Symptoms

Users are unable to install Confluence. Trying to browse to localhost:port from the browser shows a cannot connect message. The following appears in the atlassian-confluence.log:

java.lang.UnsupportedClassVersionError: Bad version number in .class file (unable to load class com.atlassian.confluence.logging.ConfluenceHomeLogAppender)
    at org.apache.catalina.loader.WebappClassLoader.findClassInternal(WebappClassLoader.java:2822)

Diagnosis

Check your java version by running this command:

java -v

Cause

Java 1.5 is not supported in Confluence 3.5 and later.

Resolution

1. Update the JVM to 1.6 or later.