javah and headers

Learn to generate headers with javac -h.

Header generation with javac -h

The old javah tool generated C header files for native methods; it is deprecated since Java 8. Instead use javac -h . Class.java. This compiles the class and writes Class.h into the same folder.

The header file contains a C function declaration for each native method: JNIEXPORT jint JNICALL Java_NativeOps_sum(JNIEnv*, jclass, jint, jint);. You implement these functions on the C side.

Contents of the header file

The generated .h file carries a C function declaration for each native method. The declaration looks like: JNIEXPORT returnType JNICALL Java_Class_method(JNIEnv* env, jclass cls, ...). JNIEXPORT marks export, JNICALL is the calling convention and JNIEnv* lets C reach the JVM.

For non-static methods the second parameter is jobject (this); for static methods it is jclass. You implement these functions in C and compile them into a shared library.

Exercise Declare two native methods and print a header marker

Define class NativeOps with native int sum(int a, int b); and native String greet(String name);. In Main's main method print System.out.println("header generated");. Do not call the methods.

Show example solution

class NativeOps {
    native int sum(int a, int b);
    native String greet(String name);
}

public class Main {
    public static void main(String[] args) {
        System.out.println("header generated");
    }
}

Test yourself

Complete the header-generating command

javac ____ NativeOps.java

Show answer

javac -h . NativeOps.java

What does javac -h produce?
- An executable JAR
- Documentation HTML
- A C header file (.h)
- A class file (.class)
Show answer

A C header file (.h) — javac -h produces a C header file for the native methods.
Match the header elements

JNIEXPORT

Export macro
JNICALL

Calling convention
JNIEnv*

C interface pointer
jclass

Class reference

Next lesson: JNIEnv

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies