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This is an improvement in Wicket 7.0 which should not affect any application migrating from Wicket 6.x. But if you use Servlet 3.0's AsyncContext in IResource in Wicket 7 then make sure that your code flushes the http response after completing the request.
API changes
Changes to org.apache.wicket.ajax.json.* WICKET-6287 - Switch from json.org to open-json RESOLVED
Because of license issues all classes in the mentioned package have been replaced by classes of open-json (https://github.com/tdunning/open-json) which means that the basic functionality is nearly the same. Some classes however (example: org.json.HTTP) are throwing WicketRuntimeExceptions because there is no corresponding implementation in open-json, yet. json.org can be used in this case (can be found in the central maven repository) if the license conditions are accepted.
DecimalConverters' number format WICKET-5853 WICKET-5861
org.apache.wicket.util.convert.converter.AbstractDecimalConverter's #getNumberFormat() and #setNumberFormat() has been removed in favour of #newNumberFormat(). Applications using the old methods should override the new one, where they configure specifics or preserve a reference to the created NumberFormat.
org/apache/wicket/core/util/io/SerializableChecker is replaced with org.apache.wicket.core.util.objects.checker.CheckingObjectOutputStream WICKET-4817
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