Will programming using JDK 7 allow the application to run on JRE 6?

Question

I am programming in Eclipse. I never change anything about the default compile setting of Eclipse, or anything else.

If I have JDK 7 installed on my computer. Should my compiled Runnable JAR have any problem running on an older JRE, assuming that the older JRE supports all my programs features?

For example, if I program an application in Eclipse, with the latest JDK installed - should the program have a problem running on a slightly older JRE?

If the answer to this is "yes" - How can I change this? How can I allow my program to run on older JREs than my JDK? Thanks

Answer 1

Your code will only run on JRE 6 of you compile it for JRE 6. Check your compiler settings (Project->Properties->Java Compiler) and make sure that you compile for Java 6, otherwise users will get an UnsupportedClassVersionError if they don't have Java 7 installed.

You also have to make sure to either not use any Java 7 features (e.g. java.nio.Path), or install the Java 6 JDK and use it to compile your application, otherwise you'll get missing method/class exceptions when running the application on Java 6 (you'll instead get compile time errors when using JDK 6).

Answer 2

The Java runtime that Eclipse uses does not have to be the same that your project uses. You can set the runtime of your project in the project's properties:

http://help.eclipse.org/juno/index.jsp?topic=%2Forg.eclipse.jdt.doc.user%2FgettingStarted%2Fqs-ProjectConfiguration.htm

Java 7 is backwards compatible, meaning older Java programs will run in this version of Java. However, Java versions are never forwards compatible. Meaning a program written in Java 7 will not necessarily compile in Java 6 and a program written in Java 6 will not necessarily compile in Java 5. That is because Java 7 contains features that Java 6 does not contain, like strings in switch statements, and Java 6 contains features Java 5 does not contain, like scripting language support (see https://en.wikipedia.org/wiki/Java_version_history for an overview).

Also lack of forwards compatibility means Java programs compiled in a newer version of Java will not run on an older version of the Java runtime. The java compiler specifically protects compiled java class files with a version identifier to not run these classes when they are compiled in newer version of Java.

If you are careful to avoid Java 7 features you can write a Java program in Java 7 that will compile and run in Java 6. If you know however that your program needs to run in Java 6 it is best to set your project's runtime to Java 6.