xstream-1.3.1.jar provided with 5.2/6.0 is not compatible with JDK 1.7

XStream x = new XStream()

a = new Allowed("a","b")
s = x.toXML(a)
x.fromXML(s)

public class Allowed {

	private String users;
	private String groups;

	public Allowed(String users, String groups) {
		this.users = users;
		if (this.users == null)
			this.users = ""; 
		this.groups = groups;
		if (this.groups == null)
			this.groups = ""; 
	}
	public String getUsers() {
		return users;
	}
	public String getGroups() {
		return groups;
	}
	public String toString() {
		return "Users=>[" + users + "]  Groups=>[" + groups+"]";
	}

}

javax.script.ScriptException: com.thoughtworks.xstream.converters.ConversionException: Cannot construct com.xxx.xxxx.Allowed as it does not have a no-args constructor : 
Cannot construct com.xxx.xxxx.Allowed as it does not have a no-args constructor ---- Debugging information ---- message : 
Cannot construct com.xxx.xxxx.Allowed as it does not have a no-args constructor cause-exception : com.thoughtworks.xstream.converters.reflection.ObjectAccessException cause-message : 
Cannot construct com.xxx.xxxx.Allowed as it does not have a no-args 
constructor class :com.xxx.xxxx.Allowed 
required-type : com.xxx.xxxx.Allowed path : /com.xxx.xxxx.Allowed 
line number : 1 -------------------------------