I know nothing about Java. One day, I needed to open the file with extension jnlp with Java Web Start to download a huge number of images. As far as I know, for a non-Java-developer like me, I just need to install Java Runtime Environment (JRE). It’s not necessary to install Java Development Kit (JDK).

First I check my Java:

> java -version
java version "15.0.1" 2020-10-20
Java(TM) SE Runtime Environment (build 15.0.1+9-18)
Java HotSpot(TM) 64-Bit Server VM (build 15.0.1+9-18, mixed mode, sharing)

Ok, I saw something like JRE and was pretty sure that it will run. But when opening the file jnlp, I saw this error:

jnlp files are supposed to be open with javaws so I checked again if javaws existed in my computer:

> which javaws
/usr/bin/javaws

It was so weird, javaws also existed. Try again with javaws:

> javaws /path/to/file.jnlp
No Java runtime present, requesting install.
Unable to locate a Java Runtime to invoke.

I had JRE but javaws was unable to allocate it. After wondering around, I found one answer that Java Web Start was depreccated in Java 9 and removed in Java 11. (this answer).

Therefore, a workaround for this is installing JRE-8. Download here or go to Oracle website to download the installer suitable for current OS: oracle

And then install JRE-8, it will work as it supposes to be.

Conclusion:

• When encountering this problem, just download JRE-8, it will be a solution.
• To check if JRE exists, use this command rather than java -version: /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin/java -version. If not exist, install JRE-8 or sooner version.