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Declaring Native Methods

This section illustrates how to declare a native method in Java and how to generate the corresponding C/C++ function prototype.

The Java Side

Our first example, Prompt.java, contains a native method that accepts and prints a Java string. The program calls the native method, which waits for user input and then returns the line the user typed in.

The Prompt class contains a main method, which is used to invoke the program, and a native method named getLine, which is declared as follows:

private native String getLine(String prompt);

Notice that the declarations for native methods are almost identical to the declarations for regular, non-native Java methods. However, you declare native methods differently, as follows:

• First, native methods must have the native keyword. The native keyword informs the Java compiler that the implementation for this method is provided in another language.
• Secondly, the native method declaration is terminated with a semicolon (the statement terminator symbol) because the Java class file does not include implementations for native methods.

The Native Language Side

Compile the Prompt.java file and then generate the .h file. First, compile the Prompt.java file as follows:

javac Prompt.java

Once you have successfully compiled Prompt.java and have created the Prompt.class file, you can generate a JNI-style header file by specifying a -jni option to javah:

javah -jni Prompt

Examine the Prompt.h file. Note the function prototype for the native method getLine that you declared in Prompt.java.

JNIEXPORT jstring JNICALL
     Java_Prompt_getLine(JNIEnv *, jobject, jstring);

The native method function definition in the implementation code must match the generated function signature in the header file. Always include JNIEXPORT and JNICALL in your native method function signatures. JNIEXPORT and JNICALL ensure that the source code compiles on platforms such as Microsoft Windows that require special keywords for functions exported from dynamic link libraries.

Native method names are concatenated from the following components:

• the prefix Java_
• the fully qualified class name
• an underscore "_" separator
• the method name

Graphically, this looks as follows:

Thus, the native code implementation for the Prompt.getLine method becomes Java_Prompt_getLine. (Remember that no package name component appears because the Prompt class is in the default package.)

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Note:  Overloaded native method names, in addition to the above components, have an extra two underscores "__" appended to the method name followed by the argument signature. To illustrate, we created a second version of the Java program, Prompt2.java, and overloaded the getLine method by adding a second argument of type int. The two getLine method names in the Prompt2.h header file look as follows:

Java_Prompt2_getLine__Ljava_lang_String_2
Java_Prompt2_getLine__Ljava_lang_String_2I

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Recall from Step 3: Create the .h File, each native method has two parameters in addition to those parameters that you declared on the Java side. The first parameter, JNIEnv *, is the JNI interface pointer. This interface pointer is organized as a function table, with every JNI function at a known table entry point. Your native method invokes specific JNI functions to access Java objects through the JNIEnv * pointer. The jobject parameter is a reference to the object itself (it is like the this pointer in Java).

Lastly, notice that the JNI has a set of type names, such as jobject and jstring, and each type corresponds to Java types. This is covered in the next section.

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