[Ant Wiki] Update of "AntOddities" by SteveLoughran

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The following page has been changed by SteveLoughran:
http://wiki.apache.org/ant/AntOddities

The comment on the change is:
why source=1.4, 1.5 required.

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+ == Why does <javac> require you to set source="1.4" for Java1.4 features, "1.5" for Java1.5? ==
+ 
+ The command line javac tool automatically selects the latest version of Java for the platform you build on. So when you build on Java1.4, it is as if -source 1.4 was passed in, while on Java1.5, you get the effect of -source 1.5
+ 
+ Yet on Ant, you have to explicitly say what version of the Java language you want to build against. If you have the assert keyword in source and omit the source="1.4" attribute, the compile will break. If you use fancy Java1.5 stuff and forget source="1.5", you get errors when you hit enums, attributes, generics or the new for stuff.
+ 
+ Q. Why is this the case? Why doesnt Ant "do the right thing?"
+ 
+ A. Because of a deliberate policy decision by the project. 
+ 
+ Ant was written, first and foremost, to build open source projects. In such a project, you don't know who builds your project; you just give out your source and rely on it compiling everywhere, regardless of which platform or JVM the person at the far end used. We also assume that a source tarball will still compile, years into the future.
+ 
+ If <javac> automatically selected the latest version of Java, then any Java1.2 code that used "enum" as a variable name, or "assert". To build the old projects, you would have to edit every build file that didnt want to use the latest Java version and force in a source="1.3" attribute. Except if they were other people's projects, you would have to struggle to get that change committed, and things like ApacheGump would never work, because all old-JVM projects would never compile straight from the SCM repository. 
+ 
+ That is why, in Ant, if you want the latest and greatest features of the Java language, you have to ask for them. By doing so you are placing a declaration in your build file what language version you want to use, both now and into the future.
+ 
+ An interesting follow-on question is then "why does javac on the command line risk breaking every makefile-driven Java project by automatically updating Java language versions unless told not to?" That is for Sun to answer, not the Ant team.
+ 

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