Eglot does not trigger a custom formatter with jdtls, while lsp-java and VS Code with the Red Hat Java plugin can do this. See https://github.com/joaotavora/eglot/discussions/1222 for more background.

I attach files
* eglot-debug.el (the minimal configuration to see the failure)
* NotHelloWorld.java (a sample Java file)
* cay-eclipse-formatter.xml (the formatting rules for jdtls)
* eglot-events.txt (the events log from eglot)
* lsp-java-debug.el (the minimal configuration to see how it works with lsp-java)
* lsp-log.txt (the log from the successful run with lsp)

Download jdtls from https://download.eclipse.org/jdtls/snapshots/?d. I
have the version from Oct. 14, 2023. Put it somewhere on your PATH

Put the NotHelloWorld.java file in some directory (I use /tmp/hello)

Put the cay-eclipse-formatter.xml file somewhere and adjust the path in eglot-debug.el and lsp-java-debug.el.

emacs -q -l eglot-debug.el /tmp/hello/NotHelloWorld.java

M-x eglot RET

Wait for jdtls to start

M-x eglot-format-buffer RET

The formatting should now use the rules in the cay-eclipse-formatter.xml
file, resulting in 8 changes but the same Allman indentation
style. Instead, it uses a K&R indentation style.

To see the proper formatting, install lsp-mode and lsp-java and start with lsp-java-debug.el. Or use VS Code with the Red Hat Java plugin and the settings.json

{
     "java.format.settings.url": "file:///data/cay/bin/cay-eclipse.formatter.xml",
     "java.format.settings.profile": "cay"
}



In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.33,
  cairo version 1.16.0) of 2023-08-15 built on cay-ThinkPad-X1-Yoga-Gen-7
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
System Description: Ubuntu 22.04.3 LTS

Configured using:
  'configure --with-tree-sitter'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG SECCOMP SOUND
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER X11 XDBE XIM XINPUT2 XPM
GTK3 ZLIB

Important settings:
   value of $LC_MONETARY: de_DE.UTF-8
   value of $LC_NUMERIC: en_US.UTF-8
   value of $LC_TIME: en_DK.UTF-8
   value of $LANG: en_US.UTF-8
   value of $XMODIFIERS: @im=none
   locale-coding-system: utf-8-unix

Major mode: Java//l

Minor modes in effect:
   eglot-inlay-hints-mode: t
   eglot--managed-mode: t
   flymake-mode: t
   tooltip-mode: t
   global-eldoc-mode: t
   eldoc-mode: t
   show-paren-mode: t
   electric-indent-mode: t
   mouse-wheel-mode: t
   tool-bar-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   font-lock-mode: t
   blink-cursor-mode: t
   line-number-mode: t
   indent-tabs-mode: t
   transient-mark-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   abbrev-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny rfc822
mml mml-sec epa derived epg rfc6068 epg-config gnus-util mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
files-x find-dired dired dired-loaddefs grep time-date cl-extra cc-mode
cc-fonts cc-guess cc-menus cc-cmds cc-styles cc-align cc-engine cc-vars
cc-defs eglot external-completion array filenotify jsonrpc ert pp ewoc
debug backtrace help-mode find-func xref flymake-proc flymake thingatpt
warnings icons compile text-property-search comint ansi-osc ansi-color
ring pcase url-util url-parse auth-source cl-seq eieio eieio-core
cl-macs password-cache json subr-x map url-vars project byte-opt gv
bytecomp byte-compile imenu cl-loaddefs cl-lib rmc iso-transl tooltip
cconv eldoc paren electric uniquify ediff-hook vc-hooks lisp-float-type
elisp-mode mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow
isearch easymenu timer select scroll-bar mouse jit-lock font-lock syntax
font-core term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit xinput2 x multi-tty make-network-process
emacs)

Memory information:
((conses 16 107858 9131)
  (symbols 48 11792 0)
  (strings 32 34568 1702)
  (string-bytes 1 1192636)
  (vectors 16 23157)
  (vector-slots 8 313544 12514)
  (floats 8 39 90)
  (intervals 56 661 0)
  (buffers 984 18))