Going Native with the NDK - Beginning Android Games - page 632

The log() method mimics the Android Java Log.logd() method. It takes a tag and a message
that will get printed to LogCat.
The copy() method is actually useful. In Chapter 8, we investigated the performance problems
for the FloatBuffer.put() method. We resorted to using a pure Java implementation that
used IntBuffer and some nasty tricks so we could speed up the copying of a float array to
a direct ByteBuffer in the Vertices class. We'll now implement a method that takes a direct
ByteBuffer and a float array and copies the array to the buffer. This is a lot faster than using the
corresponding Java APIs. We can later modify our Vertices and Vertices3 classes to use this
new functionality.

Note that both methods are static methods instead of instance methods. This means we can
call them without having an instance of the class JniTest ! This also has minor effects on our
C signatures, as we'll see in a bit.

Creating the C/C++ Header and Implementation
The first thing we do when we start to write the C/C++ code is generate the header file via the
javah JDK command-line tool. It takes a few parameters that are useful to us:
The name of the output file, which in our case is

jni/jniutils.h . The javah
tool will create the jni/ folder for us if it doesn't exist yet.
The path containing the

ï?®

ï?® .class file of the Java class for which it should
generate a C header. This will be bin/classes if we invoke javah from the
root directory of our project. It's the output path for the Eclipse compiler
when it compiles any of our Android project's source files.

ï?® com.badlogic.
androidgames.ndk.JniUtils in our case.
Open the terminal or command prompt and navigate to the root folder of the Android project.
Make sure that the NDK and JDK are in your $PATH as described earlier. Now execute the
following command:

The fully qualified name of the class, which is

javah -o jni/jniutils.h -classpath bin/classes com.badlogic.androidgames.ndk.JniUtils

This will create a file called jniutils.h in the jni/ folder of our Android project. Listing 13-2
shows its content.

Listing 13-2. jniutils.h, Containing the C Functions That Implement Our Native Methods

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_badlogic_androidgames_ndk_JniUtils */

#ifndef _Included_com_badlogic_androidgames_ndk_JniUtils
#define _Included_com_badlogic_androidgames_ndk_JniUtils
#ifdef __cplusplus
extern "C" {