Enable debug symbols for log4j from gradle - groovy

Question

My log4j statements are showing ?s for line numbers.
What I've tried
Other answers around the web say this happens when you haven't compiled debug information into your classes. In Ant for example, this would be done with <javac debug="true" .... Searching for a way to do this in gradle yields only
http://gradle.org/docs/2.3-rc-2/dsl/org.gradle.api.tasks.compile.CompileOptions.html
debug - Tells whether to include debugging information in the generated class files. Defaults to true
I verified this by putting println compileGroovy.options.debug in my build.gradle and it prints true. I also found http://forums.gradle.org/gradle/topics/compile-groovy-with-debugging-information, which recommended using tasks.withType(GroovyCompile) { options.debug = true }, which also did not work. Finally, I found compileGroovy.options.debugOptions.debugLevel, which can have source, lines, or vars. This is null by default, but setting it to source,lines did not give my line information.
Reproducing
gradle init --type groovy-library
Edit src/main/groovy/Library.groovy to have the additional lines:
import groovy.util.logging.Log4j
#Log4j
class Library {
boolean someLibraryMethod() {
log.info "XXXXXXXXXXXXX"
true
}
}
Add
compile 'log4j:log4j:1.2.17' to build.gradle
Add the file src/main/resources/log4j.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j='http://jakarta.apache.org/log4j/'>
<appender name="console" class="org.apache.log4j.ConsoleAppender">
<layout class="org.apache.log4j.PatternLayout">
<param name="ConversionPattern" value="%d %c:%L - %m%n" />
</layout>
</appender>
<root>
<level value="INFO" />
<appender-ref ref="console" />
</root>
</log4j:configuration>
And finally run gradle clean test -i and observe that you see
2015-03-30 08:47:31,996 Library:? - XXXXXXXXXXXXX
Note the ? in the message.
Version information
$ gradle -version
------------------------------------------------------------
Gradle 2.3
------------------------------------------------------------
Build time: 2015-02-16 05:09:33 UTC
Build number: none
Revision: 586be72bf6e3df1ee7676d1f2a3afd9157341274
Groovy: 2.3.9
Ant: Apache Ant(TM) version 1.9.3 compiled on December 23 2013
JVM: 1.8.0_40 (Oracle Corporation 25.40-b25)
OS: Windows 7 6.1 amd64
$ groovy -version
Groovy Version: 2.4.3 JVM: 1.8.0_40 Vendor: Oracle Corporation OS: Windows 7
$ java -version
java version "1.8.0_40"
Java(TM) SE Runtime Environment (build 1.8.0_40-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.40-b25, mixed mode)
My question is, of course, how do I get line numbers to show instead of question marks?

Answer 1

Groovy includes the line numbers in the class files. The problem is that groovy calls methods slightly differently than Java which confuses the Log4j code that determines the line number.
In particular, if you change the line
log.info "XXXXXXXXXXXXX"
to
log.error "XXXXXXXXXXXXX", new Exception('test')
you can see some reflection methods are called when groovy invokes methods. The topmost method in the stack trace I generated was sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method), which, being a native method, doesn't have a line number. This is what Log4j is looking at. Unfortunately, there's no easy was to filter the stacktrace Log4j uses or tell Log4j to look further down the stack.
One possible solution is to turn on static compilation. If you add #groovy.transform.CompileStatic to the Library class or method you'll get the correct line number.