Oracle CEO Larry Ellison Oracle CEO Larry Ellison

Google once again is set to battle Oracle in court over how it used the Java programming language in building its Android mobile operating system. Oracle, which lost a patent and copyright battle to Google in the spring of 2012, has filed an appeal in the case and oral arguments started this week.

At the heart of the appeal was the ruling by Federal Court Judge William Alsup that application programming interfaces (APIs) cannot be copyrighted. In the original case, a jury came to a split decision that Google's use of Java APIs were considered "fair use." In the end, it did not matter what the jury decided because Alsup ruled against Oracle on if APIs could be copyrighted. 

The primary issue in the appeal comes down to how Google used Java in Android. Overall, Google engineers wrote some 15 million lines of original code to build Android. But it relied on some of Java APIs (Oracle took control of the open source Java language when it bought Sun Microsystems in 2009) to build Android. Specifically at stake is the structure, sequence and organization (SSO) of the APIs, which Google basically rebuilt with their own code. 

In his original ruling, Alsup said that parts of the APIs in question were not applicable to copyright. He described it in his 2012 judgement as akin to the organizing system of a library (think, the Dewey Decimal system) and could not be copyrighted.

In many ways, the Oracle and Google standoff pits old technology companies against newer companies. In February 2013, Microsoft, EMC and Netapp filed a joint brief supporting Oracle. The App Developers Alliance and Rackspace both support Google. 

"The court's ruling, if allowed to stand, will deal a serious if not potentially staggering blow to existing incentives established by copyright law for innovation in this critical industry," the joint brief stated. 

The appeal by Oracle was long expected after the split jury decision on fair use and Judge Alsup's controversial ruling on if APIs can be copyrighted. 

A decision on the copyright claims is not expected for several months, according to Bloomberg.

Google is also facing another patent court battle from a group called the Rockstar Consortium over the implementation of Android. Rockstar is a company jointly owned by Apple, Sony, Microsoft, BlackBerry and Ericsson that formed after the group bought Nortel's patent portfolio at auction in 2011.