Measure Twice, Byte Once Making Stuff

Fixing ProGuard Warning “can’t write resource [META-INF/MANIFEST.MF]“

We're using ProGuard in our Android build and were getting a number of warnings regarding duplicate entries in the output jar. When ProGuard encounters resources in an input jar, it will by default copy the resources into the output jar. This is fine as long as you don't have multiple jars that contain resources with the same name. All jars(that I'm aware of) have manifest files, so attempting to use ProGuard with multiple input jars will cause warnings that look like: "Warning: can't write resource [META-INF/MANIFEST.MF] (Duplicate zip entry [yyy.jar:META-INF/MANIFEST.MF])".

The warnings are harmless, but a clean build is a happy build, so we might as well try and get rid of them. The ProGuard Manual actually describes what needs to be done, but it only provides a solution for the case where input jars are specified individually:

-injars  in1.jar
-injars  in2.jar(!META-INF/MANIFEST.MF)
-injars  in3.jar(!META-INF/MANIFEST.MF)

When using ProGuard in the Android build process, the default behavior is to pass in all input jars in a giant delimited(semi-colon on Windows) blob. I have done some experimentation, but was unable to determine how an input filter can be used when multiple jars(having absolute paths) are passed in this way. Fortunately, it's not too difficult to change the build process to pass in a list of input jars individually and with individual filters.

This is the relevant section of the standard Android build.xml(starting around line 700):

<!-- Build a path object with all the jar files that must be obfuscated.
	 This include the project compiled source code and any 3rd party jar
	 files. -->
<path id="project.jars.ref">
	<pathelement location="${preobfuscate.jar.file}" />
	<path refid="jar.libs.ref" />
</path>
<!-- Set the project jar files Path object into a single property. It'll be
	 all the jar files separated by a platform path-separator.
	 Each path must be quoted if it contains spaces.
-->
<pathconvert property="project.jars" refid="project.jars.ref">
	<firstmatchmapper>
		<regexpmapper from='^([^ ]*)( .*)$$' to='"\1\2"'/>
		<identitymapper/>
	</firstmatchmapper>
</pathconvert>

<mkdir   dir="${obfuscate.absolute.dir}" />
<delete file="${preobfuscate.jar.file}"/>
<delete file="${obfuscated.jar.file}"/>
<jar basedir="${out.classes.absolute.dir}"
	destfile="${preobfuscate.jar.file}" />
<proguard>
	@${proguard.config}
	-injars       ${project.jars}
	-outjars      "${obfuscated.jar.file}"
	-libraryjars  ${android.libraryjars}
	-dump         "${obfuscate.absolute.dir}/dump.txt"
	-printseeds   "${obfuscate.absolute.dir}/seeds.txt"
	-printusage   "${obfuscate.absolute.dir}/usage.txt"
	-printmapping "${obfuscate.absolute.dir}/mapping.txt"
</proguard>

The directives above create a new object that combines the location of the compiled jar along with other jars that were used during compilation. These locations are then joined into a string by the pathconvert task, which also does some transformation to ensure that paths containing spaces are quoted properly. Finally, the string from pathconvert(stored in project.jars) is passed into proguard via the statement: "-injars ${project.jars}"

This is the same section of my updated build.xml:

<!-- Set the project jar files Path object into a single property. It'll be
	 all the jar files separated by a platform path-separator.
	 Each path must be quoted if it contains spaces.
-->
<pathconvert property="project.jars" refid="jar.libs.ref" pathsep=" ">
	<firstmatchmapper>
		<regexpmapper from='^([^ ]*)( .*)$$' to='-injars "\1\2"(!META-INF/MANIFEST.MF)'/>
		<regexpmapper from='(.*)' to='-injars \1(!META-INF/MANIFEST.MF)'/>
	</firstmatchmapper>
</pathconvert>

<mkdir   dir="${obfuscate.absolute.dir}" />
<delete file="${preobfuscate.jar.file}"/>
<delete file="${obfuscated.jar.file}"/>
<jar basedir="${out.classes.absolute.dir}"
	destfile="${preobfuscate.jar.file}" />
<proguard>
	@${proguard.config}
	 -injars       ${preobfuscate.jar.file}
	 ${project.jars}
	-outjars      "${obfuscated.jar.file}"
	-libraryjars  ${android.libraryjars}
	-dump         "${obfuscate.absolute.dir}/dump.txt"
	-printseeds   "${obfuscate.absolute.dir}/seeds.txt"
	-printusage   "${obfuscate.absolute.dir}/usage.txt"
	-printmapping "${obfuscate.absolute.dir}/mapping.txt"
</proguard>

The goal of my changes is to preserve the manifest files from the original jar("preobfuscate.jar.file") and ignore(filter) the manifest files from the remaining input jar files. Since I haven't figured out how to filter input jars when passed in as a blob, I instead use the pathconvert task to transform each input jar location into a separate "-injars" statement with its own manifest filter. The property containing these locations is then passed into ProGuard directly via: "${project.jars}"

The solution isn't as clean as I would have liked, but it does fix the warnings.