One DAP client, three debuggers

June 11, 2026 · Adrian De Leon · debugging, dap

Editora can debug Java, Python, and JavaScript, and it does it with one client. The Debug Adapter Protocol (DAP) is the same idea as LSP but for debuggers: a common wire protocol so an editor doesn’t need bespoke code per language. The trick is that the three adapters are reached three completely different ways.

Three transports, one protocol

Java is layered on the existing jdtls language-server session. jdtls is started with Microsoft’s java-debug plugin, which registers vscode.java.* commands; at debug time, asking it to vscode.java.startDebugSession returns a TCP port, and the client connects a socket to it.
Python is debugpy over stdio: Editora spawns python -m debugpy.adapter and speaks DAP over the process’s stdin/stdout.
JavaScript is vscode-js-debug over a socket: spawn node dapDebugServer.js <port> and connect.

All three then speak the same protocol (via org.eclipse.lsp4j.debug), so breakpoints, stepping, watches, and the variables tree are written once. Above the client, everything is exposed as neutral records, so the UI and editor layers never import the debug library at all.

The gotchas were all about processes and threads

Getting three adapters working surfaced the same two traps as the language-server work, plus a threading one:

Undrained stderr deadlocks. Same as the LSP servers: an adapter’s stderr PIPE that nobody reads fills up and blocks it mid-handshake. Redirect.DISCARD it.
Orphaned adapters. Kill the whole descendant process tree on dispose, not just the launcher; a wrapper script otherwise leaves the real adapter running.
Don’t block the reader thread. When the initialized event arrives, you fire setBreakpoints / setExceptionBreakpoints / configurationDone. That callback runs on the DAP reader thread: so if you .join() on those requests, you deadlock, because the responses can’t be read while you’re blocking the reader. Fire and don’t wait.

Why one client was worth it

It would have been faster, short-term, to write a Java-only debugger and stop there. But DAP meant Python and JavaScript were mostly a matter of “spawn the adapter differently and shape the launch arguments”: the breakpoints, stepping, call stack, watches, inline values, and the console came along for free. The same bet LSP makes for language smarts, DAP makes for debugging: do the protocol once, get the languages cheaply.