The Java Native Interface (JNI) - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 612

private int[][] array2d;
private String some - string;

}

The native implementation of doStatic() begins

JNIEXPORT void JNICALL
Java - javatech - jni22 - JNIDemo - doStatic (JNIEnv *jenv,
jclass cls) {
...

}

Since the jclass is received in the parameter list, it is already available for any
calls to GetStaticFieldID() or GetFieldID() .
However, the native implementation of the doNonStatic() method receives
a jobject parameter instead of a jclass . The reason should be obvious. Static
methods are class methods. They are available even if the class has never been
instantiated. Said another way, static methods are independent of any particu-
lar instantiation (i.e. object) of the class in which they appear. They have no
this variable. When a static method is called, it is called without any object
of that class necessarily having been instantiated. But instance methods (i.e.
non-static methods) require an instantiation of the class (i.e. an object). For
this reason, instance methods are sometimes called object methods. Each object
method is associated with a particular object, and object methods always have an
implied this variable. So, while a native implementation of a static method
receives a reference to the class in which the native method is declared on
the Java side (the jclass parameter), the native implementation of an object
method receives a reference to the object from which it was called (the jobject
parameter).
Since a jclass reference is needed to call GetFieldID() , receiving the
jclass directly in the argument list of the native function that implements a static
native method is convenient for finding field IDs. Non-static native methods are
not so lucky. Obviously a jobject is closely related to a jclass (the former
is an instantiation of the latter), so it should be easy to find the jclass that is
associated with the jobject .Infact, it is easy. For a jobject known as jo ,
the JNI function to get the corresponding jclass is

jclass cls = jenv->GetObjectClass (jo);

Using this JNI library function, non-static native implementations can find their
jclass to use to find field IDs.
It is even possible to find field IDs for classes for which we have neither a
jclass nor a jobject reference. We later discuss the FindClass() function
that provides this service.