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Invoking the Java Virtual Machine

In JDK1.1, the Java Virtual Machine is shipped as a shared library (or dynamic link library on Win32). You can embed the Java VM into your native application by linking the native application with the shared library. The JNI supports an Invocation API that allows you to load, initialize, and invoke the Java Virtual Machine. Indeed, the normal way of starting the Java interpreter, java, is no more than a simple C program that parses the command line arguments and invoke the Java Virtual Machine through the Invocation API.

Invoking the Java Virtual Machine

As an example, we will write a C program to invoke the Java Virtual machine and call the Prog.main method defined in

public class Prog {
 public static void main(String[] args) {
  System.out.println("Hello World" + args[0]);
The C code in invoke.c begins with a call to JNI_GetDefaultJavaVMInitArgs to obtain the default initialization settings (heap size, stack size, and so on). It then calls JNI_CreateJavaVM to load and initialize the Virtual Machine. JNI_CreateJavaVM fills in two return values: Note that after JNI_CreateJavaVM successfully returns, the current native thread has bootstrapped itself into the Java Virtual Machine, and is therefore running just like a native method. The only difference is that there is no concept of returning to the Java Virtual Machine. Therefore, the local references created subsequently will not be freed until you call DestroyJavaVM.

Once you have created the Java Virtual Machine, you can issue regular JNI calls to invoke, for example, Prog.main. DestroyJavaVM attempts to unload the Java Virtual Machine. (The JDK 1.1 Java Virtual Machine cannot be unloaded, therefore DestroyJavaVM always returns an error code.)

You need to compile and link invoke.c with Java libraries shipped with JDK1.1. On Solaris, you can use the following command to compile and link invoke.c:

cc -I<where jni.h is> -L<where is> -ljava invoke.c
On Win32 with Microsoft Visual C++ 4.0, the command line is:
cl -I<where jni.h is> -MT invoke.c -link <where javai.lib is>\javai.lib

Run the resulting executable from the command line. If you get the following error message:

Unable to initialize threads: cannot find class java/lang/Thread
Can't create Java VM
This means that your CLASSPATH environment variable is not set up properly to include Java system classes. On the other hand, if the system complains that it cannot find either (on Solaris) or javai.dll (on Win32), add into your LD_LIBRARY_PATH on Solaris, or add javai.dll into your executable path on Win32. If the program complains that it can not find the class Prog, make sure the directory containing Prog.class is in the CLASSPATH as well.

If you are building a window (as opposed to a console) application on Win32, you must explicitly set the CLASSPATH slot in the Virtual Machine initialization structure. Window applications do not read the environment strings like console applications do. In general, it always a good idea to specify the CLASSPATH before you call JNI_CreateJavaVM, rather than relying on the environment variable settings when the program is run.

Attaching Native Threads

The Invocation API also allows you to attach native threads to a running Java VM, and bootstrap themselves into Java threads. This requires that the Java Virtual Machine internally uses native threads. In JDK 1.1, this feature only works on Win32. The Solaris version of Java Virtual Machine uses user-level thread support and is therefore incapable of attaching native threads. A future version of JDK on Solaris will support native threads.

Our example program, attach.c, therefore, will only work on Win32. It is a variation of invoke.c. Instead of calling Prog.main in the main thread, the native code spawns five threads, and then just waits for them to finish before it destroys the Java Virtual Machine. Each thread attaches itself to the Java Virtual Machine, invokes the Prog.main method, and finally detaches itself from the Virtual Machine before it terminates.

All local references belonging to the current thread will be freed when DetachCurrentThread is called.

Limitations of the Invocation API in JDK1.1

As mentioned above, there are a number of limitations of the Invocation API implementation in JDK1.1. These problems will be fixed in future releases of the JDK.

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