Java Native Access

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, 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.

The following table shows an overview of types mapping between Java and native code and supported by the JNA library.[2]

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

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.

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 {
    public interface CLibrary extends Library {
        CLibrary INSTANCE = (CLibrary) Native.loadLibrary(
            (Platform.isWindows() ? "msvcrt" : "c"), CLibrary.class);
        void printf(String format, Object... args);
    }

    public static void main(String[] args) {
        CLibrary.INSTANCE.printf("Hello, World\n");
        for (int i = 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.

import com.sun.jna.Library;
import com.sun.jna.Native;

/** Simple example of native C POSIX library declaration and usage. */
public class ExampleOfPOSIX {
    public interface POSIX extends Library {
	    public int chmod(String filename, int mode);
	    public int chown(String filename, int user, int group);
	    public int rename(String oldpath, String newpath);
	    public int kill(int pid, int signal);
	    public int link(String oldpath, String newpath);
	    public int mkdir(String path, int mode);
	    public int rmdir(String path);
    }

    public static void main(String[] args) {
        POSIX posix = (POSIX) Native.loadLibrary("c", POSIX.class);
	    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.

import com.sun.jna.Library;
import com.sun.jna.Native;

/** Simple example of Windows native library declaration and usage. */
public class BeepExample {
    public interface Kernel32 extends Library {
        // FREQUENCY is expressed in hertz and ranges from 37 to 32767
        // DURATION is expressed in milliseconds
        public boolean Beep(int FREQUENCY, int DURATION);
        public void Sleep(int DURATION);
    }

    public static void main(String[] args) {
	    Kernel32 lib = (Kernel32) Native.loadLibrary("kernel32", Kernel32.class);
	    lib.Beep(698, 500);
	    lib.Sleep(500);
	    lib.Beep(698, 500);
    }
}

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

• Armed Bear Common Lisp, using JNA for its CFFI implementation[3]
• JRuby, using JNA for POSIX functionality[4][5]
• Freedom for Media in Java (FMJ)[6]
• IntelliJ IDEA IDE by the company JetBrains
• OpenSearchServer, an open source search engine and web crawler
• SVNKit, a pure Java Subversion client library with optional native library integration through the JNA
• VLC media player, via the vlcj library[7]
• Cyberduck FTP, SFTP, WebDAV, Cloud Files & Amazon S3 Browser for macOS
• Log4j, a library of native log appenders[8]
• Hudson and Jenkins continuous integration servers
• Webdriver
• YAJSW (Yet Another Java Service Wrapper)
• Cassandra, an open source distributed database from the Apache Software Foundation uses JNA[9]

• JNAerator
• P/Invoke
• SWIG

• Java Native Access Web Page
• Java Native Access - Download page
• Java Native Access - User Mailing List
• Java Native Access:An easier way to access native code by Jeff Friesen, JavaWorld.com, 5 February 2008
• Protect Your Legacy Code Investment with JNA by Stephen B. Morris, Java.net, 20 May 2009
• Dasgupta, Sanjay (11 November 2009). "Simplify Native Code Access with JNA". Java.net. Archived from the original on 15 November 2009.
• JNA is now a Githubber by Daniel Doubrovkine, 20 June 2011
• Calling the Lotus Domino C-API with JNA by Jesper Kiaer 21 March 2010