The Java Native Interface (JNI) - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 613

Table 22.2 JNI functions to get Java field values.

Java field type JNI function Return type

boolean GetBooleanField() jboolean

byte GetByteField() jbyte

char GetCharField() jchar

short GetShortField() jshort

int GetIntField() jint

long GetLongField() jlong

float GetFloatField() jfloat

double GetDoubleField() jdouble

22.8.3 Getting and setting non-static field values
with the field ID
Getting the field ID is only half the battle, since what we really want is the field
itself. With a field ID in hand, we can then get and set the values of static fields
in a class or non-static fields in an object. We discuss non-static fields first. Let
us consider the some - int field of our JNIDemo class. Since it is an int ,we
get its value with the GetIntField() method:

jint my - int = jenv->GetIntField (jo, some - int - fid);
The jo parameter is a reference to the object that contains the desired field.
Since there may be multiple object instantiations, each with different values for
the fields, we must pass in a reference to the object that we're interested in.
GetIntField() also requires the field ID of the field we desire. Here we have
used the some - int - fid obtained earlier in Section 22.8.1. There are analogous
GetXxxField() functions for the other primitives as shown in Table 22.2. Each
“ Get „ method takes a jobject parameter, which must not be NULL , and a valid
field ID. Each returns a corresponding “ j „ data type to the C side as shown in
the table.
To set the value of a primitive field, we use the field ID and a “ Set „ method.
Forexample, if we want to change the value of the JNIDemo.some - int field
from a native method implementation, we use code like the following:

jint new - value = 7;
jenv- > SetIntField (jo, some - int - fid, new - value);
There are, of course, “ Set „ methods for each primitive type.
These built-in JNI methods handle all the Java primitive types. For object
types, there are general GetObjectField() and SetObjectField() func-
tions. They apply for standard objects, like the some - string String field of
JNIDemo , and for any custom objects the programmer might define. The return