The brief was filed on Friday US time.

In May last year, a jury found that Google's use of 37 Java APIs in the Android mobile operating system was covered by fair use.

Oracle gave notice at that time that it would not take the verdict lying down. In August, Oracle tried to get the verdict set aside, but was told by Judge William Alsup refused. Later in 2016, the database giant filed the necessary papers to prolong the battle.

In its opening brief on Friday, Oracle said: "When a plagiarist takes the most recognisable portions of a novel and adapts them into a film, the plagiarist commits the 'classic' unfair use," according to Ars Technica.

It added: "Google's copying in this case is the software equivalent of this classic unfair use. Google copied thousands of lines of copyrighted code from Oracle's Java programming platform.

"Google concedes it put that code to the same use in the competing Android platform, for what this Court already has deemed 'entirely commercial' purposes. And Google reaped billions of dollars while leaving Oracle's Java business in tatters."

The legal stoush has been going on since 2010 when Oracle sued Google shortly after it purchased Sun Microsystems and became the owner of Java, claiming that the search engine company had violated its copyright and patents.

That case ended in 2012 with Google being largely the victor. Justice Alsup ruled that APIs could not be copyrighted.

But an appeal gave Oracle what it wanted: a ruling that APIs can be copyrighted. This ruling has put developers at risk, as they could be sued for using APIs that they could use freely prior to the trial.

In a second trial that ended in May, a jury found that Google's use of 37 Java APIs in its Android mobile operating system was covered under fair use. As expected, Oracle was not happy with the verdict.

Chapter three has just begun. Google's response will be filed in the months ahead.