When are Java Strings interned?

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The optimization happens (or at least can happen) in both places:

  • If two references to the same string constant appear in the same class, I’d expect the class file to only contain one constant pool entry. This isn’t strictly required in order to ensure that there’s only one String object created in the JVM, but it’s an obvious optimization to make. This isn’t actually interning as such – just constant optimization.
  • When classes are loaded, the string pool for the class is added to the intern pool. This is “real” interning.

(I have a vague recollection that one of the bits of work for Java 7 around “small jar files” included a single string pool for the whole jar file… but I could be very wrong.)

EDIT: Section 5.1 of the JVM spec, “The Runtime Constant Pool” goes into details of this:

To derive a string literal, the Java
virtual machine examines the sequence
of characters given by the
CONSTANT_String_info structure.

  • If the method String.intern has
    previously been called on an instance
    of class String containing a sequence
    of Unicode characters identical to
    that given by the CONSTANT_String_info
    structure, then the result of string
    literal derivation is a reference to
    that same instance of class String.

  • Otherwise, a new instance of class
    String is created containing the
    sequence of Unicode characters given
    by the CONSTANT_String_info structure;
    that class instance is the result of
    string literal derivation. Finally,
    the intern method of the new String
    instance is invoked.

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