一個線程兩次或者多次調用start()方法會怎么樣

會拋出IllegalThreadStateException()

示例代碼如下：

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

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

　　　　t1.start();
　　　　t1.start()
　　}

}

之所以會拋出異常，是因為在start()函數里，一開始會檢查線程狀態，如果線程狀態不為0，會拋出該異常。線程的狀態初始值為0，當第一次進入start（）方法后，會修改線程狀態，使其不為0，第二次進入start()函數，那么肯定就拋出異常了

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 */
            }
        }
    }