How to pass char array from C JNI function to Java method as byte[]

Question

I'm having trouble finding the right documentation for passing a char buffer from JNI method to Java method. Here's the code

jint JNICALL Java_foo_package_MyJavaClass_myNativeMethod(JNIEnv *jenv, jobject jobj)
{
    jclass clazz = (*jenv)->GetObjectClass(jenv, jobj);
    //  MyJavaClass method:  private void addData(byte[] data)
    jmethodID mid = (*jenv)->GetMethodID(jenv, clazz, "addData", "([B)V");
    assert(mid);

    const char buf[] = { 0, 1, 2, 3, 42 };
    const size_t buf_len = sizeof buf;

    (*jenv)->CallVoidMethod(jenv, jobj, mid, buf /* obviously wrong */ );

    return 0;
}

Is CallVoidMethod the right function to use here, what's the correct thing to pass to it, how to allocate it, and how (if at all) it should be freed?

A code snippet would probably be the most compact answer, with a few words explaining how ownership of objects goes.

I think you have to change the GetMethodID param type to "([C)V". — Yohji
– Yohji, Commented Jun 6, 2014 at 8:53
@Yohji No, C is Java char, which is 16 bits and represents a unicode character. C char represents a native byte (8 bits assumed in this case), often (but not here) used to represent 8 bit character of any 8 bit encoding. — hyde
– hyde, Commented Jun 6, 2014 at 13:45
@hyde Java char holds a UTF-16 code-unit; One or two of which represents a Unicode character. Character.toChars(int) provides a succinct explanation. — Tom Blodget
– Tom Blodget, Commented Jun 7, 2014 at 3:04

Shiv Buyya · Accepted Answer · 2014-11-20 13:46:32Z

Below example works for passing char[] from C code to Java byte[].

void JNICALL Java_com_example_testapplication_MainActivity_getJNIByteArrayArg(JNIEnv    *jenv, jobject jobj)
{
jclass clazz = (*jenv)->FindClass(jenv, "com/example/testapplication/MainActivity"); // class path
jmethodID mid = (*jenv)->GetMethodID(jenv, clazz, "addData", "([B)V");// function name

jbyteArray retArray;
char data[] = {'a','b',3,4,5};
int data_size = 5;
if(!retArray)
retArray = (*jenv)->NewByteArray(jenv, data_size);

if((*jenv)->GetArrayLength(jenv, retArray) != data_size)
{
    (*jenv)->DeleteLocalRef(jenv, retArray);
    retArray = (*jenv)->NewByteArray(jenv, data_size);
}

void *temp = (*jenv)->GetPrimitiveArrayCritical(jenv, (jarray)retArray, 0);
memcpy(temp, data, data_size);
(*jenv)->ReleasePrimitiveArrayCritical(jenv, retArray, temp, 0);

(*jenv)->CallVoidMethod(jenv, jobj, mid, retArray);
}
public void addData(byte[] data) {
    System.out.println("Buyya: From C: " + new String(data));
}

Alex Barker · Accepted Answer · 2014-06-07 17:15:14Z

3

The functions you are looking for are GetByteArrayElements and ReleaseByteArrayElements.

Something like this should do the trick:

jint JNICALL Java_foo_package_MyJavaClass_myNativeMethod(JNIEnv *jenv, jobject jobj)
{
    jclass clazz = (*jenv)->GetObjectClass(jenv, jobj);
    //  MyJavaClass method:  private void addData(byte[] data)
    jmethodID mid = (*jenv)->GetMethodID(jenv, clazz, "addData", "([B)V");
    assert(mid);

    const char buf[] = { 0, 1, 2, 3, 42 };
    const size_t buf_len = sizeof buf;

    jboolean isCopy;
    jbyte *jbuf = (*jenv)->GetByteArrayElements(jenv, buf, &isCopy);

    (*jenv)->CallVoidMethod(jenv, jobj, mid, jbuf);

    (*jenv)->ReleaseByteArrayElements(jenv, buf, jbuf, 0);

    return JNI_OK;
}

edited Jun 7, 2014 at 17:15

answered Jun 6, 2014 at 21:02

Alex Barker

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2 Comments

QZHua Over a year ago

Why the buf_len is not used? I need to pass the length of the array to Java too?

Alex Barker Over a year ago

Is is not really needed for this example, and I probably forgot to delete if from whatever I copy pasted this snippet from. You will need it for dealing with the C <=> Java conversions. I am not sure what happens java overflows buf. I would suspect you get null back if isCopy is not set and and JNI_TRUE for isCopy if it is a pointer. Below ShivBuy has a good example of where buf_len is needed in conjunction with NewByteArray().

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