Java Native Access

Last updated
Java Native Access
Original author(s) Todd Fast, Timothy Wall, Liang Chen
Initial releaseMay 9, 2007 (2007-05-09)
Stable release
5.17.0 / March 16, 2025;4 months ago (2025-03-16) [1]
Repository
Written in C and Java
Operating system Windows, macOS, Android, AIX, FreeBSD, Linux, OpenBSD, Solaris, Windows Mobile
Platform Java 1.4 or later (for JNA 3.5.2 or earlier), Java 1.6 for JNA 4.0.0 and later
Size 1.83 MB (archived)
Type Software Library
License LGPL version 2.1 or later and (from version 4.0 onward) the Apache Software License, version 2.0
Website github.com/java-native-access/jna

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

Contents

Since Java 22, the Foreign Function and Memory API was provided as a standard modern alternative.

Architecture

The JNA library uses a small native library called foreign function interface library (libffi) 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, 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. [2]

Native TypeSizeJava TypeCommon Windows Types
char 8-bit integer byte BYTE, TCHAR
short 16-bit integer short WORD
wchar_t 16/32-bit character char TCHAR
int 32-bit integer int DWORD
int boolean valuebooleanBOOL
long 32/64-bit integerNativeLongLONG
long long 64-bit integer long __int64
float 32-bit FP float
double 64-bit FP double
char*C stringStringLPCSTR
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 POSIX (Linux / Unix / macOS) platforms.

importcom.sun.jna.Library;importcom.sun.jna.Native;importcom.sun.jna.Platform;/** Simple example of native library declaration and usage. */publicclassHelloWorld{publicinterfaceCLibraryextendsLibrary{CLibraryINSTANCE=(CLibrary)Native.loadLibrary((Platform.isWindows()?"msvcrt":"c"),CLibrary.class);voidprintf(Stringformat,Object...args);}publicstaticvoidmain(String[]args){CLibrary.INSTANCE.printf("Hello, World\n");for(inti=0;i<args.length;i++){CLibrary.INSTANCE.printf("Argument %d: %s\n",i,args[i]);}}}

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.

importcom.sun.jna.Library;importcom.sun.jna.Native;/** Simple example of native C POSIX library declaration and usage. */publicclassExampleOfPOSIX{publicinterfacePOSIXextendsLibrary{publicintchmod(Stringfilename,intmode);publicintchown(Stringfilename,intuser,intgroup);publicintrename(Stringoldpath,Stringnewpath);publicintkill(intpid,intsignal);publicintlink(Stringoldpath,Stringnewpath);publicintmkdir(Stringpath,intmode);publicintrmdir(Stringpath);}publicstaticvoidmain(String[]args){// It is possible to load msvcrt for its partial POSIX support on Windows...POSIXposix=(POSIX)Native.loadLibrary("c",POSIX.class);// but it will still fail on Windows due to /tmp being missing.posix.mkdir("/tmp/newdir",0777);posix.rename("/tmp/newdir","/tmp/renamedir");}}

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.

importcom.sun.jna.Library;importcom.sun.jna.Native;/** Simple example of Windows native library declaration and usage. */publicclassBeepExample{publicinterfaceKernel32extendsLibrary{// FREQUENCY is expressed in hertz and ranges from 37 to 32767// DURATION is expressed in millisecondspublicbooleanBeep(intFREQUENCY,intDURATION);publicvoidSleep(intDURATION);}publicstaticvoidmain(String[]args){Kernel32lib=(Kernel32)Native.loadLibrary("kernel32",Kernel32.class);lib.Beep(698,500);lib.Sleep(500);lib.Beep(698,500);}}

See also

References

  1. ↑ "Release 5.17.0". GitHub . 2025-03-16.
  2. ↑ "Default Type Mappings". jna.dev.java.net. Retrieved 2011-08-02.
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