Performance impact of Java class files generated with “-target 1.5” running on a 1.6 VM?-Collection of common programming errors

It shouldn’t make much difference. 1.6 files can have stack map/table structures that improve bytecode verification speed (Apache Harmony just uses a smarter algorithm).

If you were to go to 1.4 the initial loading of a class constant would be slightly slower, but that’s irrelevant given how long a class takes to load (a new form of the ldc bytecode replaces Class.forName, but the result was stored in a static field.)

In general it doesn’t matter. The runtime compiler is the important bit and that can pretty much generate the same machine code. In the past there have been made to javac to reduce the optimisation it does so as not to mangle the code for the runtime compiler, but that is independent of the -target flag.