Thursday, December 13, 2018

A class cast exception when upgrading from GWT 2.8.0 to 2.8.2 ?

I just upgraded from GWT 2.8.0 to 2.8.2 and get a class cast exception which I don't understand, please some help.

It goes wrong in the following line of code:
ZtoeslagAppConfig this.bla = new JsoZtoeslagAppConfig(getZtoeslagConfig());

All relevant code:
private <T extends JavaScriptObject & ZtoeslagAppConfig> T getZtoeslagConfig() {
return (T) UtilsJsni.getProperty(getJson(), getMiscValues().zorgtoeslag());

private JavaScriptObject getJson() {
return this.json;

public interface ZtoeslagAppConfig {
AppPage[] getPages();

public final class JsoZtoeslagAppConfig  implements ZtoeslagAppConfig {
private final JavaScriptObject jsoConfig;

public <T extends JavaScriptObject & ZtoeslagAppConfig> JsoZtoeslagAppConfig(final T jsoConfig) {

public AppPage[] getPages() {
return UtilsJsArrays.toArray(getJsoPages());

private JsArray<? extends JsoAppPage> getJsoPages() {
return UtilsJsni.getPropertyAsArray(getJsoConfig(), getMiscValues().pages());

private JavaScriptObject getJsoConfig() {
return this.jsoConfig;

During super dev mode, in Chrome the values of this.json above:
 newsletter: "825eceb44f"
 zorgtoeslag: {payments: Array(3), pages: Array(3)} 

Just before calling the constructor in: 
new JsoZtoeslagAppConfig(getZtoeslagConfig());
It is calling a gwt method for casting that seems to fail:
 static Object castTo(Object src, JavaScriptObject dstId) {
    checkType(src == null || canCast(src, dstId));

In this method, I see that dstId has the value 1891, and src is some json value.
When performing the above body it calls the following method, that I don't directly understand as I don't see the method call in the above function:
  static native boolean jsEquals(Object a, Object b) /*-{
    return a == b;
I think this method fails as a is some json and b has the value null.

What goes wrong here? 


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