Is the class file generated by javac always the same?

Question

Currently we are in the process of re-scripting all of our build system for a large project (around 2000 source files) and there has been talk of doing a binary comparison on the files to ensure that everything is correct which leads to the following question: Is the output of javac guaranteed to be same across compilations or could it be subject to change?

Another question implied that the constant pool could have a different order, but assuming we are able to control for the order of the files going into the javac call is there still a potential for differences? We are using Ant and Maven as part of the build if that may influence things as well.

Answer 1

The bytecode is absolutely not guaranteed to be the same; for one thing, compilers are allowed to perform optimizations that don't affect any guaranteed behaviors. The Java Language Specification even mentions, in a few places, optimizations that a compiler might perform; for example, of the string concatenation operator +, it notes that:

An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

[link]