| Java Native Interface Programming

## 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 `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 (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. It can be later used to, for example, destroy the Virtual Machine.
> - `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, 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 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

> 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 `libjava.so` (on Solaris) or `javai.dll` (on Win32), add `libjava.so` 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.
> - The user-level Java thread implementation on Solaris requires the Java VM 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 it is called 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.

| Java Native Interface Programming