|Step By Step|
In this step, you use the
javahutility program to generate a header file (a
.hfile) from the
HelloWorldJava class. The header file provides a function prototype for the implementation of the native method
displayHelloWorld()defined in that class.
javahnow on the
HelloWorldclass that you created in the previous steps.
javahplaces the new
.hfile in the same directory as the
.classfile. You can tell
javahto place the header files in a different directory with the
The name of the header file is the Java class name with a
.happended to the end of it. For example, the command shown above will generate a file named
The Function DefinitionLook at the header file
Java_HelloWorld_displayHelloWorld()is the function that provides the implementation for the
HelloWorldclass's native method
displayHelloWorld, which you will write in Step 4: Write the Native Method Implementation. You use the same function signature when you write the implementation for the native method.
HelloWorldcontained any other native methods, their function signatures would appear here as well.
The name of the native language function that implements the native method consists of the prefix
Java_, the package name, the class name, and the name of the Java native method. Between each name component is an underscore "_" separator. The package name is omitted when the method is in the default package. Thus, the native method
Java_HelloWorld_displayHelloWorld(). In our example, there is no package name because
HelloWorldis in the default package.
You will notice that the implementation of the native language function accepts two parameters even though, in its definition in the Java class, it accepts no parameters. The first parameter for every native method is a
JNIEnvinterface pointer. It is through this pointer that your native code accesses parameters and objects passed to it from the Java application. The
jobjectparameter is a reference to the object itself. For a non-static native method, such as the
displayHelloWorldmethod in our example, this argument is a reference to the object. For static native methods, this argument would be a reference to the method's Java class. In a sense, you can think of the
jobjectparameter as the "this" variable in C++. Our example ignores both parameters. The next lesson, Java Native Interface Programming , describes how to access the data using the JNI interface pointer
|Step By Step|