Going Native with the NDK - Beginning Android Games - page 633

#endif
/*
* Class: com_badlogic_androidgames_ndk_JniUtils
* Method: log
* Signature: (Ljava/lang/String;Ljava/lang/String;)V
*/
JNIEXPORT void JNICALL Java_com_badlogic_androidgames_ndk_JniUtils_log
(JNIEnv *, jclass, jstring, jstring);
/*
* Class: com_badlogic_androidgames_ndk_JniUtils
* Method: copy
* Signature: (Ljava/nio/ByteBuffer;[FII)V
*/
JNIEXPORT void JNICALL Java_com_badlogic_androidgames_ndk_JniUtils_copy
(JNIEnv *, jclass, jobject, jfloatArray, jint, jint);
#ifdef __cplusplus
}
#endif
#endif
Time to implement those functions. First, we create a new file called jniutils.cpp in the jni/
folder. Listing 13-3 shows its content.
Listing 13-3. jniutils.cpp, the Implementation of the JniUtils Native Methods
#include <android/log.h>
#include <string.h>
#include "jniutils.h"
We need a few C includes, namely log.h , which is provided by the NDK, string.h , and our
own jniutils.h . The first include lets us use the native Android logging functions. The second
include lets us use memcpy() . The last one imports the signatures of our native methods as well
as jni.h , which contains the JNI APIs.
JNIEXPORT void JNICALL Java_com_badlogic_androidgames_ndk_JniUtils_log
(JNIEnv *env, jclass clazz, jstring tag, jstring message) {
const char *cTag = env->GetStringUTFChars(tag, 0);
const char *cMessage = env->GetStringUTFChars(message, 0);
__android_log_print(ANDROID_LOG_VERBOSE, cTag, cMessage);
env->ReleaseStringUTFChars(tag, cTag);
env->ReleaseStringUTFChars(message, cMessage);
}
This function implements the JniUtils.log() method, which takes a JNIEnv and a jclass as
the first two parameters. The env parameter allows us to work directly with the JVM. The jclass
parameter represents the JniUtils class. Remember that our methods are static methods.
Instead of getting a jobject , as in the previous example, we get the class. The tag and message
parameters are the two strings we pass in from Java.