模式 功能
DEFAULT 立即执行协程体
ATOMIC 立即执行协程体，但在开始运行之前无法取消
UNDISPATCHED 立即在当前线程执行协程体，直到第一个 suspend 调用
LAZY 只有在需要的情况下运行

DEFAULT

默认启动一个新线程，类似于线程池

  1.             GlobalScope.launch() {
  2.                 println("1  ${Thread.currentThread()}")
  3.                 val job = GlobalScope.launch(start = CoroutineStart.DEFAULT) {
  4.                     println("2  ${Thread.currentThread()}")
  5.                 }
  6.                 delay(100)
  7.                 println("3  ${Thread.currentThread()}")
  8.                 job.join()
  9.                 println("4  ${Thread.currentThread()}")
  10.             }
  11.             Thread.sleep(1000)
  12.         }

运行结果

1  Thread[DefaultDispatcher-worker-1 @coroutine#1,5,main]
2  Thread[DefaultDispatcher-worker-2 @coroutine#2,5,main]
3  Thread[DefaultDispatcher-worker-1 @coroutine#1,5,main]
4  Thread[DefaultDispatcher-worker-1 @coroutine#1,5,main]

LAZY