Java
C/C++ implementation
Learn JNI implementation with C/C++.
JNI type mapping
JNI represents Java types with signature letters on the C side: int → I, long → J, boolean → Z, void → V, String → Ljava/lang/String;. A method signature has the form (params)return.
For example, int sum(int a, int b) has the signature (II)I. These signatures are visible with javap -s and are appended to the C function name for overloading.
Loading the native library
Before a native method is called the native library must be loaded: System.loadLibrary("nativeops");. This call is usually placed in a static block so the library is ready when the class loads. The library is searched along java.library.path.
If the library is not found an UnsatisfiedLinkError is thrown. Cross-platform projects build a separate library for each operating system.
Exercise Write the type mapper and native class
Define class TypeMapper with a static String jni(String t) method that uses a switch to return int → "I", long → "J", boolean → "Z", void → "V". Define class NativeOps with native int sum(int a, int b); and a static block calling System.loadLibrary("nativeops"); (wrapped in try). In main print System.out.println("(" + TypeMapper.jni("int") + TypeMapper.jni("int") + ")" + TypeMapper.jni("int"));.
Show example solution
class TypeMapper {
static String jni(String t) {
return switch (t) {
case "int" -> "I";
case "long" -> "J";
case "boolean" -> "Z";
case "void" -> "V";
default -> "?";
};
}
}
class NativeOps {
native int sum(int a, int b);
static {
try { System.loadLibrary("nativeops"); } catch (UnsatisfiedLinkError e) { }
}
}
public class Main {
public static void main(String[] args) {
System.out.println("(" + TypeMapper.jni("int") + TypeMapper.jni("int") + ")" + TypeMapper.jni("int"));
}
}
Test yourself
-
What is the JNI signature letter of int?
Show answer
I — int is shown with the letter I in JNI signatures.
-
Complete the method signature
String sig = "(____";
Show answer
String sig = "(II)I";
-
Match the type mapping
- int
- I
- long
- J
- boolean
- Z
- String
- Ljava/lang/String;
Next lesson: Foreign Function & Memory API
