Java™ Application Development on Linux - Dator

5.7 Going Native135is an 8-bit encoding for Unicode that coincides with ASCII in the low 7 bits,so it is often used for such conversions). You can think of the JNIEnv as a C++class pointer, or you can think of it as a structure of data and function pointers(that’s really what a C++ class is, after all). The bottom line is, it provides themeans to access and manipulate the Java environment from your native code.The second argument, jobject, is the “this” pointer. It points to theGetUser class, and it is upcast to the JNI equivalent of the Java Objecttype. If our method took parameters, they would follow these two constantarguments.JNI is a huge topic. You can read more about it in the Sun MicrosystemsJNI Tutorial, 11 or in Java 2 SDK JNI FAQ, 12 or in the JNI 1.1 Specification, 13or in the associated JDK 1.2 Update 14 or the JDK 1.4 Update. 15Even with all of this “We’re too busy to explain things to you” going onhere, we’ve got a lot more to cover before we are done. The next step in ourlittle demo is to compile the C program and create a shared library of the code.$ cc -c GetUser.c$ cc -shared -o libgetuser.so GetUser.o$ export LD_LIBRARY_PATH=.The first line compiles the native method to a .o (object) file. The secondcommand makes a shared library out of it. Now, refer back to the static initializerin Example 5.7. A static initializer is run before everything else in a class,even before main(). In this case, it uses the loadLibrary() method of theSystem class to load the shared library we just created. Note that library namingrules of the target OS are applied. The library is named getuser and ona Linux system it is assumed that that library will be in a file namedlibgetuser.so.The last line sets an environment variable, LD_LIBRARY_PATH, to providea path where Java will search for libraries. This is behavior inherited from Solaris.Linux uses ldconfig to maintain a list of shared libraries. Usually, a libraryis placed in a directory named in the file ld.so.conf and a memory cache of11. http://java.sun.com/docs/books/tutorial/native1.1/index.html12. http://java.sun.com/products/jdk/faq/jni-j2sdk-faq.html13. http://java.sun.com/products/jdk/1.2/docs/guide/jni/spec/jniTOC.doc.html14. http://java.sun.com/j2se/1.4.1/docs/guide/jni/jni-12.html15. http://java.sun.com/j2se/1.4.1/docs/guide/jni/jni-14.html