Does the JDK version being used to compile the code matter?

Question

In a conversation with someone else, they claimed that the version of JDK being used to compile the code does not matter vs. the JVM version being used to run the compiled code.

That seems strange to me - are there never instances where the JDK version matters? Is functionality always only dependent on the JVM the code is running on, and not on how the code is being compiled?

https://stackoverflow.com/a/19554706/2687324 talks about backwards compatibility. Is Java code also forwards compatible?

Answer 1

No, you can't necessarily run code compiled with a new JDK on an old JRE. Compiled classes contain a version number for the class file format; if this is newer than the runtime expects, it will refuse to load the class.

Most Java compilers support an option to target an older JRE, generating an older class file format than the compiler was built for. However, you can still run into trouble if you don't also compile against an older version of the Java runtime library. Your code might use new API that isn't in the older version of Java. The compiler, with its current version of the API, won't catch this even when you specify an older target.

For the standard javac compiler in OpenJDK, these options are -target and -bootclasspath. You might also want to set the -source option to catch usage of newer language features that require support the older class files don't provide.