Problem
The default REPL printing and reading wrt Dates is inconsistent between a minimal runtime environment (like a JVM with only the java.base module, or the deprecated JDK8 bootclasspath/bcp) and a full JVM.
There are a handful of references in Clojure to classes that live outside of the java.base module (or bcp). In such all places the referent class does not load automatically during Clojure's init. (They are loaded on demand, e.g. when the user first calls {{bean}}) The load of the core.instant namespace became conditional in Clojure 1.9, depending on the availability of the java.sql.Timestamp class. This was an accommodation made to enable lein to Clojure to run on the Java 8 bootclasspath, which lein used as a hack to avoid bytecode verification and thus load faster. (The reference to java.sql.Timestamp was verboten on the bcp) This also had the by-product of not loading read/print handlers for java.util.Date, which do exist on the bcp or java.base module.
present situation
lein has meanwhile stopped (mis)using the bootclasspath (instead skipping bytecode verification explicitly)
The notion of a bootclasspath no longer exists as of JDK9
There is now a bottom module with a clearly-defined set of classes, including java.util.Date and java.time.Instant
jlinking JVMs is a common and effective container deployment strategy
Clojure already runs fine on the bootclasspath or a JVM with only #{java.base}, but because the default data reader & clojure.instant aren't loaded, no read/print handlers for inst/j.u.Date get installed. The conditional load is about java.sql.Timestamp, which is never in default use anyways (besides the print handler) but it affects things that work fine in a reduced environment. (java.sql.Timestamp is in the java.sql module, which adds an additional 12MB to a jlinked jvm)
Great tagged value handling for dates sets Clojure's REPL apart from other dynlangs. It would be nice to have those conveniences available consistently on all modularized JDKs.
considerations
reduce the scope of the conditional load to avoid affecting date read/print handlers?
move the install of the read handler into clojure.instant, and load it unconditionally? (There was never a promise about when the handler was installed)
java.time.Instant is also in the bootclasspath or java.base module
informational
The following is a list of class references outside of the java.base module, not including the check for java.sql.Timestamp inside core.clj
clojure.core$bean -> java.beans.BeanInfo java.desktop
clojure.core$bean$fn__6987$fn__6988 -> java.beans.PropertyDescriptor java.desktop
clojure.core$resultset_seq -> java.sql.ResultSet java.sql
clojure.inspector$inspect -> java.awt.BorderLayout java.desktop
clojure.inspector$inspect_table -> java.awt.Component java.desktop
clojure.inspector$inspect_tree -> java.awt.Component java.desktop
clojure.instant$construct_timestamp -> java.sql.Timestamp java.sql
clojure.instant$print_timestamp -> java.sql.Timestamp java.sql
clojure.java.browse_ui$open_url_in_swing -> java.awt.Component java.desktop
clojure.lang.XMLHandler -> org.xml.sax.Attributes java.xml
clojure.repl$set_break_handler_BANG_ -> sun.misc.Signal JDK internal API (jdk.unsupported)
clojure.repl$set_break_handler_BANG_ -> sun.misc.SignalHandler JDK internal API (jdk.unsupported)
clojure.repl.proxy$java.lang.Object$SignalHandler$d8c00ec7 -> sun.misc.Signal JDK internal API (jdk.unsupported)
clojure.repl.proxy$java.lang.Object$SignalHandler$d8c00ec7 -> sun.misc.SignalHandler JDK internal API (jdk.unsupported)
clojure.xml$startparse_sax -> javax.xml.parsers.SAXParserFactory java.xml
clojure.xml$fn__8900 -> org.xml.sax.ContentHandler java.xml
