Add objectClass() and className() for QJniObject

This can be handy sometimes to avoid doing a env->GetObjectClass() call
to get the jclass object.

Change-Id: I015fe5ed73304338826e468e59778bcd3ceadc3b
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

src/corelib/kernel/qjnienvironment.cpp

@@ -286,12 +286,7 @@ bool QJniEnvironment::registerNativeMethods(const char *className, JNINativeMeth
 
     if (!classObject.isValid())
         return false;
-
-    jclass clazz = d->jniEnv->GetObjectClass(classObject.object());
-    const bool result = registerNativeMethods(clazz, methods, size);
-    d->jniEnv->DeleteLocalRef(clazz);
-
-    return result;
+    return registerNativeMethods(classObject.objectClass(), methods, size);
 }
 
 /*!

src/corelib/kernel/qjniobject.cpp

@@ -779,10 +779,10 @@ QJniObject::~QJniObject()
     jstring jstring = string.object<jstring>();
     \endcode
 
-    \note The returned object is still kept live by this QJniObject. To keep the
-    object live beyond the lifetime of this QJniObject, for example to record it
+    \note The returned object is still kept alive by this QJniObject. To keep the
+    object alive beyond the lifetime of this QJniObject, for example to record it
     for later use, the easiest approach is to store it in another QJniObject with
-    a suitable lifetime. Alternatively, you can make a new global reference to the
+    a suitable lifetime. Alternatively, you may create a new global reference to the
     object and store it, taking care to free it when you are done with it.
 
     \snippet jni/src_qjniobject.cpp QJniObject scope
@@ -792,6 +792,36 @@ jobject QJniObject::object() const
     return javaObject();
 }
 
+/*!
+    \fn jclass QJniObject::objectClass() const
+
+    Returns the class object held by the QJniObject as a \c jclass.
+
+    \note The returned object is still kept alive by this QJniObject. To keep the
+    object alive beyond the lifetime of this QJniObject, for example to record it
+    for later use, the easiest approach is to store it in another QJniObject with
+    a suitable lifetime. Alternatively, you may create a new global reference to the
+    object and store it, taking care to free it when you are done with it.
+
+    \since 6.2
+*/
+jclass QJniObject::objectClass() const
+{
+    return d->m_jclass;
+}
+
+/*!
+    \fn QByteArray QJniObject::className() const
+
+    Returns the name of the class object held by the QJniObject as a \c QByteArray.
+
+    \since 6.2
+*/
+QByteArray QJniObject::className() const
+{
+    return d->m_className;
+}
+
 QJniObject QJniObject::callObjectMethodV(const char *methodName,
                                          const char *signature,
                                          va_list args) const

src/corelib/kernel/qjniobject.h

@@ -68,6 +68,9 @@ public:
         return static_cast<T>(javaObject());
     }
 
+    jclass objectClass() const;
+    QByteArray className() const;
+
     template <typename T>
     T callMethod(const char *methodName, const char *signature, ...) const
     {