Only JREs and JDKs distributed or OEMed from Oracle Microsystems are supported. (That is, Fuse ESB is not guaranteed to run with gcj.) On Red Hat-derived Linux systems, gcj is the default Java distribution. In these cases, you must download and install a Oracle JRE/JDK and set the JAVA_HOME environment variable. You can also integrate the Oracle JRE/JDK into the alternatives(1) system. See the alternatives(1) man page for details.