I'm curious if Java programs that were written 10 to 15 years ago can still run on newer Java Virtual Machines without needing any updates. Has Java maintained compatibility over the years, or are there significant changes that could cause older code to break?
3 Answers
In most cases, yes! As long as your code isn't built on older Java EE packages that have been renamed, it should run pretty much the same. Just be cautious with libraries that mess around with things like sun.misc.Unsafe—those can cause problems. Straightforward command line applications from those days typically work without issues.
Generally, yes for JVMs, but no for some JREs since some APIs have been deleted. The opcodes from back then are usually still part of the JVM spec, so old bytecode can run on new JVMs. However, you might have to recreate some standard APIs that are now missing, which means it's not exactly zero effort to get everything up and running. Also, just a heads up—running ancient code might bring unexpected results, especially if that code had bugs that 'worked' by chance.
It's tough to say for sure! Java aimed for backwards compatibility, but that has loosened a bit in the past decade or so. You might run into some issues, especially if your code uses bytecode manipulation tools, which might not recognize older bytecode formats. Plus, deprecated APIs can cause hiccups, too.
That makes sense! I assumed they'd keep everything intact for backwards compatibility. It's wild to think that deprecated ops can still be a thing.