Accessing Java class members from C

In the previous examples, we have been ignoring the second argument that was automatically defined by javah for the C native function. Here, we want to access a data member of the Java object that calls the native method. This is where the second argument comes in - it is the implicit this pointer of the calling Java object (line 26).

Notice that the native method has been declared non-static (line 13). This causes javah to assign a type of jobject to the second argument (instead of the type jclass used previously). This is so that the this pointer of the Java object can be communicated to non-static native methods (the current example), or, the type of the Java class can be communicated to static native methods (the two previous examples).

To decouple JNI users from implementation details of JNI vendors, and to support portability of user code, JNI has a reflection (or meta-data) capability that must be exercised in order to access Java data members and member functions. This meta-data feature is accessed via JNI functions and JNI functions are called via the env pointer. We'll need four new JNI functions for this example:

GetObjectClass() - return a data structure that represents the object's class
GetFieldID() - return an offset into the class's data structure
GetXxxField() - return the current value of the specified attribute
SetXxxField() - set the value of the specified attribute

The Xxx above represents a Java data type (see GetDoubleField() and SetDoubleField() at lines 29, 31, and 33). The string "D" (lines 28 and 30) denotes the type double. All Java types are mapped to their own code -

B		byte
C		char
D		double
F		float
I		int
J		long
Lclassname;		a class type
S		short
V		void
Z		boolean

We'll see a lot more of this table in the next several examples as we start working with the "signatures" of constructors and other member functions.

[source: Horstmann98, pp594-595]