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Sun Microsystem's Java 2 Platform Standard Edition 5.0 (J2SE 5.0), a.k.a Java 1.5, is required remains the recommended JVM to build and run Sakai. Certain files, such as *.jsp and *.jws, require compilation so downloading and attempting to use only the runtime environment (JRE 5.0) will not suffice.  Mac OS X includes the full version of Java J2SE 5.0, pre-installed with the Java Development Kit (JDK) and the HotSpot virtual machine (VM), so Mac users should 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.

Sakai 2.6 is not compatible with Java Platform, Standard Edition 6 (Java SE 6), otherwise known as Java 1.6. Compilation errors have been reported by members of the Community experimenting with Sakai and Java SE 6. Use Java 2 Platform Standard Edition 5.0 (J2SE 5.0) instead. Sun Microsystems has reported security vulnerabilities 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).

Sakai 2.6 can be run using Java Platform, Standard Edition 6 (Java SE 6), a.k.a Java 1.6, although to-date there is little production experience that warrants recommending 2.6 users to upgrade their JVM. Should you choose to run Sakai in a Java 6 JVM you must (at a minimum) add the following environment setting to your JAVA_OPTS:

-Dsun.lang.ClassLoader.allowArraySyntax=true

To confirm that Java is both installed on your system and is the correct version for Sakai, run java -version from the command line:

$
java -version

If Java is installed, the version command will output basic version and build information . Note that Java's "internal" version number will be displayed:

java version "1.5.0_18"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_18-b02)
Java HotSpot(TM) ClientServer VM (build 1.5.0_18-b02, mixed mode, sharing)

If Java is not installed or if you are running the wrong version you can download the latest J2SE 5.0 JDK release from Sun's archive:

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Install the SDK, typically in the /opt directory, i.e. /opt/java. Install the JRE to in a different directory (usually the default directory, especially if running Windows) or you may experience runtime run-time issues.