启动线程的正确姿势
通过之前的分析，我们知道了有两种定义线程执行逻辑的方法，而创建线程只有一种方法，也就是new一个Thread类的实例。
JAVA实现多线程到底有多少种方法？
我们如何启动线程呢？其实很简单，调用start()即可。
package startThread;
public class test {
public static void main(String[] args) {
Runnable runnable = ()->{
System.out.println(Thread.currentThread().getName());
};
Thread thread = new Thread(runnable);
// 设置线程名称
thread.setName("thread_kaven");
thread.start();
}
}
输出：
thread_kaven
很显然，start()启动了线程，并且是在一个新的线程中去执行任务的，但是为什么一定要调用start()，而不直接调用该线程的执行逻辑run()呢？
我们来测试调用run()的情况。
package startThread;
public class test {
public static void main(String[] args) {
Runnable runnable = ()->{
System.out.println(Thread.currentThread().getName());
};
Thread thread = new Thread(runnable);
// 设置线程名称
thread.setName("thread_kaven");
thread.run();
}
}
输出：
main
很显然，调用run()，并没有创建新的线程去执行任务，run()还是在main线程中执行的。
我们可以得到一个简单的结论：
run()只是一个类中的普通方法，调用run()跟调用普通方法一样。
而调用start()，则会做一系列工作去创建新线程，然后在新线程中执行run()里面的任务内容。
我们可以多次调用start()吗？我们来测试一下。
package startThread;
public class test {
public static void main(String[] args) {
Runnable runnable = ()->{
System.out.println(Thread.currentThread().getName());
};
Thread thread = new Thread(runnable);
// 设置线程名称
thread.setName("thread_kaven");
thread.start();
thread.start();
}
}
输出：
thread_kaven
Exception in thread "main" java.lang.IllegalThreadStateException
at java.lang.Thread.start(Thread.java:708)
at startThread.test.main(test.java:13)
报错了，为什么不能多次调用start()呢？我们来看一下start()源码。
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/**
* 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 then
it will be passed up the call stack */
}
}
}
从源码可以知道，调用start()时，会判断threadStatus != 0，很明显，刚刚的异常就是这个地方抛出来的，通过源码可以知道threadStatus是记录线程的状态，初始默认为0，当线程启动后，该线程的状态就改变了，所以threadStatus的值也就改变了，当第二次调用start()时，就会抛出异常。
