不是需要调用start()方法以后才能调用run()方法，线程对象可以随时调用run()方法。

3.示例

public class ThreadDemo1 extends Thread {public ThreadDemo1() {this.setName("ThreadDemo1");}public static void printThreadInfo() {System.out.println("current thread name is: " + Thread.currentThread().getName());}@Overridepublic void run() {while (true) {printThreadInfo();try {Thread.sleep(100);} catch (Exception ex) {throw new RuntimeException(ex);}}}public static void main(String[] args) throws Exception {Thread t1 = new Thread(new Task(1));Thread t2 = new Thread(new Task(2));t1.start();t2.start();}
}

public class Task implements Runnable {int count;public Task(int count) {this.count = count;}@Overridepublic void run() {System.out.println("count is: " + count);}
}

上面的运行结果有可能是：

count is: 1
count is: 2

也有可能是

count is: 2
count is: 1

但是如果将

        t1.start();t2.start();

换成

        t1.run();t2.run();

那么结果一定是

count is: 1
count is: 2

再看另外一个示例，模拟两个线程运行。

public class ThreadDemo2 {public static void main(String[] args) {Thread t1 = new Thread(new C1());Thread t2 = new Thread(new C2());t1.start();t2.start();//t1.run();//t2.run();}
}class C1 implements Runnable {@Overridepublic void run() {try {for (int i = 0; i < 10; i++) {System.out.println(i);Thread.sleep(1000);}} catch (InterruptedException ex) {ex.printStackTrace();}}
}class C2 implements Runnable {public void run() {try {for (int j = 0; j > -10; j--) {System.out.println(j);Thread.sleep(1000);}} catch (InterruptedException ex) {ex.printStackTrace();}}
}

如果是调用start方法，输出为

如果调用的是run方法，结果为

4.简单小结

start方法是用于启动线程的，可以实现并发，而run方法只是一个普通方法，是不能实现并发的，只是在并发执行的时候会调用。