Reporting Server

You can get a huge amount of useful debugging information from the detailed HTML build reports. See for information on how to view these reports.

Maximum Verbosity

To output verbose debugging information to stdout set the log level to debug:

$ ./pants -ldebug ...


If you suspect that your build products are in a bad state you can remove them with ./pants clean-all.

Stopping Nailguns

Pants runs JVM processes in persistent daemons called "nailguns". If you think they may be holding bad state, you can kill them with ./pants ng-killall. They will restart on the next build.

Nailgun Errors

If nailguns stop responding, look for clues in the stdout and stderr files in .pants.d/ng/*/*.

Ivy Failures

You can see an Ivy invocation's detailed outputs in the HTML report.

You can get more verbose information from Ivy with the --resolve-ivy-args=-debug option.

This setting does not affect the output of the Ivy tasks used for bootstrapping tools.

Ivy Resolution

You can view Ivy's report of how it resolved each (transitive) third-party dependency:

$ ./pants resolve.ivy --open src/java/com/myorg/myproject/example/hello/greet

Ivy HTTP Debugging

This is a bit more involved, but gives you deeper inspection into the network traffic between Ivy and the repo.

First, you need to add commons-httpclient to the bootstrapped Ivy installation by creating a custom ivy.xml. You can start with the org.apache.ivy xml file under the ivy cache and just add a dependency for commons-httpclient as follows:

$ cat ./ivy-with-commons-httpclient.xml
<ivy-module version="2.0">
        <info organisation="org.apache.ivy" module="ivy-caller" />
        <configurations defaultconf="default" />
                <dependency org="org.apache.ivy" name="ivy" rev="2.3.0" />
                <dependency org="commons-httpclient" name="commons-httpclient" rev="3.0" />

Then, reference your modified ivy profile from pants.toml and flip on some extra logging in the commons-httpclient by setting some system properties:

ivy_profile = "ivy-with-commons-httpclient.xml"

# Enable httpcommons debugging when bootstrapping tools
jvm_options = [

# Enable httpcommons debugging when resolving 3rdparty libraries
jvm_options = [

You'll be able to see the output in the pants reporting server output by expanding the stderr nodes.

2015/03/12 03:55:56:456 PDT [DEBUG] HttpConnection - -Open connection to
2015/03/12 03:55:57:144 PDT [DEBUG] header - ->> "HEAD /maven2/org/antlr/antlr4/4.1/antlr4-4.1.pom HTTP/1.1[\r][\n]"
2015/03/12 03:55:57:144 PDT [DEBUG] HttpMethodBase - -Adding Host request header
2015/03/12 03:55:57:149 PDT [DEBUG] header - ->> "User-Agent: Apache Ivy/2.3.0[\r][\n]"
2015/03/12 03:55:57:150 PDT [DEBUG] header - ->> "Host:[\r][\n]"
2015/03/12 03:55:57:150 PDT [DEBUG] header - ->> "[\r][\n]"
2015/03/12 03:55:57:448 PDT [DEBUG] header - -<< "HTTP/1.1 200 OK[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Date: Thu, 12 Mar 2015 10:55:57 GMT[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Server: nginx[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Content-Type: text/xml[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Content-Length: 4800[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Last-Modified: Fri, 05 Jul 2013 21:35:45 GMT[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "ETag: "51d73c31-12c0"[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Accept-Ranges: bytes[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Accept-Ranges: bytes[\r][\n]"
2015/03/12 03:55:57:450 PDT [DEBUG] header - -<< "Via: 1.1 varnish[\r][\n]"
2015/03/12 03:55:57:451 PDT [DEBUG] header - -<< "Accept-Ranges: bytes[\r][\n]"
2015/03/12 03:55:57:451 PDT [DEBUG] header - -<< "Via: 1.1 varnish[\r][\n]"
2015/03/12 03:55:57:451 PDT [DEBUG] header - -<< "X-Served-By: cache-iad2127-IAD, cache-atl6233-ATL[\r][\n]"
2015/03/12 03:55:57:451 PDT [DEBUG] header - -<< "X-Cache: MISS, MISS[\r][\n]"
2015/03/12 03:55:57:451 PDT [DEBUG] header - -<< "X-Cache-Hits: 0, 0[\r][\n]"
2015/03/12 03:55:57:451 PDT [DEBUG] header - -<< "X-Timer: S1426157757.333469,VS0,VE36[\r][\n]"

Getting a stack trace from a running pants process

Pants implements faulthandler support in all execution contexts. To get a stack trace from a running pants process, send a SIGUSR2 signal like so:

$ kill -31 <pid>

For traditional pants runs, this will dump a stack trace (for all threads) to stderr. For pantsd-based runs and for pantsd itself, the traces will end up in the .pantsd/pantsd/pantsd.log file.

Questions, Issues, Bug Reports

See How to Ask

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