easy way to find references to other classes in class file

Question

The class file format as described in http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html contains all references to other classes in the the constant pool as entries of type CONSTANT_Utf8.

But these entries are not only references to classes but also class literals, names of methods, fields and what not.

In a first attempt I thought it would be sufficient to use the constant pool entries referenced by other constant_pool entries of type CONSTANT_Class, CONSTANT_NameAndType and CONSTANT_MethodType

But these don't seem to include type parameters and annotations. Further reading of the specification seems to suggest that I need to parse things like RuntimeVisibleAnnotations and similar constructs in order to identify the relevant constant pool entries. Which means I have to parse more or less the complete class file.

But the whole idea behind parsing the class file myself was that it would be simpler then using a library like ASM, because I thought it would be sufficient to interpret the constant pool.

My question is: Is there a way to reliable identify all classes referenced in a class file by just interpreting little more than the constant pool?

Answer 1

Annotation types that cannot be loaded by a class loader are ignored by this class loader and will simply appear to be invisible at runtime. I assume that this is the reason that types that are referenced by an annotation are not stored in the constant pool where the resolution of an unknown type would prohibit successful class loading. Annotations are code attributes, i.e. meta data and they should not be linked deeply into the class by avoiding a constant pool entry.

You are therefore required to also introspect RuntimeVisibleAnnotations which live outside of the constant pool. However, if the constant pool does not contain a string RunntimeVisibleAnnotations, your approach is working. ASM has however very little overhead so I would use it nevertheless.