CODEWITHSUNDEEP
javajava-native-interface
Philipp Wendler
Philipp Wendler

Reputation: 11433

How to influence search path of System.loadLibrary() through Java code?

In a Java project, I am using a third-party library that loads some native library via

System.loadLibrary("libName");

I'd like to be able to influence the search path of this method from within my application, so that the user doesn't need to specify a correct java.library.path value on the command line (this value depends on the current OS and architecture). E.g on Windows I want to set it to "lib/native/windows", on Linux 32bit to "lib/native/linux32" etc.

I tried

System.setProperty("java.library.path", ...)

but this is ignored, apparently because the JVM reads this property only once before my code is run.

I also tried to load the native libray before using the Java library that depends on it with

System.load("fullPath/lib")

This call succeeds, but there will still be an UnsatisfiedLinkError when the native library is loaded again with System.loadLibrary().

The only way I found is the following:

This works, but I find it very complicated and it is much effort because I need to add all those interfaces. Is there a simpler way?

Upvotes: 26

Views: 29224

Answers (6)

Donny
Donny

Reputation: 51

Just recently ran into this issue and using OpenJDK where a NullPointerException is thrown (as @0-0 mentioned in his comment to Samil's answer). The following works in OpenJDK and should work with Oracle JDK as well.

(Option 1) Replace the java.library.path

System.setProperty("java.library.path", newPath);
Field field = ClassLoader.class.getDeclaredField("sys_paths");
field.setAccessible(true);
field.set(ClassLoader.getSystemClassLoader(), new String[]{newPath});

(Option 2) Add to existing java.library.path

String libPath = System.getProperty("java.library.path");
String newPath;

if (libPath == null || libPath.isEmpty()) {
    newPath = path;
} else {
    newPath = path + File.pathSeparator + libPath;
}

System.setProperty("java.library.path", newPath);

Field field = ClassLoader.class.getDeclaredField("sys_paths");
field.setAccessible(true);
    
// Create override for sys_paths
ClassLoader classLoader = ClassLoader.getSystemClassLoader(); 
List<String> newSysPaths = new ArrayList<>();
newSysPaths.add(path);  
newSysPaths.addAll(Arrays.asList((String[])field.get(classLoader)));
           
field.set(classLoader, newSysPaths.toArray(new String[newSysPaths.size()]));

Upvotes: 2

Stephen C
Stephen C

Reputation: 718788

  1. There is no approved way to change the library path for a running JVM.
  2. You cannot load a native library more than once ... and you cannot unload a native library so that you can load it again: https://bugs.java.com/bugdatabase/view_bug?bug_id=4171986

Based on your comments above (particularly, the behavior of the 3rd-party library), I'd say that your best option is to get the library path right when you launch the JVM.

Note that there is a hacky way to change the library path (see https://stackoverflow.com/a/24258955/139985) but it involves nasty reflection, and it reportedly doesn't work for all Java releases. Certainly, it relies on undocumented private implementation details of ClassLoader that could change from one release to the next.

Upvotes: 6

Petr
Petr

Reputation: 47

While technically correct these answers are missleading. Setting the environment variables PATH on Windows, or LD_LIBRARY_PATH on unix will change where the jvm looks for libraries: What is LD_LIBRARY_PATH and how to use it?

on linux: export LD_LIBRARY_PATH=/usr/.../ then: java ....

Upvotes: -1

Samil
Samil

Reputation: 1013

I needed to change the dll path for my unit tests. I tried the following hack and it worked:

System.setProperty( "java.library.path", "/path/to/libs" ); 
Field fieldSysPath = ClassLoader.class.getDeclaredField( "sys_paths" );
fieldSysPath.setAccessible( true );
fieldSysPath.set( null, null );

For explanation, see the original link.

Upvotes: 5

fimez
fimez

Reputation: 408

I tried to following to load a native Growl library for a Java application on my Mac where the lib is in the root of the classpath of my application:

System.load(GrowlUtils.class.getResource("/libgrowl.jnilib").getFile().toString());

Upvotes: 1

Andreas Dolk
Andreas Dolk

Reputation: 114767

Is there a simpler way?

Yes, provide batch/script files to start the application. Then you can set the correct path in the batch/shell file or even read the value from an environment variable. Much easier then trying to do it from inside the application.

Upvotes: 0

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