As a Java application, setting up Elasticsearch has always required having Java set up and the JAVA_HOME environment variable pointing to it. See, for instance, my articles on setting up Elasticsearch on Windows and setting up Elasticsearch on Linux.

From version 7, Elasticsearch is making things a lot easier by bundling a version of OpenJDK with Elasticsearch itself.

“One of the more prominent “getting started hurdles” we’ve seen users run into has been not knowing that Elasticsearch is a Java application and that they need to install one of the supported JDKs first. With 7.0, we’re now releasing versions of Elasticsearch which pre-bundle the JDK to help users get started with Elasticsearch even faster. If you want to bring your own JDK, you can still do so by setting JAVA_HOME before starting Elasticsearch. “

— Elasticsearch 7.0.0 released | Elastic Blog

The documentation tells us more about the bundled JDK:

” Elasticsearch is built using Java, and includes a bundled version of OpenJDK from the JDK maintainers (GPLv2+CE) within each distribution. The bundled JVM is the recommended JVM and is located within the jdk directory of the Elasticsearch home directory.
“To use your own version of Java, set the JAVA_HOME environment variable. If you must use a version of Java that is different from the bundled JVM, we recommend using a supported LTS version of Java. Elasticsearch will refuse to start if a known-bad version of Java is used. The bundled JVM directory may be removed when using your own JVM.”

— Set up Elasticsearch | Elasticsearch Reference [7.2] | Elastic

Therefore, after downloading a fresh version of Elasticsearch (7.2 is the latest at the time of writing this), we notice that there is a jdk folder as described above:

On a machine with no JAVA_HOME set, Elasticsearch will, as from version 7, use this jdk folder automatically:

This means that we can now skip the entire section of setting up Elasticsearch that revolves around having a version of Java already available and setting the JAVA_HOME environment variable.

On the other hand, if you do have JAVA_HOME set, Elasticsearch will use that, and will not use the bundled JDK at all. This in turn means that if you have JAVA_HOME set incorrectly (e.g. to a directory that no longer exists), Elasticsearch fails with a misleading error that seems to indicate that it’s also looking for the bundled JDK:

"could not find java in JAVA_HOME or bundled at C:\tools\elasticsearch-7.2.0\jdk"

Therefore, if you want to use our own JDK, then make sure JAVA_HOME is set correctly. If you want to use the bundled one, then make sure JAVA_HOME is not set.