Akhilesh Shirbhate <[EMAIL PROTECTED]> wrote on 07/12/2005
05:18:20 AM:
> Can we have a look at the vmi.h and the list of 18 classes, and
> specially the two classes required for integration ?
As a follow-up to Tim Ellison's response let me provide more detail on
J9's VM Interface (VMI) and Kernel classes. Here is the list of
VM-dependent ?Kernel classes?. Tim provided some additional details about
com.ibm.oti.lang.SystemProcess and com.ibm.oti.vm.VM in a follow-up to his
original post.
java.lang
Class
ClassLoader
Compiler
Object
Package
Runtime
StackTraceElement
String
Thread
ThreadGroup
Throwable
java.lang.ref
Reference
ReferenceQueue
java.lang.reflect
AccessibleObject
Array
Constructor
Field
Method
java.security
AccessControlContext
AccessController
com.ibm.oti.lang
SystemProcess
com.ibm.oti.vm
VM
As Tim stated, the role of the VMI is to insulate the Java class library
implementation from internal knowledge of the VM. Recall that the VMI is
used by the portion of the class library that is not tightly tied to the
virtual machine (i.e: non-Kernel classes). The vast majority of
non-Kernel classes can be implemented using a combination of Java and
'pure' JNI code.
The VMI is a C-based interface, similar to JNI, which provides access to
VM features that are outside the scope of JNI. The meat of vmi.h is the
following structure declaration (somewhat simplified) which defines the
API between the class library and VM:
typedef struct VMInterfaceFunctions_* VMInterface;
struct VMInterfaceFunctions_
{
vmiError (JNICALL * CheckVersion)(VMInterface* vmi, vmiVersion*
version);
JavaVM* (JNICALL * GetJavaVM) (VMInterface* vmi);
J9PortLibrary* (JNICALL * GetPortLibrary) (VMInterface* vmi);
JavaVMInitArgs* (JNICALL * GetInitArgs) (VMInterface* vmi);
vmiError (JNICALL * GetSystemProperty) (VMInterface* vmi, char*
key, char** valuePtr);
vmiError (JNICALL * SetSystemProperty) (VMInterface* vmi, char*
key, char* value);
vmiError (JNICALL * CountSystemProperties) (VMInterface* vmi, int*
countPtr);
vmiError (JNICALL * IterateSystemProperties) (VMInterface* vmi,
vmiSystemPropertyIterator iterator, void* userData);
};
Like the JNIEnv the VMInterface is a structure that contains function
pointers and is associated 1:1 with a JavaVM.
Structurally, the VMI is built as a shared library (.dll/.so) that exports
two functions which return a VMInterface given either a JavaVM or a
JNIEnv. The class library JNI code is dynamically linked to this shared
library, and is VM-neutral. The VMI shared library can be replaced to
support different VM implementations without modifying the JNI natives
themselves.
The following sample illustrates how a JNI native could obtain a
VMInterface and make a simple VMI query. Note the intentional similarity
between the JNI and VMI usage.
jstring JNICALL Java_example_Example_getJavaHome(JNIEnv *env, jclass
clazz)
{
/* Obtain the VM Interface from the JNIEnv */
VMInterface * vmi = VMI_GetVMIFromJNIEnv(env);
char* javaHome;
/* Query the value of java.home using VMI */
if ((*vmi)->GetSystemProperty(vmi, "java.home",&javaHome) !=
VMI_ERROR_NONE)
return NULL;
return (*env)->NewStringUTF(env,javaHome);
}
The GetPortLibrary() member of the VMInterface is particularly
interesting. This function returns a handle to a J9PortLibrary structure,
which provides a platform-independent abstraction of operating system
functionality. The portability layer includes those features required by
both the virtual machine and core class library, including:
Basic I/O (files, sockets, tty)
Synchronization and threading
Memory allocation
Signal handling
Shared library support
Operating system and environmental queries
During the development of J9 we found that centralizing VM and core class
library interfaces to the operating system (and keeping these interfaces
thin) reduces the porting effort. If there is sufficient interest I can
follow up with a more detailed description of the J9 portability library.
The mailing list is probably not the right forum for posting non-trivial
pieces of code. Geir: What's the right forum for this sort of discussion?
Graeme Johnson
J9 VM Team, IBM Canada.
