Hadoop Java Versions
Hadoop requires Java 7 or a late version of Java 6. It is built and tested on both OpenJDK and Oracle (HotSpot)'s JDK/JRE
Here are the known JDKs in use or which have been tested:
1 - Hadoop works well with update 16 however there is a bug in JDK versions before update 19 that has been seen on HBase. See HBASE-4367 for details.
2 - If the grid is running in secure mode with MIT Kerberos 1.8 and higher, the Java version should be 1.6.0_27 or higher in order to avoid Java bug 6979329.
3 - Hortonworks has certified JDK 1.6.0_31 under RHEL5/CentOS5, RHEL6/CentOS6, and SLES11, with Hadoop 1.x, HBase, Pig, Hive, HCatalog, Oozie, Sqoop, and Ambari.
3 - Hortonworks has certified JDK 1.6.0_31 and Oracle 126.96.36.199 under RHEL5/CentOS5, RHEL6/CentOS6, and SLES11, with Hadoop 2.2.0, HBase 0.96, Pig, Hive, HCatalog, Oozie, Sqoop, and Ambari.
3 - Hortonworks has certified openjdk 1.7.0_09-icedtea on RHEL6 with Hadoop 2.2.0, HBase 0.96, Pig, Hive, HCatalog, Oozie, Sqoop, and Ambari.
Compressed Object pointers and Java 6
The Sun JVM has 32-bit and 64-bit modes. In a large cluster the NameNode and JobTracker need to run in 64-bit mode to keep all their data structures in memory. The workers can be set up for either 32-bit or 64-bit operation, depending upon preferences and how much memory the individual tasks need.
Using the Compressed Object References JVM feature (-XX:+UseCompressedOops) reduces memory consumed and increases performance on 64 bit Sun JVMs. This feature was first introduced in 1.6.0_14 but problems were been reported with its use on versions prior to 1.6.0_20. Several have reported success using it on 1.6.0_21 and above. It is the default in 1.6.0_24 and above on 64 bit JVMs, and appears now to be stable.
Useful tips for discovering and inspecting Sun JVM confuguration flags are in the following blog post: inspecting-hotspot-jvm-options
OpenJDK has been used to qualify Hadoop 2.2 -and the rest of the Hortonworks bundle- on RHEL6. No problems were noted.
Oracle's JRockit JVM is not the same as the Sun JVM: it has very different heap and memory management behavior. Hadoop has been used on JRockit, though not at "production" scale.
Hadoop 0.20.2 has been tested comprehensively and works with IBM Java 6 SR 8. IBM Java can be downloaded here.
OSX and Java 6
Apple Java 6 on OS/X has been used for development and local machine testing, but has never been qualified for production used for a simple reason: OS/X isn't a supported OS for production systems, especially on Hadoop 2. Consult HowToSetupYourDevelopmentEnvironment for details on setting up an OS/X based development environment.
A request for help from JVM/JDK developers
We would strongly encourage anyone who produces a JVM/JDK to test compiling and running Hadoop with it. It makes for a fantastic performance and stress test. As Hadoop is becoming a key back-end datacenter application, good Hadoop support matters.