在Java中，如果我调用一个从另一个可运行对象扩展Thread的类，哪个线程将执行？

Question

I am working on some concurrency programming and one part is bothering me.

Let's say I have some class Foo that extends Thread and implements it's own public method called bar() as well as the required run() method. If I implement multiple Foo objects, each one containing a reference to another Foo object, and inside the run() method for the Foo class is a call on the bar() method for whatever Foo object it has a reference to. If the Foo object with name " Thread-1 " calls bar() on the Foo object with name " Thread-2 ", then who is actually executing the method code in " Thread-2 "? Does execution get handed off from " Thread-1 " to " Thread-2 " to execute or does " Thread-1 " continue executing code in " Thread-2 "? If it is the second choice, how can I make it act like the first choice?

Answer 1

A new thread is started only if you invoke thread.start() (or use executor.submit(runnable) ). All other method invocations remain in the current thread.

Answer 2

Foo is just an object, imagine it does not extends Thread , what would you expect? As long as new thread is not start() ed, you are just calling an ordinary method, on behalf of the calling thread.

Moreover, if calling a method on a different Thread object would cause that thread taking over the execution, what would happen with the current thread's execution?

Answer 3

Apart from being executed when you start the thread, run() is a normal method. If you manuallly call, it behaves like any other method, meaning it is executed inside the caller's thread.

Answer 4

It will run on the thread that called it. Calling a method in a class that extends thread is no different than calling a method of any other class.

public class A extends Thread
{
     public void bar()
     {
         System.out.println(Thread.currentThread().getName());
     }

     public void run()
     {
         new A().bar();
     }

     public static void main()
     {
         A testA = new A();     
         testA.setName("parent");
         testA.start();        
     }
}

Should print 'parent';