Is it possible that the new Thread object is equal to current running thread in Java,( in context with Primordial threads )?

So I was going through the internal working of Executor Service, and in the constructor of thread there were these lines mentioned:


    Thread(ThreadGroup g, String name, int characteristics, Runnable task,
           long stackSize, AccessControlContext acc) {

        Thread parent = currentThread();
        boolean attached = (parent == this);   // primordial or JNI attached

So my question here is, how is it possible that current running thread is equal to the new instance of thread ?

Solution

The currentThread() is returning ... the currently executing thread.

In the Thread constructor, this will be the Thread object that is being constructed. This is not normally the executing thread. The Thread you are creating here normally starts executing when something calls start() on it.

However ... here are a couple of scenarios when that is not true.

One scenario occurs when bootstrapping the JVM. Bootstrapping involves creation of certain Java objects before all of the JVM infrastructure is working. One of these objects is the Thread object for the primordial thread; i.e. thread that is doing the bootstrapping. The bootstrapping code will do something like this:

Create the heap, the bootstrap classloader, and various other things.
Create a Java Thread object without running its constructor. (It can do that: it is implemented in native code.)
Tweak the runtime data structures so that currentThread() returns the Thread object.
Invoke that constructor on the Thread object with the appropriate parameters.

If you want to trawl through the OpenJDK source tree, you should be able to find the code that does this. It is complicated.

According to that comment, other scenarios apparently involve JNI. I'm not sure, but it could happen when a C or C++ application calls JNI_CreateJavaVM.