Debug Surefire

debug-surefire

Run Maven Surefire tests suspended in JDWP so you can attach a debugger and step through the code.
In headless/agent environments, attach with jdb (the JDK’s command-line debugger).

What this does

• Runs mvn test with Surefire in debug mode via -Dmaven.surefire.debug.
• Surefire launches the forked test JVM with a JDWP socket and suspend=y, meaning:
  • the forked JVM starts,
  • prints “Listening for transport dt_socket at address: ”,
  • then waits until a debugger attaches,
  • only then does it begin executing tests.

This is important: you do not attach to the mvn process itself; you attach to the forked JVM running the tests.

Quick start

1. Start a suspended test run

• Debug a test class:
  • .codex/skills/debug-surefire/scripts/debug-surefire.sh --test-class MyTest
• Debug a single test method (quote the #):
  • .codex/skills/debug-surefire/scripts/debug-surefire.sh --test 'MyTest#shouldDoThing'
• Debug a test in a specific module:
  • .codex/skills/debug-surefire/scripts/debug-surefire.sh --module core/sail/shacl --test-class ShaclSailTest
  • .codex/skills/debug-surefire/scripts/debug-surefire.sh --module rdf4j-sail-shacl --test 'ShaclSailTest#testSomething'

When the forked JVM is ready, you should see something like:

• Listening for transport dt_socket at address: 55005

2. Attach with jdb (preferred)

In a second terminal, attach to the printed port:

• Local attach (port only implies localhost):
  • jdb -attach 55005
• Explicit host+port:
  • jdb -attach localhost:55005

If you need source listing inside jdb, provide a sourcepath up front:

• jdb -sourcepath module/src/main/java:module/src/test/java -attach 55005

3. Set breakpoints / catches, then resume

Once you’re at the jdb prompt, set a breakpoint (or exception catch) and continue:

• stop in com.example.MyTest.shouldDoThing
• catch uncaught java.lang.AssertionError
• cont

Tip: right after attaching to a just-started suspended JVM, jdb may show:

• No frames on the current call stack

That’s normal. The JVM hasn’t executed into any Java frames yet. Set breakpoints first, then cont.

Notes

• The script runs a fast pre-test install (-Pquick into the repo-local .m2_repo) and then runs mvn test with Surefire in debug mode.
• Use SUREFIRE_DEBUG_PORT=8000 to change the port (default: 55005).
• Use --no-offline / --online if offline (-o) resolution fails.
• Everything after -- is passed to Maven, e.g.:
  • .codex/skills/debug-surefire/scripts/debug-surefire.sh --test-class MyTest -- -DtrimStackTrace=false -DfailIfNoTests=false -DforkCount=1 -DreuseForks=false

jdb interaction guide

This section is optimized for quickly getting signal from a failing unit test without drowning in framework/JDK internals.

Command cheat sheet (the ones you’ll actually use)

Start / resume

• cont — continue execution from current stop/breakpoint
• run — start execution when jdb launched the VM (less relevant when you used -attach)

Breakpoints

• stop in <class>.<method> — break on method entry
• stop at <class>:<line> — break at a specific line
• stop — list all breakpoints
• clear <class>.<method> / clear <class>:<line> — remove a breakpoint
• clear — list breakpoints (same idea as stop listing)

Stepping

• next — step over calls (line-level)
• step — step into calls (line-level)
• step up — run until current method returns
• stepi — step one bytecode instruction (rarely needed)

Threads / stacks

• threads — list threads
• thread <id> — select default thread
• where — stack trace for current thread
• where all — stack traces for all threads
• up / down — move the current frame up/down the stack

Inspect state

• locals — print locals in current frame
• print <expr> — evaluate/print an expression (same as eval)
• dump <expr> — more complete object dump
• set <lvalue> = <expr> — mutate a variable/field (use sparingly)

Source

• list — show source around current line
• list <line> or list <method> — show a specific region
• use <path1>:<path2> (aka sourcepath) — set where jdb looks for sources

Exceptions

• catch uncaught <exception> — break when an uncaught exception occurs
• catch caught <exception> — break when a caught exception occurs
• catch all <exception> — break for both caught+uncaught
• ignore ... — undo a catch

