The source and target attributes
of <javac> don't have any default values for
historical reasons. Since the underlying javac
compiler defaults depends on the JDK you use, you may encounter
build files that don't explicitly set those attributes and that will
no longer compile using a newer JDK. If you cannot change the build
file, Apache Ant provides two properties that help you setting
default values for these attributes. If the attributes have been
set explicitly, the properties listed here will be ignored.
Since Ant 1.7
Provides a default value for <javac>'s
and <javadoc>'s source attribute.
Since Ant 1.7
Provides a default value
for <javac>'s target attribute.