Step By Step |
In this step, you use thejavah
utility program to generate a header file (a.h
file) from theHelloWorld
Java class. The header file provides a function prototype for the implementation of the native methoddisplayHelloWorld()
defined in that class.Run
javah
now on theHelloWorld
class that you created in the previous steps.By default,
javah
places the new.h
file in the same directory as the.class
file. You can telljavah
to place the header files in a different directory with the-d
option.The name of the header file is the Java class name with a
.h
appended to the end of it. For example, the command shown above will generate a file namedHelloWorld.h
.The Function Definition
Look at the header fileHelloWorld.h
.#includejava example-1dot1/HelloWorld.hJava_HelloWorld_displayHelloWorld()
is the function that provides the implementation for theHelloWorld
class's native methoddisplayHelloWorld
, 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.If
HelloWorld
contained 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 methoddisplayHelloWorld
within theHelloWorld
class becomesJava_HelloWorld_displayHelloWorld()
. In our example, there is no package name becauseHelloWorld
is 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
JNIEnv
interface pointer. It is through this pointer that your native code accesses parameters and objects passed to it from the Java application. Thejobject
parameter is a reference to the object itself. For a non-static native method, such as thedisplayHelloWorld
method 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 thejobject
parameter 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 pointerenv
parameter.
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