====== Java and the JVM on Accelerators ======

This page is a guide to creating and deploying applications on your Accelerator developed in the Java programming language, and developed in other languages that run on top of the Java Virtual Machine (JVM).

<note tip>
**Java Overview**

If you're new to Java, the JVM, or want a general introduction to developing and deploying Java applications, start off by reading the **[[all-accelerators:kb:java|Java Overview]]** in the [[all-accelerators:kb:start|All Accelerators Knowledge Base]].
</note>

===== Version =====

An up-to-date version of Java is available "out-of-the-box" with your Accelerator.

==== Finding the Java Version ====

<code>
admin$ java -version
java version "1.6.0_01"
Java(TM) SE Runtime Environment (build 1.6.0_01-b06)
Java HotSpot(TM) Server VM (build 1.6.0_01-b06, mixed mode)
</code>

If you see the following error:

<code>
Error occurred during initialization of VM
Could not reserve enough space for object heap
</code>

try:

<code>
admin$ java -Xms16m -Xmx64m -version
</code>

For more help on command-line switches, type:

<code>java -h</code>

===== Location =====

Java and the JDK can be found in ''/usr/bin''. This is a read-only location, and not user-land software cannot/should not be installed here (see [[accelerators:kb:tour]]).

<code>
admin$ which javac
javac is /usr/bin/javac
</code>

Standard tools should be available to you from ''/usr/bin'':
  * java
  * javac
  * jar
  * javadoc
  * jdb
  * javap
  * javah
  * extcheck
  * apt
  * idlj
  * policytool

===== Warning on Java memory usage in Accelerators =====

Java automatically sets a heap size when it starts.  The default calculation can give undesirable results in Joyent Accelerators (e.g. consuming half the memory in a container for a simple application).

See [[http://discuss.joyent.com/viewtopic.php?id=15918|this thread]] for further discussion.

To avoid problems, specify heap size parameters when you launch Java:

<code>
# java -Xms16m -Xmx64m MyJavaApp
</code>

===== Installing / Upgrading Java =====

If you need a version of the JDK that is different to the version contained in ''/usr/bin'', you may be able to install an alternate version within ''/opt''. //Please contribute if you are able to make this work.//

===== Installing Java Software =====

   * [[tomcat|Tomcat]]