一个线程两次或者多次调用start()方法会怎么样

会抛出IllegalThreadStateException()

示例代码如下：

public class Main{
　　public static void main(String[] args){

　　　　Thread t1 = new Thread(()->{System.out.println("this is a thread");});

　　　　t1.start();
　　　　t1.start()
　　}

}

之所以会抛出异常，是因为在start()函数里，一开始会检查线程状态，如果线程状态不为0，会抛出该异常。线程的状态初始值为0，当第一次进入start（）方法后，会修改线程状态，使其不为0，第二次进入start()函数，那么肯定就抛出异常了

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 */
            }
        }
    }