Question

我无法找到将char缓冲区从JNI方法传递到Java方法的正确文档。这是代码

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;
}

CallVoidMethod是否在这里使用正确的函数，传递给它的正确方法是什么，如何分配它，以及如何（如果有的话）它应该被释放？

代码片段可能是最紧凑的答案，只需几句话就可以解释对象的所有权。

Answer 1

您正在寻找的功能是GetByteArrayElements和ReleaseByteArrayElements。

这样的事情可以解决问题：

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;
}

Answer 2

下面的示例适用于将char []从C代码传递到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));
}

如何将char数组从C JNI函数传递给Java方法作为byte []

2 个答案: