In the HTML version of PlayN, why does the following JSON-handling code throw an exception?

Question

I'm trying to figure out if this is a bug within HtmlJson.java or in my implementation. I just upgraded to PlayN version 1.2. My code attempts to load a Json.Object into a Json.Writer. This works in the Java version.

This is the code below. The exception is raised by line jsonWriter.object(jsonData); (note comment in code):

private void loadStoredData() {
    // storage parameters
    String storageKey = "jsonData";
    Json.Object jsonData = PlayN.json().createObject();        

    // attempt to load stored data
    String jsonString = PlayN.storage().getItem(storageKey);

    // if not loaded, create stored data
    if ( jsonString == null ) {
        DemoApi.log("stored data not found");
        jsonData.put("firstWrite", new Date().toString());

    // else display data
    } else {
        jsonData = PlayN.json().parse(jsonString);
        DemoApi.log("stored data loaded");
        DemoApi.log("data first written at " + jsonData.getString("firstWrite"));
        DemoApi.log("data last read at " + jsonData.getString("lastRead"));
        DemoApi.log("data last written at " + jsonData.getString("lastWrite"));
    }

    // update last read
    jsonData.put("lastRead", new Date().toString());

    // write data
    Json.Writer jsonWriter = PlayN.json().newWriter();
    jsonWriter.object(jsonData);        // <--- EXCEPTION RAISED HERE
    jsonString = jsonWriter.write();

    // store data as json
    PlayN.storage().setItem(storageKey, jsonString);

    // confirm
    if ( PlayN.storage().isPersisted() ) {
        DemoApi.log("data successfully persisted");
    } else {
        DemoApi.log("failed to persist data");
    }
}

The exception:

ERROR: Uncaught Exception: 
java.lang.ClassCastException: com.google.gwt.core.client.JavaScriptObject$ cannot be cast to java.lang.String
    at playn.html.HtmlJson$HtmlObject$.write$(HtmlJson.java:356)
    at com.google.gwt.core.client.JavaScriptObject$.playn_core_Json$Object_write(JavaScriptObject.java)
    at playn.core.json.JsonWriterBase.object(JsonWriterBase.java:119)
    at playn.core.json.JsonWriterBase.object(JsonWriterBase.java:90)
    at klenwell.sandbox.core.controllers.SandboxController.loadStoredData(SandboxController.java:121)

If this is a bug in my code, how might I correct it?

One additional piece of information that seems relevant. When I log the `JsonData` object, the log shows: `INFO: [object Object]`. This would seem to point to a problem with the line `if (o == null || isValueString(o))` in [HtmlJson.java](http://code.google.com/p/playn/source/browse/html/src/playn/html/HtmlJson.java?name=playn-1.2#356), specifically the `isValueString` function. It appears to be evaluating to `true` when it should be `false`. — klenwell, May 04 '12 at 03:42

klenwell · Accepted Answer · 2012-05-07T20:28:57.480

I found an alternative. Not quite as elegant as the original code, but it tested successfully in HTML5:

private void loadStoredData() {
    // storage parameters
    String storageKey = "jsonData";
    Json.Object jsonData = PlayN.json().createObject();  

    // to reset storage, uncomment this line
    //PlayN.storage().removeItem(storageKey);

    // attempt to load stored data
    String jsonString = PlayN.storage().getItem(storageKey);

    // if not loaded, create stored data
    if ( jsonString == null ) {
        DemoApi.log("stored data not found");
        jsonData.put("firstWrite", new Date().toString());

    // else display data
    } else {
        jsonData = PlayN.json().parse(jsonString);
        DemoApi.log("stored data loaded");
        DemoApi.log("data first written at " + jsonData.getString("firstWrite"));
        DemoApi.log("data last read at " + jsonData.getString("lastRead"));
        DemoApi.log("data last written at " + jsonData.getString("lastWrite"));
    }

    // update last read
    jsonData.put("lastRead", new Date().toString());

    // write data (this works in Java -- not in HTML)
    // see http://stackoverflow.com/q/10425877/1093087
    /*
    Json.Writer jsonWriter = PlayN.json().newWriter();
    jsonWriter.object(jsonData).end();
    jsonString = jsonWriter.write();
    */

    // alternative write routine
    Json.Writer jsonWriter = PlayN.json().newWriter();
    jsonWriter.object();
    for ( String key : jsonData.keys() ) {
        jsonWriter.value(key, jsonData.getString(key));
    }
    jsonWriter.end();
    jsonString = jsonWriter.write();

    // store data as json
    PlayN.storage().setItem(storageKey, jsonString);

    // confirm
    if ( PlayN.storage().isPersisted() ) {
        DemoApi.log("data successfully persisted");
    } else {
        DemoApi.log("failed to persist data");
    }
}

DemoApi.log, for the record, is a method I hatched to log output to the screen when testing in the browser. I later discovered that the PlayN.log() service logs to the Chrome console fine.

In the HTML version of PlayN, why does the following JSON-handling code throw an exception?

1 Answers1

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