The legal battle between Oracle and Google is about to come to an end. And nothing less is as stake than the future of programming. Today lawyers for both companies are set to make their closing arguments in the fight over whether Google’s use of the Java application programming interface (API)—an arcane but critically important part of the Android mobile operating system—was legal. Regardless of how the jury rules, the case has already had a permanent effect on the way developers build software.

For a case with such potentially great impact on the tech industry, it can be tough to follow. It’s dragged on for years, and the details, both technical and legal, can get deeply esoteric. But for anyone who cares about the future of business or technology, it’s a vital case to understand. So we’ll do our best to make sense of it for you.

Oracle completed its acquisition of Sun, the makers of the Java programming language and platform, in January 2010. In August of that year, Oracle sued Google for copyright and patent infringement over the company’s use of Java-related technology in Android. Among other things, Oracle claimed that Google’s use of the Java APIs in Android violated its copyright. In 2012, a Washington DC district court ruled in Google’s favor, finding that the APIs were not subject to copyright. Unfortunately for Google, an appeals court overturned that ruling and the US Supreme Court declined to hear the case.

Now Oracle and Google are back in district court determining whether Google’s use of the APIs constitute “fair use,” meaning that Google doesn’t need permission from the copyright holder in order to use them. The answer will have implications that will ripple across the software industry.

What’s At Stake

The suit won’t kill Android. Google has already committed to using only a fully open source version of Java in future versions of the operating system. But since the appeals court has already ruled that APIs are subject to copyright, that could open a whole new frontier of lawsuits aimed at startups and open source projects that have copied APIs in order to ensure their products are compatible with popular commercial products.

For example, several companies have built open source software that works with various cloud services in an attempt to make it easier for customers to easily move their applications from, say, Amazon to their own data centers. Basho and SwiftStack, to name just two, each offer storage products that are compatible with Amazon’s cloud storage service S3. Since APIs are subject to copyright, Amazon could in theory go after both companies for copyright violations.

Meanwhile, many open source operating systems, such as FreeBSD and those based on Linux, use a standard API called POSIX, which is based on the API of the venerable Unix operating system. Under the appeals court’s ruling, Micro Focus1, the current owners of Unix, could go after the makers of POSIX operating systems.

“Both of those scenarios are more likely after Oracle v. Google, regardless of how the jury decides,” says Mitch Stoltz, a senior staff attorney at the Electronic Frontier Foundation.

In that sense, the case has already dealt the software industry a heavy blow, because defending against an infringement suit can be a long and expensive ordeal, whether an infringement really exists or not. The threat alone could discourage developers from even attempting to build new software based on copyrighted APIs out of fear of litigation. If the court rules in Google’s favor on the fair use question, companies may have more hope that such legal battles would be winnable. If the courts rule for Oracle, on the other hand, reuse of APIs would be subject to the whims of the copyright holders.

It’s Still Hard to Explain What an API Is

One of the key problems for Google in this case has always been trying to explain what exactly an API is to non-technical judges and jurors. Judge William Alsup, who is presiding over the current case and also heard the original, taught himself to program in Java in order to better understand the case’s ins and outs.

In his 2012 ruling, he described APIs as being a bit like a library.

“Each package is like a bookshelf in the library,” Alsup wrote in his 2012 ruling. “Each class is like a book on the shelf. Each method is like a how-to-do-it chapter in a book. Go to the right shelf, select the right book, and open it to the chapter that covers the work you need.”

By essentially organizing its software libraries the same way Java is organized, Google made it easier for Java programmers to write software for Android. The problem is that although Oracle offers an open source version of Java, meaning anyone is free to change, modify and share the underlying code, Sun’s licensing terms prohibited companies from building their own versions of the language without approval. Alsup ruled that Google’s Android platform wasn’t really a Java implemention since they only borrowed the API, the organizational structure of the platform.

Although the appeals court ultimately disagreed with Alsup’s conclusion that the way you organize a library can’t be copyrighted, it was a pretty good metaphor even if it did paper over a few technical issues.

Google’s preferred metaphor, however, is a filing cabinet.

