Automatic Repair Method for Null Pointer Dereferences Guided by Program Dependency Graph

Abstract

Automatic program repair (APR) is an effective technique for eliminating defects. The repair of null pointer dereferences, as the most common defects, requires accurate dependencies among statements to determine where to repair and how to repair. In order to precisely identify the data and control dependencies, the program dependency graph is adopted. Based on the symmetry among a large number of patches, we propose four repair mechanisms in this passage, namely the assignment mechanism, restraint mechanism, evading mechanism, and transfer mechanism, and employ the decision tree algorithm to match the best repair mechanism for defects. The four repair mechanisms locate the accurate repair position using the program dependency graph, and generate candidate patches by reassigning the null pointer with an appropriate value, making a judgment for the null value in advance, or throwing an exception. Our method was implemented in the repair tool DTSFix, which supports the automatic repair of null pointer dereference in Java programs. The experimental result on Defects4J shows that 73% of null pointer dereferences are successfully repaired by DTSFix, and that the generated candidate patches do not contain over-fitting patches.