Question

Is there a way to enforcing loading a class after programm start? I have following case: I have a hashmap holding Name and java.lang.Class of plugin classes. in each plugin class I have a static block registring the class

static {
ClassMap.getInstance().register("name",MyPlugin.class);
}

I don’t know the name and package of in this example MyPlugin. So I want that this code will executed at programm start. How is this possible?

Answer 1

If the class object is in the map, this means that the class has already been loaded i.e. your static block has been executed already.

If on the other hand, you just have the class names (e.g. in a list) you can do his:

for(String classname : yourList){
    // wrap with try / catch
    Class.forName(className);
}

Here is a pretty good overview of what happens at class load time

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Update: what you seem to want is a kind of component scanning that will find all you plugin classes. There’s no way to do that with plain Java, but you can e.g. use the reflections library to do that. Sample code:

Reflections reflections = new Reflections("com.your.project");
Set<Class<? extends YourBaseClass>> subTypes = 
    // and all of these classes are already initialized
    reflections.getSubTypesOf(YourBaseClass.class);