Changing Java's Semantics for Handling Null Pointer Exceptions

Abstract

We envision a world where no exceptions are raised; instead, language semantics are changed so that operations are total functions. Either an operation executes normally or tailored recovery code is applied where exceptions would have been raised. As an initial step and evaluation of this idea, we propose to transform programs so that null pointer dereferences are handled automatically without a large runtime overhead. We increase robustness by replacing code that raises null pointer exceptions with error-handling code, allowing the program to continue execution. Our technique first finds potential null pointer dereferences and then automatically transforms programs to insert null checks and error-handling code. These transformations are guided by composable, context-sensitive recovery policies. Error-handling code may, for example, create default objects of the appropriate types, or restore data structure invariants. If no null pointers would be dereferenced, the transformed program behaves just as the original.We applied our transformation in experiments involving multiple benchmarks, the Java Standard Library, and externally reported null pointer exceptions. Our technique was able to handle the reported exceptions and allow the programs to continue to do useful work, with an average execution time overhead of less than 1% and an average bytecode space overhead of 22%.