Best way to build a Plugin system with Java

First you need an interface that all plugins need to implement, e.g.

public interface Plugin {
    public void load(PluginConfiguration pluginConfiguration);
    public void run();
    public void unload();
    public JComponent getConfigurationPage();
}

Plugin authors should then bundle their plugins into JAR files. Your applications opens the JAR file and could then use an attribute from JAR manifest or the list of all files in the JAR file to find the class that implements your Plugin interface. Instantiate that class, the plugin is ready to go.

Of course you may also want to implement some kind of sandboxing so that the plugin is restricted in what it can and can not do. I have created a small test application (and blogged about it) that consists of two plugins, one of which is denied access to local resources.

Use OSGi.

It is the foundation of the Eclipse plug-in system. Equinox is Eclipse's implementation (licensed EPL) and Felix is the Apache Project's implementation (licensed Apache Public License).

Eclipse provides a concrete example that OSGi can cover the points you mentioned (or you could just build your application on top of Eclipse RCP if you want a full Eclipse/SWT/JFace stack).

Since 1.6, there's been java.util.ServiceLoader which can be used if you want to code your own simple system.

But if you want anything more than basic features, use one of the existing frameworks.

There is also JPF (Java Plugin Framework).

I think that recommending OSGi for solving the above stated problem is extremely poor advice. OSGi is "the right choice" but for a scenario as the one above, I think either JPF or some homegrown minimalistic framework is sufficient.