As indicated by
SPR-16183, JDK 6 is severely outdated and effectively unmaintained in the meantime. The official end of of commercial JDK 6 support is coming up soon, with Oracle (http://www.oracle.com/technetwork/java/eol-135779.html) as well as IBM (https://developer.ibm.com/wasdev/docs/end-java-se-6-go/).
We'll have to join the club there, simply documentating that Spring Framework 4.3.x users are expected to be on recent patch releases of JDK 7, 8 or 9 these days. Let's roll this into 4.3.13's documentation and also blog about it around that time, officially declaring the end of first-class JDK 6 support on our end.
To be clear, we'll keep tolerating JDK 6 at runtime in the entire Spring Framework 4.3.x line, even in upcoming maintenance releases for the next few years. We just won't test on JDK 6 ourselves anymore and won't work around any JDK 6 specific issues, asking people to upgrade to JDK 7+ instead.