Java Native Access


Java Native Access is a community-developed library that provides Java programs easy access to native shared libraries without using the Java Native Interface. JNA's design aims to provide native access in a natural way with a minimum of effort. No boilerplate or generated glue code is required.

Architecture

The JNA library uses a small native library called foreign function interface library to dynamically invoke native code. The JNA library uses native functions allowing code to load a library by name and retrieve a pointer to a function within that library, and uses libffi library to invoke it, all without static bindings, header files, or any compile phase. The developer uses a Java interface to describe functions and structures in the target native library. This makes it quite easy to take advantage of native platform features without incurring the high development overhead of configuring and building JNI code.
JNA is built and tested on macOS, Microsoft Windows, FreeBSD / OpenBSD, Solaris, GNU with Linux, AIX, Windows Mobile, and Android. It is also possible to tweak and recompile the native build configurations to make it work on most other platforms that run Java.

Mapping types

The following table shows an overview of types mapping between Java and native code and supported by the JNA library.
C TypeSizeJava Language TypeCommon Windows Types
char8-bit integerbyteBYTE, TCHAR
short16-bitshortshort WORD
wchar_t16/32-bit charactercharWCHAR, TCHAR
int32-bit integerintDWORD
intboolean valuebooleanBOOL
long32/64-bit integerNativeLongLONG
long long, __int6464-bit integerlongLONGLONG
float32-bit FPfloat
double64-bit FPdouble
char*C stringStringLPCTSTR
void*pointerPointerLPVOID, HANDLE, LPXXX

Note: The meaning of TCHAR changes between char and wchar_t according to some preprocessor definitions. LPCTSTR follows.

Memory byte alignment for data structures

Native libraries have no standardized memory byte alignment flavor. JNA defaults to an OS platform specific setting, that can be overridden by a library specific custom alignment. If the alignment details are not given in the documentation of the native library, the correct alignment must be determined by trial and error during implementation of the Java wrapper.

Example

The following program loads the local C standard library implementation and uses it to call the printf function.
Note: The following code is portable and works the same on Windows and GNU+Linux / Unix / macOS platforms.

import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;
/** Simple example of native library declaration and usage. */
public class HelloWorld

The following program loads the C POSIX library and uses it to call the standard mkdir function.
Note: The following code is portable and works the same on POSIX standards platforms.

import com.sun.jna.Library;
import com.sun.jna.Native;
/** Simple example of native C POSIX library declaration and usage. */
public class ExampleOfPOSIX

The program below loads the Kernel32.dll and uses it to call the Beep and Sleep functions.
Note: The following code works only on Windows platforms.

import com.sun.jna.Library;
import com.sun.jna.Native;
/** Simple example of Windows native library declaration and usage. */
public class BeepExample

Adoption

Java Native Access is known to be used in the following projects: