build.xml: Make MANIFEST.MF friendlier to reproducible builds

Override the values of Created-By and Ant-Version entries to make
reproducible builds easier. This can be disabled by setting
manifest_cleanup=false.

The JAR specification gives an impression that the Created-By
entry refers to the version and vendor of the Java implementation.
Ant's default output conforms to that. On the other hand, Maven
and its plugins use strings that refer to Maven or plugin names
and versions. Thus, in the real-world the Created-By entry can
refer to different things.

build.properties

@@ -21,6 +21,10 @@ sourcever = 8
 sourcever9 = 9
 java8only = false
 
+# If true, the entries Created-By and Ant-Version in MANIFEST.MF will be
+# set to more stable values to make reproducible builds easier.
+manifest_cleanup = true
+
 # If true, javadoc is called with en_US locale to force the headings and
 # such strings from javadoc to be in English, independent of the system
 # language settings. This helps with reproducible builds.
@@ -33,6 +37,8 @@ dist_dir = ${build_dir}/dist
 dist_file = ${dist_dir}/xz-java-${version}.zip
 classes_dir = ${build_dir}/classes
 classes9_dir = ${build_dir}/classes9
+manifest_dir = ${build_dir}/manifest
+manifest_base = ${manifest_dir}/base.mf
 jar_dir = ${build_dir}/jar
 doc_dir = ${build_dir}/doc
 pom_dir = ${build_dir}/pom

build.xml

@@ -71,11 +71,32 @@
  </javac>
  </target>
 
- <target name="jar" depends="compile"
+ <target name="manifest">
+ <mkdir dir="${manifest_dir}"/>
+
+ <manifest file="${manifest_base}">
+ <!-- The exact Ant version used for the build shouldn't make
+ any difference in the produced files, so don't let it
+ affect the manifest either. -->
+ <attribute name="Ant-Version" value="Intentionally omitted"
+ if:true="${manifest_cleanup}"/>
+
+ <!-- Compiler output varies between major OpenJDK releases but
+ it can be stable between minor releases, thus the minor
+ version information can be annoying for reproducible builds.
+ java.specification.version contains only the major version.
+ It's already used by Maven Archiver for Build-Jdk-Spec. -->
+ <attribute name="Created-By" value="${java.specification.version}"
+ if:true="${manifest_cleanup}"/>
+ </manifest>
+ </target>
+
+ <target name="jar" depends="compile, manifest"
  description="Creates JAR packages">
  <mkdir dir="${jar_dir}"/>
 
- <jar destfile="${jar_dir}/xz.jar" modificationtime="${timestamp}">
+ <jar destfile="${jar_dir}/xz.jar" modificationtime="${timestamp}"
+ manifest="${manifest_base}">
  <fileset dir="${classes_dir}" includes="org/tukaani/xz/**"/>
  <zipfileset prefix="META-INF/versions/9/" dir="${classes9_dir}"
  unless:true="${java8only}"/>
@@ -104,6 +125,7 @@
  <sequential>
  <jar destfile="${jar_dir}/@{name}.jar"
  modificationtime="${timestamp}"
+ manifest="${manifest_base}"
  basedir="${classes_dir}"
  includes="@{name}.class">
  <manifest>
@@ -136,7 +158,7 @@
  </copy>
  </target>
 
- <target name="maven" depends="pom, doc, jar"
+ <target name="maven" depends="pom, manifest, doc, jar"
  description="Creates POM and JAR files for a Maven repository">
  <mkdir dir="${maven_dir}"/>
 
@@ -148,10 +170,12 @@
 
  <jar destfile="${maven_dir}/xz-${version}-javadoc.jar"
  modificationtime="${timestamp}"
+ manifest="${manifest_base}"
  basedir="${doc_dir}"/>
 
  <jar destfile="${maven_dir}/xz-${version}-sources.jar"
- modificationtime="${timestamp}">
+ modificationtime="${timestamp}"
+ manifest="${manifest_base}">
  <fileset dir="${src_dir}" includes="org/tukaani/xz/**"/>
  <zipfileset prefix="META-INF/versions/9/" dir="${src9_dir}"/>
  </jar>