3D Shooters for Quake II - Pro Android Games - page 232

// Java
package quake.jni;

public class Natives {
...
public static native int QuakeMain(String[] argv);
...
}

// in jni_quake.c
JNIEXPORT jint JNICALL Java_quake_jni_Natives_QuakeMain
(JNIEnv * env, jclass cls, jobjectArray jargv)

Note that the Java string arguments map into a jobjectArray in C, and they must be
converted to the standard char * format. Fortunately, JNI provides the utility functions
GetArrayLength , GetObjectArrayElement , GetStringUTFChars , and ReleaseStringUTFChars to
do this easily.

1. First, get the size of the Java array using GetArrayLength .

jsize clen = (*env)->GetArrayLength(env, jargv);

2. Next, allocate a C array of the same size.

char * args[(int)clen];

3. Then, loop through the size value, extracting a Java string from the

array by calling GetObjectArrayElement , then converting that Java
string into a C string using GetStringUTFChars , as shown:

int i;
jstring jrow;
for (i = 0; i < clen; i++)
{
jrow = (jstring)(*env)->GetObjectArrayElement(env, jargv, i);
const char *row = (*env)->GetStringUTFChars(env, jrow, 0);

4. Then, simply allocate space for the corresponding C string using the

malloc system call.

args[i] = malloc( strlen(row) + 1);
strcpy (args[i], row);

5. Make sure to release the Java string using ReleaseStringUTFChars

when done. Failing to do so will create a memory leak.

// free java string jrow
(*env)->ReleaseStringUTFChars(env, jrow, row);
}