ASM-NonClassloadingExtensions

Provides non-classloading alternatives to implementations within ObjectWeb ASM.

For the most part, ASM deals only with Java bytecode, and avoids using java.lang.Class and related classloading, in it's implementations. However, there are a small number of corner cases where ASM loads classes, in order to obtain more detailed information about the type.

Classloading can be problematic in some cases, such as static analysis, where it's necessary to avoid executing the code (including class initializer blocks) and where loading the classes of a large codebase can exhaust memory. Class loading can also be a problem in runtime class generation, if the class doesn't exist yet, and thus cannot be loaded, but it's still necessary to know some information about the type, such as it's superclass, or the interfaces it implements.

Examples of loading classes in ASM include:

• ClassWriter which loads classes in order to find the common superclass between two types
• SimpleVerifier which loads classes to know whether the type represents an interface, what it's superclass is, and if one type can be assigned to another

Both of these examples can be overridden, which is where ASM-NonClassloadingExtensions comes in: it provides two drop-in replacements that can achieve the same functionality without loading classes, by reading the class files instead.

Drop-in Replacements

• if you want a ClassWriter that doesn't load classes, use an instance of org.mutabilitydetector.asm.NonClassloadingClassWriter
• if you want a SimpleVerifier that doesn't load classes, use an instance of org.mutabilitydetector.asm.tree.analysis.NonClassloadingSimpleVerifier

In both cases, you should provide an implementation of TypeHierarchyReader. The default implementation:

• expects to find the bytecode using ClassLoader.getSystemResourceAsStream. For static analysis or class generation, this is unlikely to be what you want, and you need to provide a way to retrieve the relevant bytecode to scan.
• performs no caching of the classfiles it reads. Thus will likely be slower than the equivalent classloading version, since the JVM performs caching of Class instances for you implicitly.

This library makes no attempt to guess at how you want to retrieve class files, or which caching strategy suits you best, so instead provides the hooks to allow you to plug in those specific bits, and still use the library to perform the class reading and logic. Both subclassing and wrapping a TypeHierarchyReader are supported, and there's a couple of simple implementations available.

Querying type information

In order to implement these non-classloading extensions, it was necessary to replace the job performed by java.lang.Class but without using a ClassLoader to retrieve instance of Class. The implementations of these were then found to be general enough that they might just be useful for querying type information, that you could normally only get via an instance of Class. In this codebase, TypeHierarchy provides an analogue to Class, and TypeHierarchyReader provides an analogue to ClassLoader.

So if you're doing static analysis of bytecode and you need:

• to know if one type can be assigned to another
• to be able to discover an entire type hierarchy all the way back to java.lang.Object

Then ASM-NonClassloadingExtensions may help you.

Disclaimer: if all you need to know is the immediate superclass or interfaces of a type, or if the class is an interface, that's already trivial with ASM, you probably don't need another library for that. However, the implementation of TypeHierarchyReader#obtainHierarchyOf(type) can show you how to do it.