Monday, December 26, 2005

Debug information in Java .class files

I have often wanted to learn more about the class file format of Java class files. This study let me to interesting discoveries - regarding what debug information is stored in class files and how it is stored.

When we compile a Java source using the 'javac' exe, the generated class file by default contains some debug info - By default, only the line number and source file information is generated.
Hence when a stack trace is printed on the screen, we can see the source file name and the line number also printed on the screen. Also, when using log4J, I remember using a layout which can print the line number of each log statement - I bet log4J uses the debug infomation present in the class file to do this.

The javac exe also has a '-g' option - This option generates all debugging information, including local variables. To understand this better, I compiled a java source twice; once with the -g option and once without. I then decompiled both the class files to see how the decompiled output differs. The class file compiled with the debug option, showed all the local variable names same as in the original Java file, whereas the one without the debug option, had the local variables named by the decompiler as the local variable information was not stored in the class file.

Another interesting fact is that the class file can contain certain attributes that are non-standard; i.e. vendor specific. Suprised !!!...So was I..Please find below a snippet from the VM spec :

Compilers for Java source code are permitted to define and emit class files containing new attributes in the attributes tables of class file structures. Java Virtual Machine implementations are permitted to recognize and use new attributes found in the attributes tables of class file structures. However, all attributes not defined as part of this Java Virtual Machine specification must not affect the semantics of class or interface types. Java Virtual Machine implementations are required to silently ignore attributes they do not recognize.

For instance, defining a new attribute to support vendor-specific debugging is permitted. Because Java Virtual Machine implementations are required to ignore attributes they do not recognize, class files intended for that particular Java Virtual Machine implementation will be usable by other implementations even if those implementations cannot make use of the additional debugging information that the class files contain