Summary
-------

To reduce the maintenance costs of `javac`, this JEP defines a policy for
retiring old `-source` and `-target` options. In JDK 8, use of a source
or target of `1.5` or earlier will be deprecated and in JDK 9, support
for a source or target of `1.5` or earlier will be removed. In JDK 9 and
going forward, `javac` will use a "one + three back" policy of supported
source and target options. Under this policy, `javac` will still be able
to recognize and process class files of all previous JDKs, going back to
version 45.3 class files generated by JDK 1.0.2, which first shipped
in 1996.


Goals
-----

Reduce compiler maintenance costs by removing obsolete code.


Motivation
----------

By removing old source and target options, maintenance of the `javac`
compiler is simplified. Many of the source and target values supported in
JDK 8 are for release trains that have long passed their end of public
updates. Users of those releases can use the `javac` from those releases
if a recompile is necessary in the future.

The source and target options are not mandated by the Java SE
specification and are provided for programmer convenience. However,
maintenance of old options has nonzero cost. For example, when a `-source
N` option older than the source level of `rt.jar` is specified, it is not
clear how newer-than-release-`N` language artifacts in the platform
libraries should be modeled to the code being compiled. Reducing the span
of supported source and target values simplifies the design
considerations of such questions.


Description
-----------

The never-documented target options `jsr14`, `1.4.1` and `1.4.2` have
already been removed from JDK 8. Source and target values of 1.5/5 and
earlier will be deprecated in JDK 8. A warning will be printed when such
options are used. In JDK 9, those options will no longer be recognized by
`javac`. JDK 9 will implement a "one plus three back" support policy
meaning that `1.9`/`9`, `1.8`/`8`, `1.7`/`7`, and `1.6`/`6` will be
recognized in that release. That policy will continue in JDK 10.


Impact
------

  - Compatibility: This policy changes the command-line compatibility
    policy, but even with this new policy source code 10 or more years
    old should still be able to be compiled.