start()与run()启动

public class StartAndRunMethod {
    
    
    public static void main(String[] args) {
    
    
        Runnable runnable = () -> {
    
    
            System.out.println(Thread.currentThread().getName());
        };

        runnable.run();

        new Thread(runnable).start();
    }
}

-------------------
运行结果是：
main
Thread-0

• main线程调用new Thread(runnable).start();去创建Thread-0线程，并且不是一调用start就会创建新线程，而是等待CPU有空闲的时候采取创建新线程

• 使用Runnable对象调用run()方法，就是普通的方法调用，是在main线程里执行；而使用start()方法启动线程才是真正启动多线程，下面是Thread类中的start()方法：

  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".
           */
      	
      	//启动新线程检查线程状态是否为0
          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. */
      	
      	//加入线程组
          group.add(this);
  
          boolean started = false;
          try {
        
        
              //调用start0方法
              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 */
              }
          }
      }
  

能不能start()两次？

public class CantStartTwice {
    
    
    public static void main(String[] args) {
    
    
        Thread thread = new Thread();
        thread.start();
        thread.start();
    }
}

答案是不能，通过看Thread类的start()方法知道在创建线程是会检查线程状态threadStatus，如果不是新的线程就会抛出异常java.lang.IllegalThreadStateException