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Description
The Hadoop packages go through some effort (see /usr/lib/hadoop-0.20/bin/hadoop-config.sh) to guess at a good JVM to use if JAVA_HOME isn't set. For example, if both OpenJDK and Oracle JDK are installed, hadoop-config.sh will choose Oracle every time.
The zookeeper packages don't do similar things, so it's very easy to get into a state where Hadoop is using Oracle JDK and yet ZK is using OpenJDK.