The Java programming language (and its runtime) has undergone numerous changes since its release since its initial public release. These changes include:

• Changes in the Java programming language syntax and semantics
• Changes in the APIs provided by the Java standard class libraries.
• Changes in the Java (bytecode) instruction set and classfile format.

With very few exceptions (for example the enum keyword, changes to some “internal” classes, etc), these changes are backwards compatible.

• A Java program that was compiled using an older version of the Java toolchain will run on a newer version Java platform without recompilation.
• A Java program that was written in an older version of Java will compile successfully with a new Java compiler.

Compiling old Java with a newer compiler

If you need to (re-)compile older Java code on a newer Java platform to run on the newer platform, you generally don’t need to give any special compilation flags. In a few cases (e.g. if you had used enum as an identifier) you could use the -source option to disable the new syntax. For example, given the following class:

public class OldSyntax {
    private static int enum;  // invalid in Java 5 or later
}

the following is required to compile the class using a Java 5 compiler (or later):

$ javac -source 1.4 OldSyntax.java

Compiling for an older execution platform

If you need to compile Java to run on an older Java platforms, the simplest approach is to install a JDK for the oldest version you need to support, and use that JDK’s compiler in your builds.

You can also compile with a newer Java compiler, but there are complicated. First of all, there some important preconditions that must be satisfied:

• The code you are compiling must not use Java language constructs that were not available in the version of Java that you are targeting.
• The code must not depend on standard Java classes, fields, methods and so on that were not available in the older platforms.
• Third party libraries that the code depends must also be built for the older platform and available at compile-time and run-time.

Given the preconditions are met, you can recompile code for an older platform using the -target option. For example,

$ javac -target 1.4 SomeClass.java