Adding new paths for native libraries at runtime in Java

Question

Is it possible to add a new path for native libraries at runtime ?. (Instead of starting Java with the property java.library.path), so a call to System.loadLibrary(nativeLibraryName) will include that path when trying to find nativeLibraryName. Is that possible or these paths are frozen once the JVM has started ?

Answer 1

[This solution don't work with Java 10+]

It seems impossible without little hacking (i.e. accessing private fields of the ClassLoader class)

This blog provide 2 ways of doing it.

For the record, here is the short version.

Option 1: fully replace java.library.path with the new value)

public static void setLibraryPath(String path) throws Exception {
    System.setProperty("java.library.path", path);

    //set sys_paths to null so that java.library.path will be reevalueted next time it is needed
    final Field sysPathsField = ClassLoader.class.getDeclaredField("sys_paths");
    sysPathsField.setAccessible(true);
    sysPathsField.set(null, null);
}

Option 2: add a new path to the current java.library.path

/**
* Adds the specified path to the java library path
*
* @param pathToAdd the path to add
* @throws Exception
*/
public static void addLibraryPath(String pathToAdd) throws Exception{
    final Field usrPathsField = ClassLoader.class.getDeclaredField("usr_paths");
    usrPathsField.setAccessible(true);

    //get array of paths
    final String[] paths = (String[])usrPathsField.get(null);

    //check if the path to add is already present
    for(String path : paths) {
        if(path.equals(pathToAdd)) {
            return;
        }
    }

    //add the new path
    final String[] newPaths = Arrays.copyOf(paths, paths.length + 1);
    newPaths[newPaths.length-1] = pathToAdd;
    usrPathsField.set(null, newPaths);
}

Answer 2

I used this in Java 12/13 which should work for any JVM with MethodHandles:

Lookup cl = MethodHandles.privateLookupIn(ClassLoader.class, MethodHandles.lookup());
VarHandle sys_paths = cl.findStaticVarHandle(ClassLoader.class, "sys_paths", String[].class);
sys_paths.set(null);

It has the benefit of being a Java API.

It is replaces the:

    final Field sysPathsField = ClassLoader.class.getDeclaredField("sys_paths");
    sysPathsField.setAccessible(true);
    sysPathsField.set(null, null);