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JNI Programming in C++

The JNI provides a slightly cleaner interface for C++ programmers. The jni.h file contains a set of inline C++ functions. This allows the native method programmer to simply write:

jclass cls = env->FindClass("java/lang/String");

instead of:

jclass cls = (*env)->FindClass(env, "java/lang/String");

The extra level of indirection on env and the env argument to FindClass is hidden from the programmer. The C++ compiler simply expands out the C++ member function calls to their C counterparts; therefore, the resulting code is exactly the same.

The jni.h file also defines a set of dummy C++ classes to enforce the subtyping relationships among different variations of jobject types:

class _jobject {};
class _jclass : public _jobject {};
class _jthrowable : public _jobject {};
class _jstring : public _jobject {};
... /* more on jarray */

typedef _jobject *jobject;
typedef _jclass *jclass;
typedef _jthrowable *jthrowable;
typedef _jstring *jstring;
... /* more on jarray */

The C++ compiler is therefore better able than the C compiler to detect when incorrect types are passed to methods. For example, it is incorrect to pass a jobject to GetMethodID because GetMethodID expects a jclass. You can see this by examining the GetMethodID signature:

jmethodID GetMethodID(jclass clazz, const char *name, const char *sig);

The C compiler treats jclass as the same as jobject because it makes this determination using the following typedef statement:

typedef jobject jclass;

Therefore a C compiler is not able to detect that you have mistakenly passed the method a jobject instead of jclass.

The added type safety in C++ comes with a small inconvenience. Recall from Accessing Java Arrays that in C you can fetch a Java string from an array of strings and directly assign the result to a jstring, as follows:

jstring jstr = (*env)->GetObjectArrayElement(env, arr, i);

In C++, however, you need to insert an explicit conversion of the Java string to jstring:

jstring jstr = (jstring)env->GetObjectArrayElement(arr, i);

You must make this explicit conversion because jstring is a subtype of jobject, which is the return type of GetObjectArrayElement.

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