On Jun 24, 2009, at 11:31 AM, samppi wrote:
Are keywords and symbols garbage-collected? If I generated a lot of keywords or symbols, put them into a collection, and then removed them, would they disappear and free up space? I'm wondering if they're similar to Ruby symbols, which are never garbage collected.
Symbol objects are subject to garbage collection, but the "namespace" and "name" strings that identify them are not. Those strings are "interned" via the "intern" method on java.lang.String. Once a String is interned, there exists a single canonical String object that represents it throughout the remaining lifetime the JVM instance. Any two Symbols of the same namespace and name will reference those canonical namespace and name strings and will thus have identical namespaces and names.
Not interning the Symbols themselves turns out to be important in Clojure because it allows two Symbols with identical namespace and name to have different metadata.
Keyword objects are interned by Clojure and are not garbage collected. When a Keyword is created, it's placed in a ConcurrentHashMap that maps a Symbol of the same name and namespace to the Keyword object. In a given JVM instance, there is at most one Keyword object with a given name and namespace. Whenever the reader reads a Keyword's text representation, it returns the unique Keyword object associated with it.
Here's some repl playing to support this:
user=> (identical? 'a/b 'a/b)
false
user=> (identical? (name 'a/b) (name 'a/b))
true
user=> (identical? (namespace 'a/b) (namespace 'a/b))
true
user=> (= (quote #^{:tagged true} a/b) (quote a/b))
true
user=> (= (meta (quote #^{:tagged true} a/b)) (meta (quote a/b)))
false
user=> (identical? :a/b :a/b)
true
--Steve
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