Configure JDTLS to run with specific Java version

[fancywriter]

Hello. I am working on some legacy projects. Typical in Java world. I have to work on Java-17 and Java-8 code base (I know it could be worse like Java-6 or even Java-5, very typical).
This is what my JAVA_HOME is configured in the project, the project is built with gradle (but should be similar with Maven or any other tool) and it requires that older java version, it won't work with 21 or 23.

However, JDTLS requires at least Java-21 and fails to work with Java-17.

How can I configure Mason to use (I can even hardcode path to java there) to use specific JRE only for JDTLS (but use JAVA_HOME elsewhere, like for running gradle/maven or code linters from other nvim plugins) ?

I use LazyVIM.

I tried to create ~/.config/nvim/lua/plugins/jdtls.lua

return {
  "neovim/nvim-lspconfig",
  opts = {
    servers = {
      jdtls = {
        cmd = {
          "/Library/Java/JavaVirtualMachines/zulu-23.jdk/Contents/Home/bin/java",
          "-Declipse.application=org.eclipse.jdt.ls.core.id1",
          "-Dosgi.bundles.defaultStartLevel=4",
          "-Declipse.product=org.eclipse.jdt.ls.core.product",
          "-Dlog.level=ALL",
          "-Xmx1G",
          "--add-modules=ALL-SYSTEM",
          "--add-opens",
          "java.base/java.util=ALL-UNNAMED",
          "--add-opens",
          "java.base/java.lang=ALL-UNNAMED",
          "-jar",
          vim.fn.glob(vim.fn.stdpath("data") .. "/mason/packages/jdtls/plugins/org.eclipse.equinox.launcher_*.jar"),
          "-configuration",
          vim.fn.stdpath("data") .. "/mason/packages/jdtls/config_mac",
          "-data",
          vim.fn.stdpath("cache") .. "/jdtls/workspace",
        },
      },
    },
  },
}

But I see it still tries to use Java-17, completely ignores all of this. Because it's installed by mason... how to configure mason properly for this?

I just want to configure it once and forget forever (or until it stop work under <=Java-23 for instance, far future I expect).

[emont01]

I was also looking for a way to do this, but it seems the only reasonable option is to use a custom script:

1. Create a custom script, something like:

#!/bin/sh
export JAVA_HOME="/path/to/java/21"
exec /full/path/to/mason/bin/jdtls "$@"

2. User your script when calling jdtls, for example:

/path/to/jdtls/script.sh \
  --jvm-arg=-javaagent:/path/to/lombok/lombok.jar \
  -configuration /path/to/config/folder \
  -data /path/to/workspace

I am using nvim-jdtls with lazyvim and this is my actual configuration plugins/java.lua

return {
  {
    "mfussenegger/nvim-jdtls",
    opts = {
      cmd = (function()
        -- Use vim.fn.expand to expand ~ in JAVA_HOME path
        vim.env.JAVA_HOME = vim.fn.expand("~/.asdf/installs/java/adoptopenjdk-21.0.6+7.0.LTS/")

        local jdtls_bin = vim.fn.stdpath("data") .. "/mason/bin/jdtls"
        local jdtls_pkg = vim.fn.stdpath("data") .. "/mason/packages/jdtls"
        local workspace = vim.fn.stdpath("cache") .. "/nvim/jdtls/workspace"

        -- Determine platform config folder
        local uv = vim.uv or vim.loop
        local os_name = uv.os_uname().sysname
        local config_folder = "config_linux"

        if os_name == "Darwin" then
          config_folder = "config_mac"
        elseif os_name == "Windows_NT" then
          config_folder = "config_win"
        end

        local cmd = {
          jdtls_bin,
          "--jvm-arg=-javaagent:" .. jdtls_pkg .. "/lombok.jar",
          "-configuration",
          jdtls_pkg .. "/" .. config_folder,
          "-data",
          workspace,
        }

        -- Notify the full command for debugging
        vim.schedule(function()
          vim.notify(
            "Starting jdtls with command:\n" .. table.concat(cmd, " "),
            vim.log.levels.INFO,
            { title = "jdtls cmd" }
          )
        end)

        return cmd
      end)(),
    },
  },
}