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How to Add Logging to Ant Builds

The Apache Ant build tool is a great aid in automating builds and deployments.
When the build fails, however, diagnosing the problem can be painful -
especially if it only occurs intermittently. Logging details of the Ant
build can therefore be useful.

By default Ant writes informational messages to the console. While
this is usually sufficient for a successful build, more detail is often
useful in the case of failed builds. Relying on the console output for
information about the build has limitations – there might be too much
output to fix in the console's buffer, or you might clear or close the
console. The solution is to write Ant's output to a log file. This can
be done using the <record> task, which allows you to
specify a log level of "verbose" or "debug" to provide more details than
the standard "info" log level. This task should be the first one
performed by the build for any target to ensure that a log is always
produced. To accomplish this, I put the <record> task
at the start of a target named "init" which I ensure is the first
dependency of every other target. An example is given below:

<target name="init">
<record name="build.log" loglevel="verbose" append="false"/>

Normally I prefer to use the "verbose" level for the log file, as the
"debug" level provides so much additional detail that it is only really
useful for debugging bad behavior within Ant code. Notice that I
specified the option append="false" which ensures that at
the start of the build any existing log file is deleted and a new one
created. This makes it easier to see what happens for an individual
execution of the build and avoids having the log become too large. This
can be a problem, however, if you need to refer back to the log from a
prior build to resolve problems or for auditing purposes. I have had
situations where after a build fails I look at the build log for the
problem, run some other ant target (such as clean) before I have
finished resolving the problem, then realize that I have wiped the
record of the problem.

One solution to this issue is to create a separate log file for each
execution of the build. This can be done by using a build timestamp as
part of the log file name, as shown in the below example:

<target name="init">
<format property="timestamp" pattern="yyyy-MM-dd_HH-mm-ss"/>
<property name="build.log.dir" location="${basedir}/buildlogs"/>
<mkdir dir="${build.log.dir}"/>
<property name="build.log.filename" value="build_${timestamp}.log"/>
<record name="${build.log.dir}/${build.log.filename}"
loglevel="verbose" append="false"/>
<echo message="Build logged to ${build.log.filename}"/>

The log files are written to a buildlogs subdirectory to avoid cluttering the main directory. For some reason the <record> task requires an absolute path when specifying a directory, so the build.log.dir property is prefixed with ${basedir}.
After setting up the logging, a message is written to the console
displaying the log filename used to make it easier to look up the log
file if a problem occurs. This approach will cause log files to
accumulate within the log directory. If this becomes a problem, you
could add some Ant logic to delete log files older than a certain date. I
will leave the implementation of this as an exercise to the reader.
Hint: use a <delete> task with a <fileset> that contains a <date> selector.