IdentityHashMap in Google Web Toolkit (GWT): String Identity

Problem

IdentityHashMap in Google Web Toolkit (2.2.0) JavaScript applications does not seem to work correctly, if String objects are used as key.

Source Code:

final IdentityHashMap<Object, List<String>> idsFromObjects = new IdentityHashMap<Object, List<String>>();

final String node1 = “Node 1”;

final String id = “value”;

final ArrayList<String> ids = new ArrayList<String>();

ids.add(id);

idsFromObjects.put(node1, ids);                    

final String node1b = new String(“Node 1”);

GWT.log(“Values: “+idsFromObjects.get(node1b));

GWT.log(“Contains key?: “+idsFromObjects.containsKey(node1b));

GWT.log(“Equals?: “+String.valueOf(node1b == node1));

Output (Hosted Mode):

Values: null

Contains Key?: false

Equals?: false

Output (Compiled JavaScript):

Values: [value]

Contains Key?: true

Equals?: true

Note: This problem does not exist in hosted mode.

Solution

It seems there is no direct solution. Since the identity test node1b == node1 shows that the two Strings are actually identical for JavaScript, this might be a feature of JavaScript. Maybe JavaScript is keeping track of all the strings created, and does not allocate new memory, when an identical string is created; unlike Java when using the constructor new String(…).

It also appears the Google Web Toolkit JRE emulation, keeps a kind of cache for String hash codes (see the class String.HashCache). A cache is even for their identity hash maps – see System.identityHashCode() in this commit). This might be a result on JavaScripts handling of strings.

However, it is unfortunate that in this scenario running code in GWT’s hosted mode leads to different results than running the compiled JavaScript.

A possible workaround is to wrap strings in a simple proxy.

public class StringNode {

private final String value;

public StringNode(String value) {

this.value = value;

}

}

Note: This class is not serializable for GWT RPC. See this post for possible workarounds for serialization for immutable objects.

Resources

Google Groups ‘IdentityHashMap emulation

SVN trunk IdentityHashMap

Commit Message Google Web Toolkit Trunk, Stable identity hash values for strings?

String.HashCache JavaDoc

String.HashCache Source (as part of class String)

GWT Issue 631: adding String “watch” to HashSet will break GWT application in Firefox

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