同一个线程不要调用2次Thread.start()

同一个线程，如果调用start()两次会报异常：Exception in thread "main" java.lang.IllegalThreadStateException

看源码：

/* Java thread status for tools,
 * initialized to indicate thread 'not yet started'
 */

private volatile int threadStatus = 0;

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 */
        }
    }
}

只有new Thread()时线程调用init函数初始化时threadStatus = 0，调用start()函数后，线程的状态都大于0.

线程状态：

public enum State {
    /**
     * Thread state for a thread which has not yet started.
     */
    NEW,

    /**
     * Thread state for a runnable thread.  A thread in the runnable
     * state is executing in the Java virtual machine but it may
     * be waiting for other resources from the operating system
     * such as processor.
     */
    RUNNABLE,

    /**
     * Thread state for a thread blocked waiting for a monitor lock.
     * A thread in the blocked state is waiting for a monitor lock
     * to enter a synchronized block/method or
     * reenter a synchronized block/method after calling
     * {@link Object#wait() Object.wait}.
     */
    BLOCKED,

    /**
     * Thread state for a waiting thread.
     * A thread is in the waiting state due to calling one of the
     * following methods:
     * 

     *   
{@link Object#wait() Object.wait} with no timeout
     *   
{@link #join() Thread.join} with no timeout
     *   
{@link LockSupport#park() LockSupport.park}
     * 
     *
     * 
A thread in the waiting state is waiting for another thread to
     * perform a particular action.
     *
     * For example, a thread that has called Object.wait()
     * on an object is waiting for another thread to call
     * Object.notify() or Object.notifyAll() on
     * that object. A thread that has called Thread.join()
     * is waiting for a specified thread to terminate.
     */
    WAITING,

    /**
     * Thread state for a waiting thread with a specified waiting time.
     * A thread is in the timed waiting state due to calling one of
     * the following methods with a specified positive waiting time:
     * 

     *   
{@link #sleep Thread.sleep}
     *   
{@link Object#wait(long) Object.wait} with timeout
     *   
{@link #join(long) Thread.join} with timeout
     *   
{@link LockSupport#parkNanos LockSupport.parkNanos}
     *   
{@link LockSupport#parkUntil LockSupport.parkUntil}
     * 
     */
    TIMED_WAITING,

    /**
     * Thread state for a terminated thread.
     * The thread has completed execution.
     */
    TERMINATED;
}