If I install an older, deprecated Java runtime, will that mess up applications that use the current Java version?

Question

I own a really old version of Photoshop. After I upgraded to El Capitan when I attempt to launch Photoshop I get a dialog saying...

To open "Adobe Photoshop CS5.1.app" you need to install the legacy Java SE 6 runtime.

This is my current system Java version that came with the OS -->

java -showversion
java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01, mixed mode)

I downloaded the old, deprecated Java 6 DMG, but I haven't installed it yet.

My question is, will it replace the Apple's distributed Java?

And if so, is that likely to break applications that are expecting an up-to-date Java version?

Or will it just make the older JRE available as an alternative for applications that need it?

Answer 1

So long as you've downloaded the Java 6 installer from Apple it will work without affecting your current Java installation.

Answer 2

For this question: "...is that likely to break applications that are expecting an up-to-date Java version?"

It's possible that some of your applications will not work. If the developer is using a feature in a specific version of Java and you go to a lower numbered version, that feature might now work. I want to emphasize 'might'. Nobody knows for sure if what you have on your machine will be impacted. Of course, and this is most important, you'll lose any security updates issued with the newer versions.

Here's what Oracle has to say about downgrading:

Most applications written for earlier versions of Java will run without problems in newer versions of Java. However, there are some cases where choices made by the developer to tie an application to a specific Java version, or inability to update, can cause some applications not to work with the latest Java versions.