Invoking the Java Virtual Machine

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Trail: Java Native Interface
Lesson: Invoking the Java Virtual Machine

Invoking the Java Virtual Machine

The JDK1.1 ships the Java Virtual Machine as a shared library (or dynamic link library on Win32). You can embed the Java Virtual Machine 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 invokes the Java Virtual Machine through the Invocation API.

Invoking the Java Virtual Machine

To illustrate, we will write a C program that invokes the Java Virtual Machine and calls the Prog.main method defined in Prog.java:

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, such as 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:

jvm refers to the created Java Virtual Machine. You can use this to destroy the Virtual Machine at a later time, for example.
env is a JNI interface pointer that the current thread can use to access Java features, such as calling a Java method.

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, any local references that you subsequently create 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 the 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 libjava.so 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

Those working in the MacOS environment should refer to the JManager API, which is part of the MacOS Runtime for Java . You use the JManager API to embed Java applications into your application.

Run the resulting executable from the command line. It is possible that you may get the following error message:

Unable to initialize threads: cannot find class java/lang/Thread
Can't create Java VM

This error message indicates that you have set the wrong value for the vm_args.classpath variable.

You might get a different system error indicating that it cannot find either libjava.so (on Solaris) or javai.dll (on Win32). If this is the case, add libjava.so into your LD_LIBRARY_PATH on Solaris, or add javai.dll into your executable path on Win32.

The program may report an error that it cannot find the class Prog. If so, make sure the directory containing Prog.class is in the vm_args.classpath variable as well.

Attaching Native Threads

The Invocation API also allows you to attach native threads to a running Java Virtual Machine and have the threads 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 the Java Virtual Machine uses user-level thread support and is therefore incapable of attaching native threads. In the future, the JDK on Solaris will support native threads.

Our example program, attach.c, therefore, will only work on Win32. This example program is a variation of invoke.c. Instead of calling Prog.main in the main thread, the native code spawns five threads and then 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. Note that the third argument to AttachCurrentThread is reserved and should be set to NULL.

When you call DetachCurrentThread, you free all local references belonging to the current thread.

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.

The user-level Java thread implementation on Solaris requires the Java Virtual Machine to redirect certain Solaris system calls. The set of redirected system calls currently includes read, readv, write, writev, getmsg, putmsg, poll, open, close, pipe, fcntl, dup, create, accept, recv, send, and so on. This may cause undesirable effects on a hosting native application that also depends on these system calls.
You cannot attach a native thread to the user-thread based Java Virtual Machine on Solaris. AttachCurrentThread simply fails on Solaris (unless your code calls it from the main thread that created the Virtual Machine).
You cannot unload the Java Virtual Machine without terminating the process. The DestroyJavaVM call simply returns an error code.

These problems will be fixed in future releases of the JDK.

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