throw

(throw ex)

Signals ex. An exception -- an ex-info, a throwable construction ((Exception. "m")), a caught exception or runtime error, a host Throwable on the interpreter and the JVM -- signals as itself, so catch sees its class, message, data and cause; a catch of a host Throwable binds that very object. Anything else is the oracle's ClassCastException (nil its NullPointerException), whose message is the value's Clojure-notation rendering: a thrown string keeps the string as its message, a thrown map prints as the map (the oracle's message names the two classes).