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I recently discovered the usefulness of dynamically loading classes into a JVM and thought I would document the discovery.

Java has a class called a URL class loader (URLClassLoader). You can create one using a URL and install it as the current thread's class loader. Any references to new classes will use the URL class loader first before looking at the parent class loader.

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<br />
URL[] urls = ...<br />
ClassLoader originalClassLoader = Thread.currentThread().getClassLoader();<br />
ClassLoader newClassLoader = new URLClassLoader(urls, originalClassLoader);</p>
<p>try {<br />
    Thread.currentThread().setContextClassLoader(newClassLoader);<br />
    // write code to load new classes<br />
} finally {<br />
    Thread.currentThread().setCLassLoader(originalClassLoader);<br />
}<br />

The example and the way I used this was for loading in JNDI initial contexts. See this tutorial for an example.
One can use this to load groups of classes together from a local jar file.
I found it useful to extend this a little with the ability to walk a file system searching for jar files to load using a single class loader.

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<br />
public class JarSeekingURLClassLoader extends URLClassLoader {</p>
<p>    public JarSeekingURLClassLoader(File file, ClassLoader parent) throws MalformedURLException {<br />
        super(makeUrls(file), parent);<br />
    }</p>
<p>    private static URL[] makeUrls(File file) throws MalformedURLException {<br />
        List&lt;URL&gt; urls = new ArrayList&lt;URL&gt;();<br />
        urls.add(file.toURI().toURL());<br />
        File[] jarFilesAndDirs = file.listFiles(new FilenameFilter() {<br />
            @Override<br />
            public boolean accept(File dir, String name) {<br />
                return dir.isDirectory() || name.endsWith(&quot;.jar&quot;);<br />
            }<br />
        });<br />
        if (jarFilesAndDirs != null) {<br />
            for (File jarOrDir : jarFilesAndDirs) {<br />
                if (jarOrDir.isDirectory()) {<br />
                    urls.addAll(Arrays.asList(makeUrls(jarOrDir)));<br />
                } else {<br />
                    urls.add(jarOrDir.toURI().toURL());<br />
                }<br />
            }<br />
        }<br />
        return urls.toArray(new URL[urls.size()]);<br />
    }</p>
<p>    public JarSeekingURLClassLoader(File file) throws MalformedURLException {<br />
        super(makeUrls(file));<br />
    }</p>
<p>    public JarSeekingURLClassLoader(File file, ClassLoader parent, URLStreamHandlerFactory factory) throws MalformedURLException {<br />
        super(makeUrls(file), parent, factory);<br />
    }<br />
}<br />

One can of course use these two patterns together to arrange to dynamically load a directory full of JAR files using a single class loader.

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<br />
URL[] urls = ...<br />
ClassLoader originalClassLoader = Thread.currentThread().getClassLoader();<br />
ClassLoader newClassLoader = new JarSeekingURLClassLoader(new File(Config.getDynamicLibraryLocation());</p>
<p>try {<br />
    Thread.currentThread().setContextClassLoader(newClassLoader);<br />
    // write code to load new classes<br />
    Class.forName(&quot;com.dynamic.library.class&quot;);<br />
} finally {<br />
    Thread.currentThread().setCLassLoader(originalClassLoader);<br />
}<br />

This trick is handy if you need to simplify your compile time dependencies.

This is of course getting close to building a dynamic loading system like OSGi, but sometimes small concepts are more efficient.

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