JVM_ENTRY on OpenJDK source code

Question

I read codes that throw exception in hotspot OpenJDK 7 and track which classes using them. They are ended up in classes that implement JVM_ENTRY(result_type, header) , for example in jvm.cpp , java.cpp , etc. My question is what is the function of JVM_ENTRY and how does it works ?

Below is the example in jvm.cpp

JVM_ENTRY(jobject, JVM_AllocateNewObject(JNIEnv *env, jobject receiver, jclass currClass, jclass initClass))
  JVMWrapper("JVM_AllocateNewObject");
  JvmtiVMObjectAllocEventCollector oam;
  // Receiver is not used
  oop curr_mirror = JNIHandles::resolve_non_null(currClass);
  oop init_mirror = JNIHandles::resolve_non_null(initClass);

  // Cannot instantiate primitive types
  if (java_lang_Class::is_primitive(curr_mirror) || java_lang_Class::is_primitive(init_mirror)) {
    ResourceMark rm(THREAD);
    THROW_0(vmSymbols::java_lang_InvalidClassException());
  }

  // Arrays not allowed here, must use JVM_AllocateNewArray
  if (Klass::cast(java_lang_Class::as_klassOop(curr_mirror))->oop_is_javaArray() ||
      Klass::cast(java_lang_Class::as_klassOop(init_mirror))->oop_is_javaArray()) {
    ResourceMark rm(THREAD);
    THROW_0(vmSymbols::java_lang_InvalidClassException());
  }

  instanceKlassHandle curr_klass (THREAD, java_lang_Class::as_klassOop(curr_mirror));
  instanceKlassHandle init_klass (THREAD, java_lang_Class::as_klassOop(init_mirror));

  assert(curr_klass->is_subclass_of(init_klass()), "just checking");

  // Interfaces, abstract classes, and java.lang.Class classes cannot be instantiated directly.
  curr_klass->check_valid_for_instantiation(false, CHECK_NULL);

  // Make sure klass is initialized, since we are about to instantiate one of them.
  curr_klass->initialize(CHECK_NULL);

 methodHandle m (THREAD,
                 init_klass->find_method(vmSymbols::object_initializer_name(),
                                         vmSymbols::void_method_signature()));
  if (m.is_null()) {
    ResourceMark rm(THREAD);
    THROW_MSG_0(vmSymbols::java_lang_NoSuchMethodError(),
                methodOopDesc::name_and_sig_as_C_string(Klass::cast(init_klass()),
                                          vmSymbols::object_initializer_name(),
                                          vmSymbols::void_method_signature()));
  }

  if (curr_klass ==  init_klass && !m->is_public()) {
    // Calling the constructor for class 'curr_klass'.
    // Only allow calls to a public no-arg constructor.
    // This path corresponds to creating an Externalizable object.
    THROW_0(vmSymbols::java_lang_IllegalAccessException());
  }

  if (!force_verify_field_access(curr_klass(), init_klass(), m->access_flags(), false)) {
    // subclass 'curr_klass' does not have access to no-arg constructor of 'initcb'
    THROW_0(vmSymbols::java_lang_IllegalAccessException());
  }

  Handle obj = curr_klass->allocate_instance_handle(CHECK_NULL);
  // Call constructor m. This might call a constructor higher up in the hierachy
  JavaCalls::call_default_constructor(thread, m, obj, CHECK_NULL);

  return JNIHandles::make_local(obj());
JVM_END

I found definition of JVM_ENTRY at hotspot/src/share/vm/runtime/interfaceSupport.hpp

#define JVM_ENTRY(result_type, header)                               \
extern "C" {                                                         \
  result_type JNICALL header {                                       \
    JavaThread* thread=JavaThread::thread_from_jni_environment(env); \
    ThreadInVMfromNative __tiv(thread);                              \
    debug_only(VMNativeEntryWrapper __vew;)                          \
    VM_ENTRY_BASE(result_type, header, thread)

Answer 1

JVM_ENTRY is a preprocessor macro that adds some boilerplate code that is common for all functions of HotSpot JVM API. This API is a connection layer between the native code of JDK class library and the JVM.

What JVM_ENTRY macro does:

1. Gets the pointer to current JavaThread from JNIEnv* argument.
2. Performs thread state transition from _thread_in_native to _thread_in_vm , blocking at safepoint if necessary..
3. Cleans JNI local references on the exit from the function.
4. Takes care of debug tracing and verification in debug builds of JVM.