Effective and Efficient Compilation of Run-Time Generics in Java

Abstract

After several years from Sun Microsystems' call-for-proposals for adding generics to the Java Programming Language, JDK 1.5 will be finally shipped with a compile-time support for generics. However, differently e.g. from the current implementation of .NET Common Language Runtime, run-time support to generics -also commonly referred to as reification of type parameters -is not provided, leading to a number of well-known inadequacies which might potentially be critical. In this paper we present the EGO compiler (Exact Generics on Demand). This is the result of a project developed in collaboration with Sun Microsystems as an effort to provide run-time generics in a smooth way, without requiring any change on the JVM or on any other run-time support. The core solution is a sophisticated translation of code based on the type-passing style, where run-time type information is automatically created on a by-need basis, limiting as most as possible run-time overhead while retaining interoperability with legacy Java code. We present the main aspects of this development, from basic design to implementation and deployment issues. Many relevant aspects that typical raise when implementing advanced type systems over a mainstream programming language are discussed, shading light to some effective implementation techniques.

On the reification of Java wildcards

“…Moreover, the reference to a descriptor is also stored locally to where it has been used, e.g. as static field of a client class, leading to a particularly space-and time-efficient double-caching mechanism [33]. The details of this kind of management are encapsulated into a method $crCLA(), automatically added by the EGO compiler to a generic class (C), which takes the instantiation of the type parameters (T) and yields the corresponding descriptor of the current class (C<T>), also taking care of other tasks such as registering the descriptor and setting the descriptor for its direct supertype.…”

“…inner classes-though solutions at the JVM level have obviously better performance and coherence, and may be more appealing for an official release of the language. The EGO compiler (Exact Generics on-Demand) [33] is the result of a project developed in collaboration with Sun Microsystems with the goal of evaluating a smooth support to runtime generics, which would not require changes on the JVM or on any other component of the Java Runtime Environment (JRE). The solution conceived and developed is a sophisticated translation of code based on the type-passing style [34,32] -also known as lifting of type parameters -where runtime type information is automatically created on a by-need basis, and cached for future utilisation.…”

“…non-termination of subtyping, hidden existential types, and so on. Secondly, we apply this analysis to a specific reification technique, namely the EGO compiler [33]. In EGO, the compiler automatically creates the necessary code to make sure that generic type information is created once and on a by need basis, it is stored into object instances, and -thanks to specific caching techniques -it is quickly retrieved when necessary in type-dependent operations.…”

“…On the other hand, generics can be reified by the compiler, which can automatically create additional code running on the existing JVM, and simulating a layer of runtime generics. Two approaches following this direction are NextGen [4,20] and EGO (Exact-Generics on Demand) [33]. Though a JVM-based approach inevitably leads to a better-engineered solution (with greatest performance and coherence), as developed e.g for .NET [26], compile-time approaches are still an interesting alternative that require no JVM modification and is more easily deployed.…”

“…Several solutions have been studied to address this problem-a rather complete list of references is [19,1,4,22,14,13,34,32,33,20,2]. On the one hand, the JVM can be redesigned to directly represent generic types [14], possibly limiting the modifications to the class loader only, as shown e.g.…”