java - 线程未保持 Activity 状态 - IllegalStateException

Question

这是我使用线程制作的一个小程序的片段。

JOptionPane.showMessageDialog(null, "Before: " + thread.isAlive());
if (!thread.isAlive()) {
    JOptionPane.showMessageDialog(null, "Thread is not alive.");
    thread.start();
}
JOptionPane.showMessageDialog(null, "After: " + thread.isAlive());

此代码可使用按钮激活。当我第一次按下按钮时，我正确地得到“之前:假”，然后是“之后:真”。当我再次按下按钮时，我错误地得到“Before: false”，然后是“After: true”，但期望 Before: true，因为我没有销毁线程或覆盖变量。

我相信这就是导致我收到 IllegalStateException 的原因(如果我也错了，请纠正我!)

谁能向我解释一下我做错了什么？

编辑:

public class SomeClass extends Applet
{

private ClassThatExtendsThread thread;

public void init()
{
    super.init();

    //Some UI elements are created here.

    thread = new ClassThatExtendsThread (/*there are some parameters*/);
} 

Answer 1

一旦线程完成运行，它就被认为是死亡的。此时在同一线程上调用 isAlive 将始终返回 false 结果。 JavaDoc for Thread确实提到了这一点:

public final boolean isAlive()Tests if this thread is alive. A thread is alive if it has been started and has notyet died.

If you are not re-instantiating your thread instance in-between calls to your code snippet then you will definitely get an IllegalStateException. This is because you attempt to start a thread that has already been terminated:

if (!thread.isAlive()) {
    JOptionPane.showMessageDialog(null, "Thread is not alive.");
    thread.start();
}

请注意，为了将来引用，您可以通过 getState 方法查询线程状态，这至少有助于分析错误。