Common Constants

[Clayton Cramer]

We have started to use a tool called Your Kit Java Profiler. It shows something that just dumbfounds me, because I would have thought Java was smarter than this. It shows that we have 26 MB of heap devoted to duplicate copies of the empty string "". Unlike every C compiler that I have ever, used it would appear that if you have multiple strings that are identical, and immutable (you are referencing them as "" in expressions like str = i + ""), each reference produces its own copy of that empty string. The painful solution is to define an empty string in one file:

public final static String s = "";

and then replace all the "" references with s.

Is there really no way to tell the Java compiler to combine immutable constants?

[Winston Gutkowski]

Clayton Cramer wrote:Is there really no way to tell the Java compiler to combine immutable constants?

Have a look at String.intern().

However, if you say that there's 26Mb of this, I don't think that's the full extent of your problem.

Winston

[Mike Simmons]

This sounds very suspicious, as the Java Language Specification has always required that string literals and other String constant expressions be interned in the first place. It's possible that you've got a compiler/JVM which is somehow violating this I suppose. But it seems more likely you're misinterpreting what YourKit is telling you. Perhaps there are other objects being created by the expressions you're using. For example, for

String str = i + ""

it's probably generating something like this:

String str = new StringBuilder("").append(i).toString();

You can probably do better with

String str = String.valueOf(i);

[Clayton Cramer]

It is certainly the case that we have vast numbers of i + "" expressions scattered about the code, and that might explain why we have 26 MB of "". Each new StringBuilder("") would presumably create a distinct copy, because a StringBuilder object is mutable.

[Jeff Verdegan]

26 MB of StringBuilder containing "", yes, could theoretically make sense, although they should be short-lived, so I'm not sure why you'd have 26 MB live at once.

26 MB of String objects containing "" sounds very wrong.

[Jesper de Jong]

Do you have code that uses the String constructor with a literal, like this:

String something = new String("");
String another = new String("hello");

If you do that a lot, you would be creating a lot of unnecessary String objects, and it prevents the compiler from reusing String objects. Never use new String(...) with a string literal as its argument, it's never necessary.