学习Thread的第一天就知道要调用Thread的start方法,不要调用Thread额run方法,为什么呢?
新建一个Thread类的实例,然后调用run方法,相当于调用普通的方法,属于当前线程的一个方法执行,可以重复多次调用,run方法运行结束, 此线程终止, 而CPU再运行其它线程。
run()方法当作普通方法的方式调用,程序还是要顺序执行,还是要等待run方法体执行完毕后才可继续执行下面的代码;如果直接用run方法, 这只是调用一个方法而已, 程序中依然只有主线程这一个线程, 其程序执行路径还是只有一条。上面的结论可以得出线程的模型,线程栈是指某时刻时内存中线程调度的栈信息,当前调用的方法总是位于栈顶。当调用Thread的start方法时会生成新的线程,放在单独的线程栈中。
调用start的方法有什么不同呢,调用了start的方法会创建线程,并自动调用run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.
添加进线程组
添加到group中的线程都是stared*/
group.add(this);
boolean started = false;
try {
start0();
started = true;
} finally {
try {
if (!started) {
//如果失败会被移除线程组
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
private native void start0();
根据上面Thread 中start方法的注释,调用Thread的start方法后会创建一个线程(不同于调用start的当前线程的线程),此时线程还没有运行,是可运行的状态(新建线程的状态)。线程最终会调用本地方法start0,它的内部最终会调用run方法。JVM会在线程启动时调用run方法,此时的线程才处于运行状态。run方法结束时,线程终止。
一个线程只能调用start()方法一次,多次启动一个线程是非法的,因为线程的状态变化从new到running不可逆转,且只会发生一次。
看源码 if (threadStatus != 0)
throw new IllegalThreadStateException();
当重复调用时threadStatus != 0 会抛出异常。
Thread 被添加进了group, ThreadGroup group;
线程一定属于某一个线程组中,线程组中可以有线程对象,也可以有线程组,组中还可以有线程,是一种类似属性结构的结构。
Thread 方法:
activeCount 获取当前线程所有活动线程的估计数,因为线程数量是实时改变的。
enumerate把所有活动线程放入到tarray中。
获取当前main线程内的所有的线程:
package com.sync.demo;
import java.util.concurrent.Callable;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
public class Demo1 {
public static void main(String[] args) {
ThreadDemo demo1 = new ThreadDemo();
demo1.setName("demo1");
demo1.start();
Thread thread2 = new Thread(new RunnableDemo());
thread2.setName("demo1");
thread2.start();
FutureTask<String> thread3 = new FutureTask<>(new Callable<String>() {
@Override
public String call() throws Exception {
try {
Thread.sleep(3000);
} catch (Exception e) {
// TODO: handle exception
}
return "demo3";
}
});
new Thread(thread3).start();
//or Executors.newSingleThreadExecutor().execute(thread3);
findAllThread();
}
public static class ThreadDemo extends Thread{
@Override
public void run() {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("demo1");
}
}
public static class RunnableDemo implements Runnable{
@Override
public void run() {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("demo2");
}
}
public static Thread[] findAllThread(){
ThreadGroup currentGroup =Thread.currentThread().getThreadGroup();
//找到最终的父线程组
while (currentGroup.getParent()!=null && currentGroup.getParent().getName().equals("main")){
currentGroup=currentGroup.getParent();
}
//活动线程的估计数
int noThreads = currentGroup.activeCount();
Thread[] threadArray = new Thread[noThreads];
//复制到指定数组中。
currentGroup.enumerate(threadArray);
for (Thread thread : threadArray) {
System.out.println("线程数量:"+noThreads+" 线程id:" + thread.getId() + " 线程名称:" + thread.getName() + " 线程状态:" + thread.getState());
}
return threadArray;
}
}
Result:
线程数量:4 线程id:1 线程名称:main 线程状态:RUNNABLE
线程数量:4 线程id:11 线程名称:demo1 线程状态:TIMED_WAITING
线程数量:4 线程id:12 线程名称:demo1 线程状态:TIMED_WAITING
线程数量:4 线程id:13 线程名称:Thread-2 线程状态:TIMED_WAITING
demo1
demo2