Is the creation of Java class files deterministic?

Question

When using the same JDK (i.e. the same javac executable), are the generated class files always identical? Can there be a difference depending on the

Answer 1

Firstly, there's absolutely no such guarantee in the spec. A conforming compiler could stamp the time of compilation into the generated class file as an additional (custom) attribute, and the class file would still be correct. It would however produce a byte-level different file on every single build, and trivially so.

Secondly, even without such nasty tricks about, there's no reason to expect a compiler to do exactly the same thing twice in a row unless both its configuration and its input are identical in the two cases. The spec does describe the source filename as one of the standard attributes, and adding blank lines to the source file could well change the line number table.

Thirdly, I've never encountered any difference in build due to the host platform (other than that which was attributable to differences in what was on the classpath). The code which would vary based on platform (i.e., native code libraries) isn't part of the class file, and the actual generation of native code from the bytecode happens after the class is loaded.

Fourthly (and most importantly) it reeks of a bad process smell (like a code smell, but for how you act on the code) to want to know this. Version the source if possible, not the build, and if you do need to version the build, version at the whole-component level and not on individual class files. For preference, use a CI server (such as Jenkins) to manage the process of turning source into runnable code.