An instance of java.lang.Thread is not a thread; it can be used to represent a thread of execution in the JVM but the JVM is perfectly capable of creating threads without using the Thread class at all.
This is what happens with the main thread: the JVM creates it, and an instance of java.lang.Thread is created to represent it later.
In the Hotspot JVM there’s a lot of threading related code in the Threads class defined in src/share/vm/runtime/thread.hpp and src/share/vm/runtime/thread.cpp. The startup of the JVM calls the static Threads::create_vm function, which is already running in a thread set up by the operating system. Within that function we find:
(src/share/vm/runtime/thread.cpp)
3191 // Attach the main thread to this os thread
3192 JavaThread* main_thread = new JavaThread();
3193 main_thread->set_thread_state(_thread_in_vm);
3194 // must do this before set_active_handles and initialize_thread_local_storage
3195 // Note: on solaris initialize_thread_local_storage() will (indirectly)
3196 // change the stack size recorded here to one based on the java thread
3197 // stacksize. This adjusted size is what is used to figure the placement
3198 // of the guard pages.
3199 main_thread->record_stack_base_and_size();
3200 main_thread->initialize_thread_local_storage();
The JavaThread class is apparently used for bookkeeping; it associates an OS or VM thread with a Java Thread object. The Java object apparently doesn’t exist yet. The code then goes on to initialize various other things, and later on still in the same function we find this:
3335 // Initialize java_lang.System (needed before creating the thread)
3336 if (InitializeJavaLangSystem) {
3337 initialize_class(vmSymbols::java_lang_System(), CHECK_0);
3338 initialize_class(vmSymbols::java_lang_ThreadGroup(), CHECK_0);
3339 Handle thread_group = create_initial_thread_group(CHECK_0);
3340 Universe::set_main_thread_group(thread_group());
3341 initialize_class(vmSymbols::java_lang_Thread(), CHECK_0);
3342 oop thread_object = create_initial_thread(thread_group, main_thread, CHECK_0);
3343 main_thread->set_threadObj(thread_object);
3344 // Set thread status to running since main thread has
3345 // been started and running.
3346 java_lang_Thread::set_thread_status(thread_object,
3347 java_lang_Thread::RUNNABLE);
In other words, we it initializes the System, ThreadGroup, and Thread classes, then creates an instance of Thread referenced by thread_object (line 3342), and sets the Thread instance for the main JavaThread.
If you wonder what the create_initial_thread does, apparently it allocates the Thread instance, stores a pointer to the JavaThread (C++) object in the private eetop field of the Thread instance, sets the thread priority field to normal, calls the Thread(ThreadGroup group,String name) constructor, and returns the instance:
967 // Creates the initial Thread
968 static oop create_initial_thread(Handle thread_group, JavaThread* thread, TRAPS) {
969 klassOop k = SystemDictionary::resolve_or_fail(vmSymbols::java_lang_Thread(), true, CHECK_ NULL);
970 instanceKlassHandle klass (THREAD, k);
971 instanceHandle thread_oop = klass->allocate_instance_handle(CHECK_NULL);
972
973 java_lang_Thread::set_thread(thread_oop(), thread);
974 java_lang_Thread::set_priority(thread_oop(), NormPriority);
975 thread->set_threadObj(thread_oop());
976
977 Handle string = java_lang_String::create_from_str("main", CHECK_NULL);
978
979 JavaValue result(T_VOID);
980 JavaCalls::call_special(&result, thread_oop,
981 klass,
982 vmSymbols::object_initializer_name(),
983 vmSymbols::threadgroup_string_void_signature(),
984 thread_group,
985 string,
986 CHECK_NULL);
987 return thread_oop();
988 }
Now, this is what the Hotspot VM does. Other implementations, like IBM J9, Oracle JRockit, or Azul Zing probably do something similar though.