java.lang.UnsupportedClassVersionError in eclipse

Question

Hi all, I am not a Java programmer really, so I am posting this question. The exception is being thrown java.lang.UnsupportedClassVersionError in my main class in an eclipse project. If I comment out the imports that this class has, it compiles and runs fine. If I put the imports back in, it does not work. Does this mean that the libraries I am importing were compiled with a newer or older version of java than I have? when i do java -version on the system i get 1.5_07

I could've sworn this was actually working last week, but maybe some setting in eclipse got tweaked? Is the Java Build Path in eclipse what I need to look for to check the JRE and compiler versions?

Answer 1

Eclipse is not using JDK compiler, but has its own compiler which can produce bytecode for any JRE. You need to make sure that compiler settings in your project are set to the same or lower version as the JRE you have registered in Eclipse. See Window / Preferences / Java / Installed JREs and also check what JRE is used in your launch configuration (see Run menu for that).

Answer 2

In Eclipse, the compiler version is set independent of the JRE version. That means you can set the compiler to Java version 1.6 and use the JRE 1.5. In this case, you compiled classes cannot be run.

You can check the preferences in Window / Preferences / Java / Compiler for the default compiler compliance level, or the properties of your project for a project specific compiler compliance level. Compare that level with the JRE used in your project (Project / Properties / Java Build Path -> Libraries / JRE System Library) and in your program's launch configuration (via the Run / Run Configurations... menu).

Answer 3

If your imported classes come from an external library (i.e., not compiled from sources within your project), then you should validate that the JRE used for your project is new enough for this library.

Answer 4

UnsupportedClassVersionError means that the Java runtime environment you are using doesn't recognise the version of a class file that you are trying to execute. The most common cause for this error is trying to use a class file compiled for a newer Java version on an older Java version - for example, you are trying to use a class compiled for Java 6 on a Java 5 runtime environment.

As Eugene explained, Eclipse has its own built-in compiler, it does not use the compiler from the JDK - so that's how you can end up with Java 6 class files even if you're running on Java 5.

Christian explains how to set the Java class file version in Eclipse.