Recompilation of dependencies with Maven - possible? Any performance boost?

Question

I was thinking about dependencies in Maven. Maven downloads them but it is unknown for what target version of JVM are they compiled for and with what compiler. This raises two questions:

• Would dependency recompilation bring faster dependency libraries? I tried to search for this, but have not found sufficient answer. I found out that for 1.6 there is Split bytecode verification, that is done when compiling with target 1.6.
  There is also a question Are Java 6's performance improvements in the JDK, JVM, or both? where it is mentioned that newer versions of javac might generate more optimized code.
• Is it possible with Maven to perform recompilation of depending libraries? Would it be possible to configure Maven to download sources, put there information about 1.6 target and perform mvn clean install?
  I am aware of Maven Dependency plugin and dependency:sources goal. That could be used for source download.
  There is also Maven Replacer Plugin allowing replacing of text in files. As stated in its Issue 58 there was implemented XPath support for it.
  Would it be possible to implement it with these plugins for dependency and also for its dependencies to perform it? I am not sure on how to perform it on the dependencies - perhaps with Maven Replacer Plugin injecting the configuration into unpacked dependencies pom.xml?
  Or is there a simpler way to configure target java version with build profile in user's settings.xml that would take precedence of project settings and therefore avoiding pom.xml modification?

Answer 1

The javac does next to no optimisations and those it has have been there from the earliest days. (And could even be considered historical) If you have code built by Java 1.0 you might find an improvement but anything from the last tens years is likely to be as optimal in byte code as it is today.

Most of the optimisation is done in the JVM itself, and you should find that Java 6 update 30 runs faster than Java 6 update 0 even for the exactly the same code.

Answer 2

Let's think for a moment what a Maven dependency is. In the most general sense, it's a piece of software developed by someone else, living its own life cycle that:

• is packed in an archive,
• which exists in a repository
• under a version descriptor (group, artifact, version).

That's all Maven guarantees. Note that it even does not say it's Java (could be resources, or native libraries, for example). Not much. Far too little to demand automatic recompilation of dependencies.