I have no idea

ftplugin/java.lua

@@ -0,0 +1,6 @@
+local config = {
+    cmd = {'/path/to/jdt-language-server/bin/jdtls'},
+    root_dir = vim.fs.dirname(vim.fs.find({'.gradlew', '.git', 'mvnw'}, { upward = true })[1]),
+}
+require('jdtls').start_or_attach(config)
+