Clojure: debugging and introspecting Java Code

One of the limitations of Cider is that it does not support debugging Java(or it is not very easy to do so). This page provides information on how to do that using dap-mode As a bonus, it will also enable navigation through java code like finding definitions, references, implementations, etc.

Java code navigation

1. Follow the guides on installing lsp-java. If you are Spacemacs user you need to add the following layers dap,lsp, (lsp-java :variables java-backend 'lsp). lsp-java automatically downloads all required server-side components(JDT Language Server, Java Debug Adapter and JUnit Test Runner).
2. Go to the root of your project and do

lein pom

3. Open a file from your project and do C-u- M-x - lsp which will prompt you to select a server to start, select jdtls server. Once the server has started(the initialization may take some time since JDT LS is downloading sources) you may call xref-appropos and open type a Java Type like ArrayList or PersistentList. When you are in java file you can inspect documentation, look for class implementations, references and so on.
4. Invoke xref-apropos, type a Java class name and then you can set breakpoints, go to definitions, find references, etc.

Implementations of clojure.lang.IReduce

Debugging

1. Configure cider to start nrepl with remote debugging enabled and do cider-jack-in

(setenv "JAVA_OPTS" "-Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=1044")

2. Call dap-debug and select Java Attach configuration and enter port 1044 (the same port as the port above.).
3. Go to java class and place a breakpoint using dap-breakpoint-toggle.

Debugging in progress