Is Java bytecode compatible with different versions of Java?

Question

If I compile an application using Java 5 code into bytecode, are the resulting .class files going to be able to run under Java 1.4?

If the latter can work and I'm trying to use a Java 5 framework in my Java 1.4 application, is there anything I should watch out for?

Answer 1

Nope! .class files are forward compatible only. Java 5 introduced new classfile attributes and format changes, to handle Varargs, enums, and generics. Java 4 would just fail when processing these.

However, there is the unsupported -target jsr14 option on javac that generates JDK 1.4-compatible bytecode for some Java 5 language features.

Also, there are projects (e.g. Retroweaver, Retrotranslator) that convert Java 5 classfiles into Java 4 files.

EDIT: I found this good resource: Using Java 5 language features in earlier JDKs

Answer 2

No. You can only move upwards - Java 1.4 bytecode will work on Java 5 runtimes. However, if you aren't using any functionality not found in Java 1.4, you can compile using the -target and -source options of javac.

If you want to use Java 5 to write an application that can be run on Java 4, you can't use any features that weren't present before Java 5.