Oracle's Sun Java SE 6, a.k.a Java 1.6, is the preferred version to use with the Sakai CLE, because it has been tested the most thoroughly. Java SE 7, a.k.a Java 1.7 should also be compatible. Certain files, such as *.jsp and *.jws, require compilation so downloading and attempting to use only the JDK, the JRE with only the run time environment (JRE 6.0 or JRE 7.0) will not suffice. Mac OS X 10.6 (Snow Leopard) and later includes the full version of Java SE 6 so Mac users do not need to install Java. If you find Sun's version and naming conventions confusing, see Sun Java SE Naming and Versions for an outline of their practices.
Oracle's Sun Java J2SE 5.0 (a.k.a Java 1.5) has completed the EOL process and is no longer supported. If are still running Java 1.5 please note that security vulnerabilities exist in JDK/JRE 5.0 updates 1.5.0_17 and earlier. Sun recommends that you install JDK/JRE 5.0 Update 18+ (the final update is JDK/JRE 5.0 Update 22).
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If Java is not installed or you are running the wrong version you can download the Java SE 6.0 JDK or the Java SE 7.0 JDK release from Oracle's Sun archive:
http://javawww.sunoracle.com/technetwork/java/javase/downloads/index.jsparchive-139210.html
Install the JDK, typically in the /opt directory, i.e. /opt/java. Install the JRE in a different directory (usually the default directory, especially if running Windows) or you may experience run-time issues.