別想當(dāng)然

問(wèn)題

Java 線程調(diào)用start()后會(huì)立即執(zhí)行run()方法嗎？

我們?cè)陂_(kāi)發(fā)中，經(jīng)常和線程打交道，有些東西總是司空見(jiàn)慣，想當(dāng)然地認(rèn)為某些事情理所當(dāng)然...

但是今天偶然發(fā)現(xiàn)一個(gè)有趣的現(xiàn)象：

class Test {

public static void main(String[] args) {

System.out.println("hello https://tool.lu/");

Runnable runnable = new Runnable(){

@Override

public void run() {

System.out.println("--run()--");

}

};

Thread thread = new Thread(runnable);

thread.start();

System.out.println("--after start()--");

}

}

運(yùn)行結(jié)果如下：

運(yùn)行結(jié)果

看到運(yùn)行結(jié)果之后，我愣了一下，WTF?

跟我想的有點(diǎn)不一樣啊，

理論上來(lái)講，線程調(diào)用start()之后，將會(huì)調(diào)用該線程的run();

我又趕緊去查看了Java的官方文檔：JavaAPI

Thread的start()

Cause this thread to begin execution ; the Java Virtual Machine calls the run method of this thread.

通過(guò)上面我們可以發(fā)現(xiàn)start()就是使該線程開(kāi)始運(yùn)行，JVM會(huì)調(diào)用線程的run()方法。

那我們只能從源碼中找答案啦。

/**

* Causes this thread to begin execution; the Java Virtual Machine

* calls the run method of this thread.

*

* The result is that two threads are running concurrently: the

* current thread (which returns from the call to the

* start method) and the other thread (which executes its

* run method).

*

* 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".

*/

// Android-changed: throw if 'started' is true

if (threadStatus != 0 || started)

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);

started = false;

try {

nativeCreate(this, stackSize, daemon);

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 */

}

}

}

private native static void nativeCreate(Thread t, long stackSize, boolean daemon);

關(guān)鍵代碼已經(jīng)貼出，但是似乎關(guān)鍵在于nativeCreate()方法。該方法屬于c方法，有時(shí)間再追蹤一下。

思考

通過(guò)代碼測(cè)試我們發(fā)現(xiàn)線程調(diào)用start()方法，并不會(huì)立刻執(zhí)行run()方法。但是兩者之間的時(shí)間差是多少呢？

測(cè)試用例,有興趣的朋友can have a try.

把運(yùn)行結(jié)果給出來(lái)：

運(yùn)行結(jié)果

通過(guò)我的多次測(cè)試，run()和start()的時(shí)間差一般都在[0,8]之內(nèi)，當(dāng)然運(yùn)行足夠多次會(huì)發(fā)現(xiàn)時(shí)間差會(huì)更大。

總結(jié)

雖然該用例看似沒(méi)什么卵用，但不親自上手嘗試，還真會(huì)回答錯(cuò)...有趣有趣...

需要注意的是如果我們?cè)诰€程中創(chuàng)建對(duì)象，而在start()方法后直接使用該對(duì)象，就會(huì)出現(xiàn)該問(wèn)題測(cè)試用例。

夫小惑易方，大惑易性...

總結(jié)

以上是生活随笔為你收集整理的java线程立刻执行_Java 线程调用start()后会立即执行run()方法吗？的全部?jī)?nèi)容，希望文章能夠幫你解決所遇到的問(wèn)題。

如果覺(jué)得生活随笔網(wǎng)站內(nèi)容還不錯(cuò)，歡迎將生活随笔推薦給好友。