Queryll: Java Database Queries through Bytecode Rewriting
When interfacing Java with other systems such as databases, programmers must often program in special interface languages like SQL. Code written in these languages often needs to be embedded in strings where they cannot be error-checked at compile-time, or the Java compiler needs to be altered to directly recognize code written in these languages. We have taken a different approach to adding database query facilities to Java. Bytecode rewriting allows us to add query facilities to Java whose correctness can be checked at compile-time but which don't require any changes to the Java language, Java compilers, Java VMs, or IDEs. Like traditional object-relational mapping tools, we provide Java libraries for accessing individual database entries as objects and navigating among them. To express a query though, a programmer simply writes code that takes a Collection representing the entire contents of a database, iterates over each entry like they would with a normal Collection, and choose the entries of interest. The query is fully valid Java code that, if executed, will read through an entire database and copy entries into Java objects where they will be inspected. Executing queries in this way is obviously inefficient, but we have a special bytecode rewriting tool that can decompile Java class files, identify queries in the bytecode, and rewrite the code to use SQL instead. The rewritten bytecode can then be run using any standard Java VM. Since queries use standard Java set manipulation syntax, Java programmers do not need to learn any new syntax. Our system is able to handle complex queries that make use of all the basic relational operations and exhibits performance comparable to that of hand-written SQL.