The Java Native Interface (JNI) - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 610

Table 22.1 JNI Type Signatures.

Java type Signature

boolean Z

byte B

char C

short S

int I

long J

float F

double D

type [] [ type

Class

L fully-qualified-class;

replacing dots, and ends with a semicolon. So the String field's type sig-
nature is “ Ljava/lang/String; „ . The type signature for a custom object
is similar. It just specifies the custom path to the object instead of a path
inside the java.lang package. So the type signature of MyCustomObject
is “ Ljavatech/jni22/MyCustomObject; „ .Anarray of object types is
merely prefixed with a left square bracket, just like arrays of primitives.
The complete mapping of Java types to JNI type signatures is given in
Table 22.1. Notice that the type signature does not differ for static and non-static
fields. That difference is handled by using either the GetStaticFieldID() or
GetFieldID() function, respectively. With this table and the rules given above,
one can generate the type signature of any field in any class. However, rather than
manually using the table and rules, it is often easier to use the javap tool to dis-
play the signatures automatically. This tool operates on a compiled class file, and
the -s option is used to generate type signatures. By default, javap generates
only the signatures for the default, protected, and public access scope classes and
fields. Use the -private option to show all classes and fields. With our java-
tech.jni22.JNIDemo class

described above compiled into build/

classes ,welaunch javap as follows:

javap -s -private -classpath build/classes
javatech.jni22.JNIDemo

where we fully specify the class name ( javatech.jni22.JNIDemo )onthe
command line and also set the classpath to point to the build/classes direc-
tory. The output from javap is

1 Compiled from " JNIDemo.java "
2 public class javatech.jni22.JNIDemo extends
java.lang.Object {