Avoid duplicate Strings in Java

Question

I want to ask a question about avoiding String duplicates in Java.

The context is: an XML with tags and attributes like this one:

<product id="PROD" name="My Product"...></product>

With JibX, this XML is marshalled/unmarshalled in a class like this:

public class Product{
private String id;
private String name;
// constructor, getters, setters, methods  and so on
}

The program is a long-time batch processing, so Product objects are created, used, copied, etc.

Well, the question is: When I analysed the execution with software like Eclipse memory analyzer (MAT), I found several duplicated Strings. For example, in the id attribute, the PROD value is duplicated around 2000 instances, etc.

How can I avoid this situation? Other attributes in Product class may change their value along the execution, but attrs like id, name... don't change so frequently.

I have readed something about String.intern() method, but I haven't used yet and I'm not sure it's a solution for this. Could I define the most frequent values in those attributes like static final constants in the class?

I hope I'd have expressed my question in a right way. Any help or advice is very appreciated. Thanks in advance.

Answer 1

interning would be the right solution, if you really have a problem. Java stores String literals and a lot of other Strings in an internal pool and whenever a new String is about to be created, the JVM first checks, if the String is already in the pool. If yes, it will not create a new instance but pass the reference to the interned String object.

There are two ways to control this behaviour:

String interned = String.intern(aString); // returns a reference to an interned String
String notInterned = new String(aString); // creates a new String instance (guaranteed)

So maybe, the libraries really create new instances for all xml attribute values. This is possible and you won't be able to change it.

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intern has a global effect. An interned String is immediatly available "for any object" (this view doesn't really make sense, but it may help to understand it).

So, lets say we have a line in class Foo, method foolish:

String s = "ABCD";

String literals are interned immediatly. JVM checks, if "ABCD" is already in the pool, if not, "ABCD" is stored in the pool. The JVM assigns a reference to the interned String to s.

Now, maybe in another class Bar, in method barbar:

String t = "AB"+"CD";

Then the JVM will intern "AB" and "CD" like above, create the concatenated String, look, if it is intered already, Hey, yes it is, and assign the reference to the interned String "ABCD" to t.

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Calling "PROD".intern() may work or fail. Yes, it will intern the String "PROD". But there's a chance, that jibx really creates new Strings for attribute values with

String value = new String(getAttributeValue(attribute));

In that case, value will not have a reference to an interned String (even if "PROD" is in the pool) but a reference to a new String instance on the heap.

And, to the other question in your command: this happens at runtime only. Compiling simply creates class files, the String pool is a datastructure on the object heap and that is used by the JVM, that executes the application.