How do I load and use native library in java?

Question

I've got a java class, calling a native method and trying to load library:

import java.io.UnsupportedEncodingException; public class Main { public static native String getMyString(String s); /** * @param args * @throws UnsupportedEncodingException */ public static void main(String[] args) throws UnsupportedEncodingException { // TODO Auto-generated method stub // System.out.println("here!"); String s2 = getMyString("string text"); for (Byte b : s2.getBytes("UTF-8")) { System.out.print(b); System.out.print(","); } } static { System.loadLibrary("mylib.so"); } }

The "mylib.so" is in the directory, where Main.class is located.

When I run java Main I get following exception:

Exception in thread "main" java.lang.UnsatisfiedLinkError: no mylib.so in java.library.path at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1856) at java.lang.Runtime.loadLibrary0(Runtime.java:845) at java.lang.System.loadLibrary(System.java:1084) at Main.<clinit>(Main.java:24)

What should I change for this to wark?

I've tried setting library full path without success

Unless you've coded for JNI, you're probably not going to get much further even after you successfully load your .so. Please check out the Sun JNI tutorial: java.sun.com/developer/onlineTraining/Programming/JDCBook/… — paulsm4
– paulsm4, Commented Aug 2, 2012 at 18:40

Reimeus · Accepted Answer · 2012-08-02 18:40:51Z

12

Do the following:

Use System.loadLibrary("mylib");
Copy mylib.so to libmylib.so
Run java -Djava.library.path=/root/ Main

edited Aug 2, 2012 at 18:40

answered Aug 2, 2012 at 18:33

Reimeus

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3 Comments

vbezhenar Over a year ago

Also use -Djava.library.path=path-to-lib-file when starting java.

Arsen Zahray Over a year ago

I made the changes and run the program with java Main -Djava.library.path=/root/, without any success

codesniffer Over a year ago

Note that the path passed to -Djava.library.path must be an absolute path.

muru · Accepted Answer · 2017-05-25 01:12:02Z

"How to load native library"

public final class NativeLibsLoaderUtil { private static final String JAVA_LIBRARY_PATH = "java.library.path"; private static final String SYS_PATHS = "sys_paths"; private NativeLibsLoaderUtil() { } private static void addLibsToJavaLibraryPath(final String tmpDirName) { try { System.setProperty(JAVA_LIBRARY_PATH, tmpDirName); /* Optionally add these two lines */ System.setProperty("jna.library.path", tmpDirName); System.setProperty("jni.library.path", tmpDirName); final Field fieldSysPath = ClassLoader.class.getDeclaredField(SYS_PATHS); fieldSysPath.setAccessible(true); fieldSysPath.set(null, null); } catch (IllegalAccessException | NoSuchFieldException e) { LOGGER.error(e.getMessage(), e); } } }

Where tmpDirName is a directory where you store your library. Or you can modify above class and use temp directory from your system property, like this:

/** * Temporary directory system property name */ private static final String JAVA_IO_TMPDIR = "java.io.tmpdir"; /** * * @return */ private static File getTempDir() { final String tmpDirName = System.getProperty(JAVA_IO_TMPDIR); final File tmpDir = new File(tmpDirName); if (!tmpDir.exists()) { tmpDir.mkdir(); } return tmpDir; }

!But first you have to copy there your native lib :)

Then to load native library call "addLibsToJavaLibraryPath" method in static block in "most root" class before any class constructor was executed.

static { NativeLibsLoaderUtil.addLibsToJavaLibraryPath("/tmp"); }

Community · Accepted Answer · 2017-05-23 12:24:29Z

You should add the so to library path: -Djava.libarary.path= (this is in the java command).

if you run from eclipse: How to add native library to "java.library.path" with Eclipse launch (instead of overriding it)

Gil · Accepted Answer · 2022-09-07 21:25:57Z

If you compiled opencv, on installation you should have seen something like:

make install: -- Up-to-date: /usr/local/share/java/opencv4/libopencv_java460.so -- Up-to-date: /usr/local/share/java/opencv4/opencv-460.jar

make a hard link to the /usr/lib/ folder:

$ sudo ln /usr/local/share/java/opencv4/libopencv_java460.so /usr/lib/libopencv_java460.so

And then just run:

System.loadLibrary(Core.NATIVE_LIBRARY_NAME);

that he will get the *.so

twid · Accepted Answer · 2012-08-21 12:28:03Z

-7

As Reimeus answered. Or you can use System.load("/Library/Path/libsample.so");

answered Aug 21, 2012 at 12:28

twid

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1 Comment

user207421 Over a year ago

... which makes your answer redundant.

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