I was just getting the following output from XStream when trying to serialize an object which has a collection property:

<addresses class="java.util.Arrays$ArrayList">
<a class="address-array">
<address>

It had to step through the code to see what was going on.

The key is in the subtlety of the type of the list that I was putting in there. Notice that actually its not an java.util.ArrayList its actually an inner class of the Arrays, called, happily $ArrayList, so I had been staring at it the whole time and not noticed.

When XStream serializes its doing the following:

        Class actualType = newObj.getClass();
        Class defaultType = mapper.defaultImplementationOf(fieldType);
        if (!actualType.equals(defaultType)) {
            String serializedClassName = mapper.serializedClass(actualType);
            if (!serializedClassName.equals(mapper.serializedClass(defaultType))) {
                String attributeName = mapper.aliasForSystemAttribute(“class”);
                if (attributeName != null) {
                    writer.addAttribute(attributeName, serializedClassName);
                 }
            }
        }

From version 1.3.1 – AbstractReflectionConverter:127

So its asking the mapper what the default implementation of the type is. The type in this case was java.util.List and the default implementation is of course java.util.ArrayList. It is not seeing java.util.Arrays$ArrayList as the same thing and so thinks it needs to specify it.

Now you might be asking, how come I have a strange inner class version of ArrayList ? Well the thing is, I thought I was being clever in the set up of my test data and im using:

       addresses = Arrays.asList(
                new AddressBuilder().addressType(“home”).streetAddress(“1 The street”).build(),
                new AddressBuilder().addressType(“work”).streetAddress(“2 The street”).build(),
                new AddressBuilder().addressType(“holiday”).streetAddress(“3 The street”).build()
        );

To set up my test data. asList does this:

 public static  List asList(T… a) {
	return new ArrayList(a);
    }
    private static class ArrayList extends AbstractList
	implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
	private Object[] a;

Where it uses its own implementation of ArrayList.

Phew, another fun couple of hours spent there.