The company’s attorneys wheeled out a real-life filing cabinet back in 2012 to try to explain that the labels on the files are the API and that what you put inside the file folders is the real, copyrightable stuff. It didn’t go over that well.

“When the person in the row ahead of you is buying tickets on StubHub during testimony, you know you’re in the weeds,” The Verge reported from the courtroom gallery at the time.

It’s not clear why, then, that Google’s attorneys brought back the filing cabinet last week instead of adopting Judge Alsup’s more eloquent explanation. It certainly doesn’t seem to have gone over any better this time around. Motherboard’s tech savvy contributing editor Sarah Jeong–who already knew what an API was–wrote that she couldn’t follow the attorney’s explanation. That’s not a good sign for Google.

Oracle Drudged Up Some Emails That Make Google Look Pretty Bad

Oracle’s attorneys discovered two emails that suggest Google knew full well the APIs were copyrighted. According to em>Ars Technica, Android founder Andy Rubin wrote an email that read: “Wish them luck. Java.lang.apis are copyrighted.” And Sun, he wrote, gets to decide who gets a license to use them.

In an email to Rubin, engineer Tim Lindholm wrote, “We conclude that we need to negotiate a license for Java under the terms we need.”

The pair of emails don’t necessarily prove that the company was in the wrong. They weren’t sent by Google’s lawyers, and they don’t prove that the APIs aren’t subject to fair use. But they certainly don’t look good for Google.

Sun’s Blessing

Not everything has gone badly for Google. Former Sun CEO Jonathan Schwartz, who ran the company from 2006 to 2010, told the court that Sun was always free and open and that Sun was always okay with Google’s use of the APIs.

Schwartz’s testimony is consistent with his public statements around the time of Android’s launch. “I just wanted to add my voice to the chorus of others from Sun in offering my heartfelt congratulations to Google on the announcement of their new Java/Linux phone platform, Android,” he wrote on the Sun website in 2007.

Oracle lawyer Peter Bick sought to undermine Schwartz’s testimony, Ars Technica reported, by reading from emails Schwartz sent at the time in which he referred to Android as “lame” and speculated that Google may have played it fast and loose with the licensing rules. Bick also managed to work in a few barbs about Schwartz’s leadership, pointing out that Oracle didn’t offer Schwartz a senior position after the acquisition. Bick also noted that Schwartz had once been named one of the worst CEOs of all time by Glassdoor. In other words, Bick was trying to portray Schwartz’s decision not to pursue legal action against Android as mistake made by a lousy CEO even though Android was “devastating” to Java.

What the Future Could Look Like

Even if the jury rules in Oracle’s favor, all is not lost for the future of software development. Many newer development platforms, including Google’s Go language and Apple’s Swift, are licensed under more liberal terms than Java and allow for-profit companies to use and modify them.

Meanwhile, Microsoft has open sourced its venerable .NET programming platform, and both startups and big tech companies like from Cisco, Intel to IBM are coming together to create open source technology standards that could help prevent future legal battles like this one.

And just because Oracle win could unleash a torrent of new lawsuits over APIs doesn’t mean those lawsuits will ever manifest. Amazon didn’t respond to our questions about whether and how it would license its Amazon Web Services (AWS) APIs if the courts rule in Oracle’s favor, but Gartner analyst Raj Bala told us it wouldn’t really be in the company’s best interest to stop people from using its APIs. “AWS controls portions of the roadmaps of every storage vendor that implements S3 API compatibility including IBM, Google and EMC amongst others,” he says. “Why would AWS be upset at this enormous power? For one’s proprietary protocol to be the de-facto standard is an enviable position from a vendor’s perspective.” And Google, one of Amazon’s biggest competitors in the cloud business, has already open sourced some of its key cloud technologies, such as Kubernetes.

These more open technologies should give developers of all stripes confidence that they can build their software without being sued. Companies that embrace this openness will rule the future, regardless of what the jury decides in this case.

1Correction at 5/23/2016 at 2:00 PM ET: An earlier version of this article referred to AT&T as Unix’s owner. Novell, now a division of Micro Focus, actually owns the copyright to the older versions of Unix

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The Oracle-Google Case Will Decide the Future of Software