How do I compile a .java with support for older versions of Java?

You can specify the target version of the compiler lower than the build java version (all answers basically said that) but you should not: it is quite messy as it only works if you also supply the class libraries for the actual target version. If you do that you are much better off with simply using this old Java version to compile. Much cleaner and less stuff to setup. For Java 9 the options are a bit nicer but you still need an old JDK lying around for the rt.jar.

What you can do is in your ide and dedvelopment cycle use the -target/-bootclasspath trick to get faster compiler and better warnings, but IT and release builds then should be done with the target version.

You can use javac -target 1.5 <source files>.

If you're using a build system, Eclipse or some other IDE to build jars, please specify which one.

Yes, you can set the version of compiler at compile time. And compile your java code into old versions of java.

From Oracle article : http://docs.oracle.com/javase/1.5.0/docs/tooldocs/windows/javac.html

Cross-Compilation Example

Here we use javac to compile code that will run on a 1.4 VM.

% javac -target 1.4 -bootclasspath jdk1.4.2/lib/classes.zip \
             -extdirs "" OldCode.java

You might also need following parameter to set denote the version of your code.

-source release

-Specifies the version of source code accepted.

As of JDK 9, javac support a new option for cross-compiling

javac --release N ...

which is equivalent to

javac -source N -target N –bootclasspath rtN.jar