certificate - Java SecurityException: signer information does not match

I recompiled my classes as usual, and suddenly got the following error message. Why? How can I fix it?

java.lang.SecurityException: class "Chinese_English_Dictionary"'s signer information does not match signer information of other classes in the same package
    at java.lang.ClassLoader.checkCerts(ClassLoader.java:776)

Answer

This happens when classes belonging to the same package are loaded from different JAR files, and those JAR files have signatures signed with different certificates - or, perhaps more often, at least one is signed and one or more others are not (which includes classes loaded from directories since those AFAIK cannot be signed).

So either make sure all JARs (or at least those which contain classes from the same packages) are signed using the same certificate, or remove the signatures from the manifest of JAR files with overlapping packages.