在运行时将jar文件下载并添加到classpath而不进行反射

Question

We have a standalone java-application. This application has lots of functionality, but most customers does only use a small bit of this functionality. We want to distribute the application with just the "normal" minimum of third-party jar-files. In the eclipse project all external jar-files are in the class path. So no reflection is used to instanciate classes. So when a user are going to use a more advanced function, eg taking webcamera pictures, we will then download all the javacv jar files and then somehow add these to the classpath/classloader before calling our class that takes a picture. Is this possible? I allready have a working solution for a very small module that downloads some jar-files on the fly and adds them in a custom classloader, but I then have to painfully instanciate every class and call every method by reflection. Eg:

URLClassLoader loader = new URLClassLoader(urls,w.getClass().getClassLoader());
Class grabberClass = Class.forName("org.bytedeco.javacv.FrameGrabber",true,loader);
Object grabber = grabberClass.getMethod("createDefault", int.class).invoke(null, 0);
grabberClass.getMethod("start", null).invoke(grabber, null);
//And so on...

This is not what we want. All our source code is just fine, and I don't want to obfuscate it by turning it into reflection-calls. But how may I use my old code and still be able to add the jar-files at runtime?

Answer 1

There is no need for reflection. Try running your application without the javacv jar files in the classpath. If you never touch the webcam functions, and if you haven't coded your application in a monolithic manner, you should not have any problems, because Java doesn't try to load a class unless and until some other class refers to it in code (or inherits from it, or uses it in a method signature or as a field type).

As for downloading the .jar files on demand, consider using Java Web Start and making them lazy components in a .jnlp file:

<!DOCTYPE jnlp PUBLIC "-//Sun Microsystems, Inc//DTD JNLP Descriptor 6.0.10//EN"
                      "http://java.sun.com/dtd/JNLP-6.0.10.dtd">
<jnlp version="1.6"
      codebase="http://www.example.com/ourstandaloneapp/"
      href="OurStandaloneApp.jnlp">
    <information>
        <title>Our Stand-alone App</title>
        <vendor>Example, Inc.</vendor>
        <homepage href="http://www.example.com/ourstandaloneapp/"/>
        <description>A stand-alone application with many capabilities.</description>
        <icon kind="default" href="standalone.gif"/>
        <offline-allowed/>
    </information>

    <security>
        <all-permissions/>
    </security>

    <resources>
        <j2se version="1.7+"/>
        <jar href="OurStandaloneApp.jar" main="true"/>
        <jar href="http://search.maven.org/remotecontent?filepath=org/bytedeco/javacv/0.10/javacv-0.10.jar"
             download="lazy"/>
    </resources>

    <application-desc/>
</jnlp>

A Web Start application doesn't have to be downloaded/installed from the web. You can actually package it on a CD or flash drive, with a shortcut or script on the installation media that executes javaws -codebase file:/path/to/media -import OurStandAloneApp.jnlp . See here for details. (Why that page was removed for Java 6 and later, I don't know. It's pretty valuable.)