The Java home setting does not change the JDK used to start the server

[nulliver]

What happened?

My $JAVA_HOME environment variable points to JDK 8 because I'm working on several projects that require Java 8. Since JDTLS requires Java 17+, I added the following to my Zed configuration:

{
  "lsp": {
      "jdtls": {
        "initialization_options": {
          "settings": {
            "java": {
              "home": "/Users/username/.sdkman/candidates/java/21.0.4-tem"
            }
          }
        }
      }
  }
}

but it doesn't work, it throws the following error, seems that it's still using the $JAVA_HOME version:

Language server error: jdtls

oneshot canceled
-- stderr--
Traceback (most recent call last):
  File "/Users/username/Library/Application Support/Zed/extensions/work/java/jdt-language-server-1.40.0/bin/jdtls", line 17, in <module>
    jdtls.main(sys.argv[1:])
    ~~~~~~~~~~^^^^^^^^^^^^^^
  File "/Users/username/Library/Application Support/Zed/extensions/work/java/jdt-language-server-1.40.0/bin/jdtls.py", line 87, in main
    java_executable = get_java_executable(known_args)
  File "/Users/username/Library/Application Support/Zed/extensions/work/java/jdt-language-server-1.40.0/bin/jdtls.py", line 44, in get_java_executable
    raise Exception("jdtls requires at least Java 17")
Exception: jdtls requires at least Java 17

If I change my $JAVA_HOME to Java 21, the plugin starts the server correctly.

If I start the server directly with the following command, the server starts fine, it uses the -Djava.home argument I pass instead of the $JAVA_HOME environment variable so I don't think it's a JDTLS issue:

java \
-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=1044 \
-Djava.home=/Users/username/.sdkman/candidates/java/21.0.4-tem \
-Declipse.application=org.eclipse.jdt.ls.core.id1 \
-Dosgi.bundles.defaultStartLevel=4 \
-Declipse.product=org.eclipse.jdt.ls.core.product \
-Dlog.protocol=true -Dlog.level=ALL -Xmx1G \
-jar "/Users/username/Library/Application Support/Zed/extensions/work/java/jdt-language-server-1.40.0/plugins/org.eclipse.equinox.launcher_1.6.900.v20240613-2009.jar" \
-configuration ./config_mac -data /Users/username/code/java-project

I was able to reproduce this in two computers: a M3 MacBook Pro (macOS Sequoia) and an M1 MacBook Pro (macOS Sonoma).

Am I configuring the wrong setting?

What did you expect to happen?

The plugin should start JDTLS using the lsp.jdtls.initialization_options.settings.java.home as specified in the README.

[valentinegb]

Everything from lsp.jdtls.initialization_options onwards is sent straight to the language server, so any peculiarities can be attributed to JDTLS and not the Zed Java extension. The extension only handles lsp.jdtls.settings.

[valentinegb]

I should note, though, that lsp.jdtls.initialization_options.settings.java.home is meant to be the version of Java to use for the project, not for running JDTLS itself. So, you should set that option to Java 8 and set your Java home to Java 21.

[nulliver]

Understood. Is there a way to use a different version of Java to start the server instead of it using $JAVA_HOME?

[valentinegb]

I believe you could launch Zed from a terminal with JAVA_HOME=/path/to/java21 zed. I would recommend giving Zed's documentation a look.

[nulliver]

I look into this a bit more and per the eclipse.jdt.ls documentation referenced in the README, the Java home setting, if provided, should be used to launch the server too:

/**
 * Specifies the folder path to the JDK (8 or more recent) used to launch the Java Language Server.
 * On Windows, backslashes must be escaped, i.e."java.home":"C:\\Program Files\\Java\\jdk1.8.0_161"
 */
home?: string;

And I think that the way it's supposed to be. The Sublime Text and Visual Studio Code extensions implement it that way and I'm able to use a different JDK from what i have in the $JAVA_HOME env variable to launch the server.

[valentinegb]

Alright, that all sounds reasonable to me. I’ll get that implemented when I can 👍