Programmers may use Java diagnostic tools to determine the efficiency of their programs, to find bottlenecks, to study program behavior, or for many other reasons. Some of the common diagnostic tools that we examined are profilers and the Java Virtual Machine (JVM) options that make some internal JVM information available to users. Information produced by these tools varies in degrees of clarity, accuracy, and usefulness. We also found that running some of these tools in conjunction with a program may affect the program's behavior, creating what we refer to as an "observer effect". We examine several tools and discuss their level of usefulness and the extent to which they impact program behavior. Additionally, we discovered program instability, i.e. a tendency of a program to change its behavior when executed with different monitoring tools or multiple times with the same tool. We discuss potential causes for instability based on information obtained via running the HotSpot JVM with an option for logging its internal compilation and optimization process.
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