JsInterop: Casting native javascript objects in gwt

[Brian Pedersen]

I have a dispatch method, exposed through JsInterop, which can take various kinds of payloads.

Once called from native javascript, I need to cast the payload, but this doesn't quite work as I would like it too.

Here is a simplified example.

GWT:

@JsExport

@JsType

@JsNamespace("foo")

public class Dispatcher {

public String dispatch(String action, Payload payload){

if("action1".equals(action))

return dispatchWithCast(payload);

return "Unknown action: " + action;

}

public String dispatchWithCast(Payload payload){

ConcretePayload w = (ConcretePayload) payload;

return w.foo;

}

}

@JsExport

@JsType

public class Payload {}

@JsExport

@JsType

public class ConcretePayload extends Payload {

public String foo;

public int bar;

}

Javascript:

var test = new foo.Dispatcher();

alert("Dispatch: " + test.dispatch("action1", {"foo": "a", "bar": 2})); // Uncaught java.lang.ClassCastException

Everything works just fine if I replace the Payload type in the Dispatcher class with ConcretePayload, and removes the cast.

I guess this this may be beyond the capabilities of the transpiler, I just need to figure out an alternative approach.

Any ideas?

(By the way, I am using the sso linker.)

[Thomas Broyer]

You may want to use GWT 2.8.0-SNAPSHOT and the new JsInterop annotations; as your usecase maybe is just not supported with the "old" annotations.

[Brian Pedersen]

I am using GWT 2.8.0-SNAPSHOT, but the new annotations does not work for me.

I tried using @JsType(namespace = "acme", name = "MyJavaScriptObject"), as described in the new version of the JsInterop document, but apparently the namespace and name attributes are not there in the jar file.

Same thing with the @JsExport and @JsNamespace, I currently have to use them or things stop working.

My gwt-user-2.8.0-SNAPSHOT.jar, which contains the annotations, is from today.

This is the pom I am using: https://github.com/bitwerk/jspoc/blob/master/pom.xml

/Brian

[Brian Pedersen]

Ahhh, I just realized that the new annotations are in a different package, jsinterop.annotations instead of com.google.gwt.core.client.js :)

/Brian

[Jens]

I am using GWT 2.8.0-SNAPSHOT, but the new annotations does not work for me.

I tried using @JsType(namespace = "acme", name = "MyJavaScriptObject"), as described in the new version of the JsInterop document, but apparently the namespace and name attributes are not there in the jar file.

Same thing with the @JsExport and @JsNamespace, I currently have to use them or things stop working.

The new annotations have a different package: jsinterop.annotations. Also @JsExport does not exist anymore in the new JsInterop version, you just use @JsType.

To activate the new annotations you also have to change the JsInterop mode via -jsInteropMode JS_RC

-- J.

[Brian Pedersen]

Thank's for pointing that out. So i switched to the new annotations in jsinterop.annotations and activated the new mode with -jsInteropMode JS_RC.

The cast still doesn't work though :(

Instead of a java.lang.ClassCastException, I now get a  Uncaught [object Object] during the cast.

Once again, everything works if I use the concrete type and remove the cast.

POM: https://github.com/bitwerk/jspoc/blob/master/pom.xml

Javascript: https://github.com/bitwerk/jspoc/blob/master/src/main/webapp/jspoc.html

GWT: https://github.com/bitwerk/jspoc/tree/master/src/main/java/dk/bitwerk/client

/Brian

[Brian Pedersen]

It seems the generated function com_google_gwt_lang_Cast_canCast__Ljava_lang_Object_2Lcom_google_gwt_core_client_JavaScriptObject_2Z only supports strings, maps, numbers and booleans.

function com_google_gwt_lang_Cast_canCast__Ljava_lang_Object_2Lcom_google_gwt_core_client_JavaScriptObject_2Z(src_0, dstId){

  return typeof src_0 === 'string' && !!com_google_gwt_lang_Cast_stringCastMap[dstId]
      || src_0.java_lang_Object_castableTypeMap && !!src_0.java_lang_Object_castableTypeMap[dstId]
      || typeof src_0 === 'number' && !!com_google_gwt_lang_Cast_doubleCastMap[dstId]
      || typeof src_0 === 'boolean' && !!com_google_gwt_lang_Cast_booleanCastMap[dstId];

}

/Brian

[Brian Pedersen]

I didn't get that quite right, the generated function seem to fail doing the lookup in the castableTypeMap, which must be it's way of assuring that the types are actually castable to the target type.

[Cristiano Costantini]

jsinterop.annotations

Truly,

If one day I'll find the man who continue using non standard packages in GWT, I'll take all his keyboards and "hack" so that each key will output "org.gwtproject" as a prefix to any key stroke!

:-D

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[Goktug Gokdogan]

You can only implement java contract (e.g. PayLoad class) in javascript if they are marked as isNative. See the related section in the new jsinterop doc.

--

[Brian Pedersen]

Thank you Gortug for making my aware of the isNative attribute. Unfortunately the cast issue remains.

I have taken the example from the new jsinterop doc, and modified it to illustrate this: https://github.com/bitwerk/jspoc/tree/master/src/main/java/com/acme

// in Java

package com.acme;

@JsType(isNative = true)

interface Foo {

}

@JsType(isNative = true)

class Baz implements Foo{

  public int qux;

}

class Bar {

  @JsMethod

  public static int action(Foo foo) {

    Baz baz = (Baz) foo;
    return baz.qux;

  }

}

// in JavaScript

var baz = { qux: 42 };

com.acme.Bar.action(baz); // will return 42!

The image below demonstrates the issue at runtime.

I can't seem to find anywhere inside the generated javascript, where a class literal is assigned to the value 33.

/Brian

[Brian Pedersen]

 I hate not being able to edit my posts on google groups :)

The comment "// will return 42!" should have contained "// Should return 42, but throws 'Uncaught [object Object]'!"

[Brian Pedersen]

Here is another variant, failing with the same exception

//In Java

package com.acme;

@JsType

public class IntParser {

  public static int parseArray(String[] args) {

    return Integer.parseInt(args[0]);

  }

}

//In Javascript
var num = com.acme.IntParser.parseArray(["42"]); // Should return 42, but throws 'Uncaught [object Object]'!

[Goktug Gokdogan]

This should work. I need to take a look why it fails.

[Brian Pedersen]

Thanks.

If it is any help, my code is here: https://github.com/bitwerk/jspoc/tree/master/src/main/java/com/acme

/Brian

[Goktug Gokdogan]

I couldn't reproduce the IntParser problem with following test:

  @JsType(namespace = JsPackage.GLOBAL)
  public static class IntParser {
    

public static int parseArray(String[] args) {
      return Integer.parseInt(args[0

]);
    }
  }

  public void testStringArray() {
    assertEquals(42, callIntParser());
  }

  private native int callIntParser() /*-{
    return $wnd.IntParser.parseArray(["42"]);
  }-*/;

​

Also in your earlier example, you are casting { qux: 42 } to Baz which will fail because ({ qux: 42 } instanceof com.acme.Baz) is not true as { } is instance of global Object. You can change Baz to point to that:

@JsType(isNative = true, name="Object", namespace=GLOBAL)

class Baz implements Foo {

  public int qux;

}