Load jar dynamically at runtime?

Question

My current java project is using methods and variables from another project (same package). Right now the other project\'s jar has to be in the classpath to work correctly.

Answer 1

I needed to load a jar file at runtime for both java 8 and java 9+. Here is the method to do it (using Spring Boot 1.5.2 if it may relate).

public static synchronized void loadLibrary(java.io.File jar) {
    try {            
        java.net.URL url = jar.toURI().toURL();
        java.lang.reflect.Method method = java.net.URLClassLoader.class.getDeclaredMethod("addURL", new Class[]{java.net.URL.class});
        method.setAccessible(true); /*promote the method to public access*/
        method.invoke(Thread.currentThread().getContextClassLoader(), new Object[]{url});
    } catch (Exception ex) {
        throw new RuntimeException("Cannot load library from jar file '" + jar.getAbsolutePath() + "'. Reason: " + ex.getMessage());
    }
}

Answer 2

Indeed this is occasionally necessary. This is how I do this in production. It uses reflection to circumvent the encapsulation of addURL in the system class loader.

/*
     * Adds the supplied Java Archive library to java.class.path. This is benign
     * if the library is already loaded.
     */
    public static synchronized void loadLibrary(java.io.File jar) throws MyException
    {
        try {
            /*We are using reflection here to circumvent encapsulation; addURL is not public*/
            java.net.URLClassLoader loader = (java.net.URLClassLoader)ClassLoader.getSystemClassLoader();
            java.net.URL url = jar.toURI().toURL();
            /*Disallow if already loaded*/
            for (java.net.URL it : java.util.Arrays.asList(loader.getURLs())){
                if (it.equals(url)){
                    return;
                }
            }
            java.lang.reflect.Method method = java.net.URLClassLoader.class.getDeclaredMethod("addURL", new Class[]{java.net.URL.class});
            method.setAccessible(true); /*promote the method to public access*/
            method.invoke(loader, new Object[]{url});
        } catch (final java.lang.NoSuchMethodException | 
            java.lang.IllegalAccessException | 
            java.net.MalformedURLException | 
            java.lang.reflect.InvocationTargetException e){
            throw new MyException(e);
        }
    }