如何从GWT JSNI访问静态方法ID

Question

I fail to get the request_id from MyJavaObject in GWT JSNI like this example but got undefined value.

Please help.

JAVA CLASS

package com.my.app;

    class MyJavaObject
    {
        public final int request_id;

        public MyJavaObject(int request_id)
        {
            this.request_id = request_id;
        }

        public static final MyJavaObject MY_REQUEST = new MyJavaObject(13);
    }

GWT JSNI

public static native void expose()/*-{

    var val = @com.my.app.MyJavaObject::MY_REQUEST.request_id;

}-*/;

Answer 1

You're trying to access a non-static member variable from a static method without any reference to this object

Read the doc here: http://www.gwtproject.org/doc/latest/DevGuideCodingBasicsJSNI.html#methods-fields , your method should be written like that:

public static native void expose()/*-{

    var val = this.@com.my.app.MyJavaObject::request_id;
    // Do something with val...
}-*/;

Edit after comment:

Maybe this could work:

public static native void expose()/*-{

    var val = @com.my.app.MyJavaObject::MY_REQUEST.@com.my.app.MyJavaObject::request_id;
    // Do something with val...
}-*/;

if it does not, pass your object as a parameter:

public static native void expose( MyJavaObject obj)/*-{

    var val = obj.@com.my.app.MyJavaObject::request_id;
    // Do something with val...
}-*/;

and either call expose( MY_REQUEST ) directly or create a helper function

public static native void expose() {
    return expose( MY_REQUEST );
}

Answer 2

The problem is that while you are referencing the full type where the MY_REQUEST field lives, you are not referencing the full type where the request_id lives. Since JS has a very different concept of type hierarchy, when we reference Java from JS, we need to be very specific.

When you referenced MY_REQUEST , you did so by using the @classname::fieldname syntax. You must also do this to get the request_id field:

public static native void expose()/*-{
  var myRequest = @com.my.app.MyJavaObject::MY_REQUEST;
  var id = myRequest.@com.my.app.MyJavaObject::request_id;
  //do something useful with these values...
}-*/;

This could also be achieved in one line, but tends to be unreadable, so I'd avoid it.