When are Java Strings interned?

Question

Inspired by the comments on this question, I'm pretty sure that Java Strings are interned at runtime rather than compile time - surely just the fact that classes can be compiled at different times, but would still point to the same reference at runtime.

I can't seem to find any evidence to back this up. Can anyone justify this?

Answer 1

It's both:

• If two references to the same string constant appear in the same class, the class file will only contain one string pool entry
• When classes are loaded, the string pool for the class is added to the intern pool

(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.