Strange problem with my ClassFactory when a non used jar file is missing.

Question

Hello,
I have a small problem regarding a class factory and I don't really understand how to solve it.
If you see the example below we can imagine that I have a classFactory who can provide 3 different types of classes.
public class ClassFactory {
public ClassFactory() {
public static Service getClass(String szType) {
if(szType.equals("1")) {
return new Class1();
} else if (szType.equals("2")) {
return new Class2();
} else if (szType.equals("3")) {
return new Class3();
else {
return null;
My problem is the following. Let�s imagine that Class1 and Class2 do not use specific jar file but Class3 uses a jar file which is not always present in old installation.
I thought that it was possible to get this factory running correctly if the customer only uses Class1 and Class2 same if specific jar file used by Class3 was not present but this is not the case.
I remarked that if the specific jar file used in Class3 was not present in my classpath the ClassFactory is blocking (same if we do not try to get a "Class3").
My problem is that I want to keep my code backward compatible without asking the customer to add a new jar file only used in new installation.
Can anyone of you explain me why it is reacting like this and how can I solve this problem?
Thank in advance for your reply.
Alain.

Answer 1

There's a standard way of handling these things now. You place a text file in each jar that contributes candidate concrete classes with the FQN of an implementing class on each line. Your factory class then collects all these using getResources() on the context class loader and loads the classes dynamically.
The text file goes in the diretory META-INF/services and the file name is the FQN of the interface or abstract class they all implement.
Typically there's a small factory class for each implementation and you ask each, in turn, whether they accept the specifications the caller requested of your factory (typically static getInstance() method).
Or you could, for example, put the selection name/index or whatever in each class as an annotation.