Openjdk and Java webstart
Several parties (RedHat, AdoptOpenJDK & Karakun) are currently working on an open source replacement of Oracle WebStart. The repo can be found at karakun/OpenWebStart. You can find additional information about the project on openwebstart.com.
Q: Do the latest OpenJDK retain support for Java WebStart?
No. Not Oracle JDK. Not OpenJDK. Not (AFAIK) any other Java implementations based on the latest OpenJDK sources.
- WebStart support was deprecated in Oracle JDK / JRE distros with Java 9 and removed in Java 11.
- According to Wikipedia, OpenJDK has never included WebStart. It was Oracle JDK/JRE specific.
There is now a replacement for classic Java Web Start in the form of OpenWebStart. (The OpenWebStart 1.0.0 release was announced on Nov 18th 2019.)
The other alternative is to move away from Java WebStart / JNLP to an alternative such as building platform-specific executables using jlink and distributing them in some other way.
Read Java Client Roadmap Update, a white paper from Oracle, update 2020-05.
Q: When building a Java program using OpenJDK in Eclipse can it run on systems that have regular Oracle JRE installed?
Yes, provided that the version of Java installed is new enough. But note that Oracle stopped releasing JRE releases after Java 8. It is now only JDKs if you are using an Oracle Java release, or an OpenJDK build from the OpenJDK download site.
(By the way, Eclipse is not relevant to the question. Java is the same language no matter what IDE or command line tool you use to build.)
Q: Then what benefit is it to use OpenJDK over Oracle?
See the following:
- What is the reason to use OpenJDK?
- OpenJDK vs Java HotspotVM
Note that there are other alternatives. Read "Java is Still Free".
I ask because we can't count on a client PC having a non Oracle JRE installed.
Then you should be looking at jlink. It creates self-contained executables. That means that you don't depend on having any JRE installed on the client PC.