Aspect-oriented software development (AOSD) is gaining popularity with the wider adoption of languages such as AspectJ. However, though the state-of-the-art aspect-oriented programming environment (such as AJDT in the Eclipse IDE) provides powerful capabilities to check the syntactic or grammar errors in AspectJ programs, it fails to detect potential semantic defects in aspect-oriented software systems. In this paper, we present XFindBugs, an eXtended FindBugs for AspectJ, to help programmers find potential bugs in AspectJ applications through static analysis. XFindBugs supports 17 bug patterns to cover common error-prone features in an aspect-oriented system, and integrates the corresponding bug detectors into the FindBugs framework. We evaluate XFindBugs on a number of large-scale open source AspectJ projects (306,800 LOC in total). In our evaluation, XFindBugs confirms 7 reported bugs and finds 257 previously unknown defects. Our experiment also indicates that the bug patterns supported in XFindBugs exist in real-world software systems, even for mature applications by experienced programmers.