This document briefly describes the work done to support Java (and other Language Server Protocol-based completion engines).
The original PR implemented native support in ycmd for the Java language, based on jdt.ls. In summary, the following key features were added:
- Installation of jdt.ls (built from source with
build.py
) - Management of the jdt.ls server instance, projects etc.
- A generic (ish) implementation of a Language Server Protocol client so far as is required for jdt.ls (easily extensible to other engines)
- Support for the following Java semantic engine features:
- Semantic code completion, including automatic imports
- As-you-type diagnostics
- GoTo including GoToReferences
- FixIt
- RefactorRename
- GetType
- GetDoc
See the trello board for a more complete picture.
Key goals:
- Support Java in ycmd and YCM; make it good enough to replace eclim and javacomplete2 for most people
- Make it possible/easy to support other lsp servers in future (but, don't suffer from yagni); prove that this works.
An overview of the objects involved can be seen on [this card][design]. In short:
- 2 classes implement the language server protocol in the
language_server_completer.py
module: LanguageServerConnection
- an abstraction of the comminication with the server, which may be over stdio or any number of TCP/IP ports (or a domain socket, etc.). Only a single implementation is included (stdio), but implementations for TCP/IP were written originally and dropped in favour of stdio's simplicity.LanguageServerCompleter
- an abstract base for any completer based on LSP, which implements as much standard functionality as possible including completions, diagnostics, goto, fixit, rename, etc.- The
java_completer
itself implements theLanguageServerCompleter
, boots the jdt.ls server, and instantiates aLanguageServerConnection
for communication with jdt.ls.
The overall plan and some general discussion around the project can be found on the trello board I used for development.
LSP is by its nature an asyncronous protocol. There are request-reply like
requests
and unsolicited notifications
. Receipt of the latter is mandatory,
so we cannot rely on their being a bottle
thread executing a client request.
So we need a message pump and despatch thread. This is actually the
LanguageServerConnection
, which implements thread
. It's main method simply
listens on the socket/stream and despatches complete messages to the
LanguageServerCompleter
. It does this:
- For
requests
: similarly to the TypeScript completer, using pythonevent
objects, wrapped in ourResponse
class - For
notifications
: via a synchronisedqueue
. More on this later.
A representation of this is on the "Requests and notifications" page of the [design][], including a rough sketch of the thread interaction.
While it is perhaps regrettable to do general processing directly in the message pump, there are certain notifications which we have to handle immediately when we get them, such as:
- Initialisation messages
- Diagnostics
In these cases, we allow some code to be executed inline within the message pump thread, as there is no other thread guaranteed to execute. These are handled by callback functions and state is protected mutexes.
See the 'initialisation sequence' tab on the [design][] for a bit of background.
In standard LSP, the initialisation sequence consists of an initialise request-reply, followed by us sending the server an initialised notification. We must not send any other requests until this has completed.
An additional wrinkle is that jdt.ls, being based on eclipse has a whole other
initialisation sequence during which time it is not fully functional, so we have
to determine when that has completed too. This is done by jdt.ls-specific
messages and controls the ServerIsReady
response.
In order for none of these shenanigans to block the user, we must do them all asynchronously, effectively in the message pump thread. In addition, we must queue up any file contents changes during this period to ensure the server is up to date when we start processing requests proper.
This is unfortunately complicated, but there were early issues with really bad UI blocking that we just had to get rid of.
Language server protocol requires that the client can apply textEdits, rather than just simple text. This is not an optional feature, but ycmd clients do not have this ability.
The protocol, however, restricts that the edit must include the original requested completion position, so we can perform some simple text manipulation to apply the edit to the current line and determine the completion start column based on that.
In particular, the jdt.ls server returns textEdits that replace the entered text for import completions, which is one of the most useful completions.
We do this super inefficiently by attempting to normalise the TextEdits into insertion_texts with the same start_codepoint. This is necessary particularly due to the way that eclipse returns import completions for packages.
We also include support for "additionalTextEdits" which allow automatic insertion of, e.g., import statements when selecting completion items. These are sent on the completion response as an additional completer data item called 'fixits'. The client applies the same logic as a standard FixIt once the selected completion item is inserted.
Diagnostics in LSP are delivered asynchronously via notifications
. Normally,
we would use the OnFileReadyToParse
response to supply diagnostics, but due to
the lag between refreshing files and receiving diagnostics, this leads to a
horrible user experience where the diagnostics always lag one edit behind.
To resolve this, we use the long-polling mechanism added here (ReceiveMessages
request) to return diagnostics to the client asynchronously.
We deliver asynchronous diagnostics to the client in the same way that the language server does, i.e. per-file. The client then fans them out or does whatever makes sense for the client. This is necessary because it isn't possible to know when we have received all diagnostics, and combining them into a single message was becoming clunky and error prone.
In order to be relatively compatible with other clients, we also return diagnostics on the file-ready-to-parse event, even though they might be out of date wrt the code. The client is responsible for ignoring these diagnostics when it handles the asynchronously delivered ones. This requires that we hold the "latest" diagnostics for a file. As it turns out, this is also required for FixIts.
jdt.ls is based on eclipse. It is in fact an eclipse plugin. So it requires an eclipse workspace. We try and hide this by creating an ad-hoc workspace for each ycmd instance. This prevents the possibility of multiple "eclipse" instances using the same workspace, but can lead to unreasonable startup times for large projects.
The jdt.ls team strongly suggest that we should re-use a workspace based on the
hash of the "project directory" (essentially the dir containing the project
file: .project
, pom.xml
or build.gradle
). They also say, however, that
eclipse frequently corrupts its workspace.
So we have a hidden switch to re-use a workspace as the jdt.ls devs suggest. In testing at work, this was mandatory due to a slow SAN, but at home, startup time is not an issue, even for large projects. I think we'll just have to see how things go to decide which one we want to keep.
There is no GetType in LSP. There's only "hover". The hover response is
hilariously server-specific, so in the base LanguageServerCompleter
we just
provide the ability to get the hover
response and JavaCompleter
extracts the
appropriate info from there. Thanks to @bstaletic for this!
FixIts are implemented as code actions, and require the diagnostic they relate
to to be send from us to the server, rather than just a position. We use the
stored diags and find the nearest one based on the request_data
.
What's worse is that the LSP provides no documentation for what the "Code
action" response should be, and it is 100% implementation-specific. They just
have this command
abstraction which is basically "do a thing".
From what I've seen, most servers just end up with either a WorkspaceEdit
or a
series of TextEdits
, which is fine for us as that's what ycmd's protocol looks
like.
The solution is that we have a callback into the JavaCompleter
to handle the
(custom) java.apply.workspaceEdit
"command".
Annoyingly, jdt.ls sometimes returns references to .class files within jar
archives using a custom jdt://
protocol. We can't handle that, so we have to
dodge and weave so that we don't crash.
Much like the initialisation sequence, the LSP shutdown sequence is a bit fiddly. 2 things are required:
- A
shutdown
request-reply. The server tides up and prepares to die! - An
exit
notification. We tell the server to die.
This isn't so bad, but jdt.ls is buggy and actually dies without responding to
the shutdown
request. So we have a bunch of code to handle that and to ensure
that the server dies eventually, as it had a habbit of getting stuck running,
particularly if we threw an exception.