Android / Java Multithreading - Thread doesn't continue until main thread done its work

Question

The title might seem kind of vague, but I can't think of another one, so I hope I can make my problem clear:

I'm using an Android WebView and a JavascriptInterface. Communication from my app (Java) towards the WebView (JavaScript) is done like so:

webview.loadUrl("javascript:function()");

This is an asynchronous call and should be handled afterwards by the JavaBridge-Thread.

The other way (Javascript towards Java) works bei calling my JavascriptInterface mainInterface from my Javascript code. I can call Java methods declared with @JavascriptInterface in my JavascriptInterface. This call is also executed by the JavaBridge-Thread.

Before going into more detail, here are the important codes:

ModelActivity.java

public boolean loadPropertyIntoWebview(final String element, final String property, final String value){
    final int requestId = jsCallIndex.incrementAndGet();
    webview.loadUrl("javascript:setProperty('"+element+"', '"+property.toLowerCase()+"', '"+value+"', "+requestId+")");

    return waitForJsReturnValue(requestId, 1000);
}

private boolean waitForJsReturnValue(int index, int timeout){
    long start = System.currentTimeMillis();

    while(true){
        long elapsed = System.currentTimeMillis() - start;
        if(elapsed > timeout){
            System.out.println("JS RESPONSE TIMEOUT");
            break;
        }

        synchronized (jsReturnValueLock) {
            String value = jsReturnValues.remove(index);

            if(value != null){
                if(value.toLowerCase().equals("true")){
                    return true;
                }else{
                    System.out.println("JS BAD RESPONSE");
                    return false;
                }
            }

            long toWait = timeout - (System.currentTimeMillis() - start);
            if (toWait > 0)
                try {
                    System.out.println("WAIT NOW FOR "+toWait+" ms on index "+index);
                    jsReturnValueLock.wait(toWait);
                } catch (InterruptedException e) {
                    break;
                }
            else{
                System.out.println("JS RESPONSE TIMEOUT");
                break;
            }
        }
    }
    return false;
}

public void processJsReturnValue(int index, String value){
    System.out.println("RECEIVED ANSWER "+value+" for index "+index+" WAIT FOR UNLOCK");
    synchronized (jsReturnValueLock) {
        System.out.println("UNLOCKED FOR INDEX "+index);
        jsReturnValues.put(index, value);
        jsReturnValueLock.notifyAll();
    }
}

JavascriptInterface.java

@JavascriptInterface
public void onClick(String elementId, String property, String value){
    final String[] info = elementId.split("_");

    dbHandler.addOrUpdateOutVariable(Integer.parseInt(info[0]), Integer.parseInt(info[1]), property, value);

    ((ModelActivity)context).runOnUiThread(new Runnable() {

        @Override
        public void run() {
            actionManager.fireClickEvent(Integer.valueOf(info[0]), Integer.valueOf(info[1]));
        }
    });
}

@JavascriptInterface
public void processJsReturnValue(String index, String value){
    try{
        ((ModelActivity)context).processJsReturnValue(Integer.parseInt(index), value);
    }catch(NumberFormatException e){
        e.printStackTrace();            
    }
}

What I basically want to do, is get a return value from my JavaScript function setProperty() (ModelActivity -> loadPropertyIntoWebview).

During initialization the method loadPropertyIntoWebview() is called multiple times from the Main-Thread, and always receives an answer from the JavaBridge-Thread via both processJsReturnValue() functions.

The other case, which won't work, is when the user pushes a button in my webview, and invokes

onClick(String elementId, String property, String value)

in my JavascriptInterface. The function call

actionManager.fireClickEvent(Integer.valueOf(info[0]), Integer.valueOf(info[1]));

goes through several classes and functions which I can't explain in detail, but in last instance it will call

loadPropertyIntoWebview(final String element, final String property, final String value) .

This call also comes from the main-Thread, because I explicitly said so in my onClick function:

((ModelActivity)context).runOnUiThread(new Runnable() {

    @Override
    public void run() {
        actionManager.fireClickEvent(Integer.valueOf(info[0]), Integer.valueOf(info[1]));
    }
});

Debugging revealed the following:

During initilization and after calling

webview.loadUrl("javascript:setProperty('"+element+"', '"+property.toLowerCase()+"', '"+value+"', "+requestId+")");

the JavaBridge-Thread immediatley returns into processJsReturnValue() .

When invoked through the user it won't do that. Then it won't go there until the main thread is done with my timedout while(true) loop.

I hope this whole thing isn't too confusing, and someone understands my problem and even has an idea on what's going wrong here.

Answer 1

Try to separate onClick and processJsReturnValue as two different JavascriptInterface classes, create their instances and register them independently. Not sure but seems like now they block each other.