Changing Java Library Path at Runtime

The java.library.path system property instructs the JVM where to search for native libraries. You have to specify it as a JVM argument using -Djava.library.path=/path/to/lib and then when you try to load a library using System.loadLibrary("foo"), the JVM will search the library path for the specified library. If it cannot be found you will get an exception which looks like:

Exception in thread "main" java.lang.UnsatisfiedLinkError: no foo in java.library.path
	at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1734)
	at java.lang.Runtime.loadLibrary0(Runtime.java:823)
	at java.lang.System.loadLibrary(System.java:1028)

The java.library.path is read only once when the JVM starts up. If you change this property using System.setProperty, it won't make any difference.

Here is the code from ClassLoader.loadLibrary which shows how the path is initialised:

if (sys_paths == null) {
    usr_paths = initializePath("java.library.path");
    sys_paths = initializePath("sun.boot.library.path");
}

As you can see from the code above, the usr_paths variable is only initialised if sys_paths is null, which will only be once.

So, how can you modify the library path at runtime? There are a couple of ways to do this, both involving reflection. You should only do this if you really have to.

Option 1: Unset sys_paths
If you set sys_paths to null, the library path will be re-initialised when you try to load a library. The following code does this:

/**
 * Sets the java library path to the specified path
 *
 * @param path the new library path
 * @throws Exception
 */
public static void setLibraryPath(String path) throws Exception {
    System.setProperty("java.library.path", path);

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

Option 2: Add path to usr_paths
Instead of having to re-evaluate the entire java.library.path and sun.boot.library.path as in Option 1, you can instead append your path to the usr_paths array. This is shown in the following code:

/**
* 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);
}