Question

Ok , I know the two standard ways to create a new thread and run it in Java :

1. Implement Runnable in a class, define run() method, and pass an instance of the class to a new Thread. When the start() method on the thread instance is called, the run method of the class instance will be invoked.

2. Let the class derive from Thread, so it can to override the method run() and then when a new instance’s start() method is called, the call is routed to overridden method.

In both methods, basically a new Thread object is created and its start method invoked. However, while in the second method, the mechanism of the call being routed to the user defined run() method is very clear, (it’s a simple runtime polymorphism in play), I don’t understand how the call to start() method on the Thread object gets routed to run() method of the class implementing Runnable interface. Does the Thread class have an private field of Type Runnable which it checks first, and if it is set then invokes the run method if it set to an object? that would be a strange mechanism IMO.

How does the call to start() on a thread get routed to the run method of the Runnable interface implemented by the class whose object is passed as a parameter when constructing the thread?

Answer 1

The Thread keeps a reference to the Runnable instance, and calls it in the base implementation of run.

You can see this in the source:

// passed into the constructor and set in the init() method
private Runnable target;
...
// called from native thread code after start() is called
public void run() {
    if (target != null) {
        target.run();
    }
}