Specializing Generic Java']Java Data Structures

The Collections framework is an essential utility in virtually every Java application. It offers a set of fundamental data structures that exploit Java Generics and the Object type in order to enable a high degree of reusability. Upon instantiation, Collections are parametrized by the type they are meant to store. However, at compile-time, due to type erasure, this type gets replaced by Object, forcing the data structures to manipulate references of type Object (the root of the Java type system). In the bytecode, the compiler transparently adds type checking instructions to ensure type safety, and generates bridge methods to enable the polymorphic behavior of parametrized classes. This approach can introduce non-trivial runtime overheads when applications extensively manipulate Collections. We propose the Java Collections Specializer (JCS), a tool we have developed to deliver truly specialized Collections. JCS can generate ArrayLists, ConcurrentHashMaps and HashMaps with true type specialization that incur no performance penalties due to bridge methods or type checking instructions. JCS offers the possibility to easily extend its use to other Collection data structures. Since the specialized data structures extend and inherit from the generic counterpart's superclasses and interfaces, the specialized versions can be used in most places where generic versions are employed. The programmer uses JCS to generate specializations ahead of time. These are generated under the java.util package, and need only be added to the class path and integrated into the application logic. We show that the specialized data structures can improve the runtime performance of data intensive workloads by up to 14% for read use-cases and 42% for write use-cases.