Reduce noise

• exclude <pattern>,<pattern>,... — don’t report step/method events for matching classes
• exclude none — clear exclusions

Automation

• monitor <command> — run a command every time the program stops (e.g., monitor where)
• read <file> — execute commands from a file
• !! — repeat last command
• <n> <command> — repeat command n times (e.g., 10 next)

Efficient workflow for failing JUnit tests

1) Add exclusions immediately (so stepping doesn’t become a swamp)

Right after connecting, set exclusions to avoid stepping into JDK/framework code:

• exclude java.*,javax.*,jdk.*,sun.*,com.sun.*,org.junit.*,org.junit.jupiter.*,org.assertj.*,org.hamcrest.*,org.mockito.*,org.apache.maven.*

You can always clear this later:

• exclude none

2) Break where it matters

Common breakpoint patterns:

• Break at the failing test method:

  • stop in com.example.MyTest.shouldDoThing

• Break inside the code under test:

  • stop in com.example.service.FooService.doWork

• Break at a specific suspicious line:

  • stop at com.example.service.FooService:123

If the method is overloaded, specify argument types:

• stop in com.example.FooService.doWork(int,java.lang.String)

Note: jdb supports deferred breakpoints. If the class isn’t loaded yet, it will still accept the breakpoint and activate it when the class loads.

3) Break on the failure, not just your guess

For unit tests, breaking on the thrown assertion/error is often faster than guessing a line.

Useful catches:

• Classic Java assertions / many test failures:

  • catch uncaught java.lang.AssertionError

• Common in JUnit 5 assertions:

  • catch uncaught org.opentest4j.AssertionFailedError

• NPE hunting:

  • catch uncaught java.lang.NullPointerException

You can remove a catch with ignore:

• ignore uncaught java.lang.AssertionError

Tip: catch all java.lang.Throwable is the nuclear option. It works, but it can get loud.

4) Resume and drive

Once breakpoints/catches are set:

• cont

When you hit a breakpoint:

• where to see the call stack
• list to see nearby source
• locals to see local variables
• print someVar or print this.someField to inspect
• next to step over, step to step into

5) Find the “real” test thread quickly

Surefire + JUnit can spin up multiple threads (and Maven itself has its own). When in doubt:

• threads
• where all

Then pick the thread that’s in your test/code-under-test:

• thread <id>
• where

6) Keep the JVM count predictable

Surefire forking and parallelism can make debugging confusing. If you see multiple JVMs / inconsistent behavior, pass Maven flags to keep it single and fresh:

• -DforkCount=1 -DreuseForks=false

Example:

• .codex/skills/debug-surefire/scripts/debug-surefire.sh --test 'MyTest#shouldDoThing' -- -DforkCount=1 -DreuseForks=false

7) Use “thread-only” breakpoints when concurrency matters

By default, jdb breakpoints suspend all threads, which can create deadlocks in concurrent tests. You can tell jdb to suspend only the thread that hits the breakpoint:

• stop thread in com.example.concurrent.Worker.run

(You can also target a specific thread id with stop thread <thread_id> in ... if needed.)

jdb startup customization (optional but powerful)

jdb will execute startup commands from jdb.ini or .jdbrc in either user.home or user.dir.

This is handy for keeping your default exclusions/catches consistent, e.g.:

• exclude java.*,javax.*,jdk.*,sun.*,com.sun.*,org.junit.*,org.assertj.*
• catch uncaught java.lang.AssertionError

You can also keep a project-local command file and load it on demand:

• read .codex/skills/debug-surefire/jdb.cmds

Troubleshooting

• jdb can’t connect

  • Confirm the port printed by the suspended JVM matches what you used in jdb -attach.
  • Confirm you’re attaching to the forked JVM port (the one that prints “Listening for transport…”), not Maven’s PID.
  • If running remotely (CI box / container / VM), ensure the port is reachable or forwarded.

• Breakpoints don’t hit

  • Use fully qualified class names.
  • If overloaded, include argument types.
  • Use classes to see what’s loaded.
  • Try stop at <class>:<line> to avoid signature mismatch.

• list can’t find source

  • Set sourcepath:
    • use module/src/main/java:module/src/test/java
  • Or launch jdb with -sourcepath ... from the start.