According to [1] LDAP connections can be pooled even if used with custom socket factories. Unfortunately it's not the way it works. As can be seen from [2] classnames of the factories are being passed to a comparator rather than factories themselves. That leads to a situation when at a runtime ClassCastException is silently swallowed and 'false' from the comparison is returned. And a custom compare() method is never executed. Thus connections are always treated as different and therefore never pooled.

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[1] http://docs.oracle.com/javase/6/docs/technotes/guides/jndi/jndi-ldap.html#pooling
[2] http://hg.openjdk.java.net/jdk6/jdk6/jdk/file/619655c4e72d/src/share/classes/com/sun/jndi/ldap/ClientId.java#l191