If I compiled a java file in the newest JDK, would an older JVM be able to run the .class files?
That depends on three things:
The actual Java versions you are talking about. For instance, a 1.4.0 JVM can run code compiled by a 1.4.2 compiler, but a 1.3.x JVM cannot1.
The compilation flags used. There is a
-targetcompiler flag that tells it to generate code that will run on an older (target) JVM. And the
-sourcecompiler flag tells it to only accept the older JVM’s language features. (This approach won’t always work, depending on the Java language features used by your code. But if the code compiles it should work.)
The library classes that the class file uses. If it uses library classes that don’t exist in the older class libraries, then it won’t run … unless you can include a JAR that back-ports the classes2. You can avoid this problem by using the
-bootclasspathoption to compile your code against the APIs of the older version of Java.
Does the bytecode depend on the version of the java it was created with?
Yes, modulo the points above.
1 – The Java 8 JVMS states this: “Oracle’s Java Virtual Machine implementation in JDK release
1.0.2 supports class file format versions
45.3 inclusive. JDK releases
1.1.* support class file format versions in the range
45.65535 inclusive. For k ≥ 2, JDK release
1.k supports class file format versions in the range
2 – A backport could be problematic too. For example: 1) Things which depend on native code support would most likely require you to implement that native code support. 2) You would most likely need to put any back-port JAR file onto the bootclasspath when you run the code on the older JVM.