automatically interning of string literals

Question

In the source code of com.sun.org.apache.xerces.internal.impl.XMLScanner at line 183 and 186

183    protected final static String fVersionSymbol = "version".intern();

186    protected final static String fEncodingSymbol = "encoding".intern();

Why "version" and "encoding" are explicitly interned by using intern() while they are string literals and would get automatically interned?

+1 Good question! The effect is the same, however the bytecode generated with and without is different. I guess we will never know unless we ask directly to the authors so this can't be answered. I would say this was premature optimization. — OscarRyz
– OscarRyz, Commented Nov 2, 2012 at 14:23

Joachim Sauer · Accepted Answer · 2012-11-02 14:29:59Z

I've tracked down the change to revision 318617 in the Apache Xerces SVN Repository (this is the project where this XML parser was initially developed, as the package name suggests).

The relevant part of the commit message is:

Trying to improve the use of symbol tables. Many predefined Strings are added to symbol tables every time the parser is reset. For small documents, this would be a significant cost. Now since we call String#intern for Strings in the symbol table, it's sufficient to use String#intern for those predefined symbols. This only needs to be performed once.

As you noted, the .intern() should not be necessary (and should have no visible effect) on a conforming JVM implementation.

My guess is that

either the author was not aware of the fact that string literals will always be interned
or it was a conscious decision to ward against a misbehaving JVM implementation

In the second case I'd expect some note of that in a comment or in the comment message, however.

One side-effect of that .intern() call is that initializers are no longer constant expressions and the fields will not be inlined by other classes referencing them.That will ensure that the class XMLScanner is loaded and its field read. I don't think this is relevant here, however.

from commit message:...Many predefined Strings are added to symbol tables every time the parser is reset...Now since we call String#intern for Strings in the symbol table... it's sufficient to use String#intern for those predefined symbols. This only needs to be performed once.--It is clear that author do not want to add PREDEFINED String SYMBOLS from code again and again to symbol table whenever parser is reset. So it is most likely that author is not aware of the fact that string literals automatically get interned.

T.J. Crowder · Accepted Answer · 2012-11-02 14:14:45Z

4

I don't believe there's any good reason for that, for the reason you identified: Literals are always automatically interned, as defined by the String class:

All literal strings and string-valued constant expressions are interned. String literals are defined in section 3.10.5 of the The Java™ Language Specification.

answered Nov 2, 2012 at 14:14

T.J. Crowder

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2 Comments

a Learner Over a year ago

so is it a example of bad coding?

T.J. Crowder Over a year ago

@aLearner: I suppose. I probably wouldn't put it quite that strongly, but at the very least if there's a good reason for doing it (which I doubt) I would expect a comment explaining what it is.

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