The Java Native Interface (JNI) - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 616

are used much like the analogous GetFieldID() and GetStaticFieldID()
functions. Instead of a field type signature, they are passed a method signature.
22.9.1 The method ID and method signature
Method signatures use an extension of the notation described in Section 22.8.1
for type signatures. A method signature begins with a parenthesized list of its
parameters, each using the type signature notation. After the closing paren-
thesis appears the return type of the method (in type signature form). There
are no commas or other separators between the parameter types. For exam-
ple, the signature “ (II)F „ describes a method that takes two int parame-
ters and returns a float - such as float some - method (int a, int
b) . The letter V is used for a void return type. If there are no parameters,
then the method descriptor begins with “ () „ , not “ (V) „ ,which is an error
since there is no void type in Java. The same “ L „ notation is used for object
types and “ [ „ notation for arrays. For example, a method that takes an array
of Java String types as a parameter and returns a String is described as
“ ([Ljava/lang/String;)Ljava/lang/String; „ .Aswith type signa-
tures, the easiest and most error-free way to determine method signatures is to use
javap -s as described above. If we add the following method to our JNIDemo
class:
int callback (int x) {
return 2*x;
}
that method's signature is “ (I)I „ either by inspection or by examining the output
of javap . To get the method ID, we use
jmethodID callback - mid = jenv- > GetMethodID (cls,
" callback " , " (I)I " );
The function arguments are a jclass reference to the class where the method
resides, the name of the method, and the method's signature. There is a corre-
sponding GetStaticMethodID() function for static methods.
GetMethodID() can fail in three ways, the most likely being NoSuchMe-
thodError in which no method with the given name and signature can be
found in the class referenced by cls , most likely due to a mispelled name or a
bad signature. The other two possible error conditions are ExceptionInIni-
tializerError and OutOfMemoryError .Ifany of these occur, the function
returns NULL which we must handle in the usual way:
if (callback - mid == NULL)
return NULL;