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Visualizing Classes as they Appear in a Running JVM

Note: This feature is experimental.

In addition to displaying class files from file system and JARs (which is the default option), Bytecode Visualizer 3.5 and Bytecode Visualizer 3.4SR1 are also able to visualize classes as they are loaded in a running Java Virtual Machine (JVM).

This preference may be set under

Window > Preferences > Dr. Garbage > Bytecode Visualizer > General

Window > Preferences > Dr. Garbage > Bytecode Visualizer > General

This may be important in situations when a class loaded in a running JVM differs from its file system counterpart. There are several motivations why this may be done:

  • Code Optimization
  • Logging and Tracing
  • Profiling (Performance Analysis)
  • Obfuscation

And there are several ways how this can be accomplished:

  • -Xprep - preprocessor as a property
  • Custom Class Loader
  • JVMTI or the older JVMPI

No matter how and why a class was changed after its compilation, Bytecode Visualizer 3.5 is able to display its actual appearance within a running JVM. To achieve this, Bytecode Visualizer uses Java Debug Interface (JDI).

In the following, some limitations of JDI are addressed.

JDI vs. Filesystem

  • Some information cannot be accessed over JDI: Attributes, Exceptions in signatures
  • Retrieving a class over JDI is only possible, when the given class has already been loaded into the JVM. If you are trying to view a class which has not been loaded yet, Bytecode Visualizer falls back to retrieving it from the file system. From where the class has actually been retrieved can be seen in the header of Bytecode Visualizer. The same holds for inner types: Only those ones can be listed which were already loaded.
  • Eclipse does not reopen the viewer if there is one already showing the given class. If the code of the given class has been changed inbetween, you will not see the actual state.

Capabilities of Distinct JDI and JVM Versions

JDI capabilities of Bytecode Visualizer 3.5 can be used when you run Eclipse with Java 5 or Java 6. Dr. Garbage strongly recommends using Java 6.
If you are - for some reason - forced to run Bytecode Visualizer with Java 5, you will have to do without some information:

  • Class file format version
  • Constant Pool

Other Limitations

  • Bytecode Visualizer is currently not able to display classes which do not have any counterpart in the build path of an Eclipse project. This holds e.g. for synthetic classes which are completely generated at runtime. We are working on a fix. Betatesters with real world class-generation scenarios are welcome - please contact us.