Question

I want to make an application that can dynamically load plug-ins but I’ve not found any literature on Internet.

The difficult thing is: I don’t know the name in advance.

For example I’ve a Plugin interface:

public interface Plugin {
    public static Plugin newPlugin();
    public void executePlugin(String args[]);
}

So that every Class implementing Plugin in the jar file are instantiated in a list:

Method method = classToLoad.getMethod ("newPlugin");
mylist.add(method.invoke(null);

1. First problem is, I cannot have a static method in an interface.
2. Second problem is, I don’t know how to find all classes that implement an interface

Thanks for your help.

Answer 1

So it seems like you want to dynamically discover Classes that implement a specific interface (e.g., Plugin) at runtime. You have basically two choices for this:

1. Use a component framework like osgi
2. Use Java’s internal discovery process (ServiceLoader)

Since there are many good tutorials on osgi (also small ones), I will not detail that here. To use Java’s internal discovery process, you need to do the following:

• Bundle all “new” classes that you wish to discover into a jar file
• Create a new file inside the jar file: META-INF/services/package.Plugin You must use the full package qualifier here
• This file is a simple text file and contains the fully qualified name of each class implementing Plugin in that jar-file
• Place that jar file into the classpath of your (potentially already running) application
• Discover the services:

Service discovery is done like this:

ServiceLoader<Plugin> loader = ServiceLoader.load(Plugin.class)
for (Plugin p : loader) {
    // do something with the plugin
}

There are more details here: http://docs.oracle.com/javase/7/docs/api/java/util/ServiceLoader.html

As for static methods in interfaces: not possible. The semantics of that would also be somewhat weird as static methods are accessible without an instance of a class, and interfaces just define the methods, without any functionality. Thus, static would allow to call Interface.doSomething() whereas the interface does not define any functionality, this leads just to confusion.

edit:

added description what should be in the meta-file