另一个线程的代码如何在主线程上运行？

Question

Lately, I was confused by some Android API. Here is some simplified explanation, cause the code is a little long which is totally right, but just confuse me.

I have two thread: UI thread and another HandlerThread called AThread .

mResponseHander is a Handler created in UI thread which obviously associtated to the UI thread's looper. Then I pass the mResponseHandler to the AThread .

AThread is a HandlerThread that does some image downloading task. In AThread , I wrote some code like this:

mResponseHandler.post(new Runnable() {
        @Override
        public void run() {

            if (mRequestMap.get(target) != url) {
                return;
            }

            mRequestMap.remove(target);
            mThumbnailDownloadListener.onThumbnailDownloaded(target, bitmap);
        }
    });

Besides, the variables mRequestMap and mThumbnailDownloadListener and others are only defined in the AThread .

I know when I call mResponseHandler.post(new Runnable) , the Runnable will later run on the UI thread cause the mResponseHandler is associtated to the UI thread's looper.

Here is the question: why the code above running in the UI thread is still right when the variables mRequestMap and mThumbnailDownloadListener and others are not defined in the UI thread but only defined in the AThread ?

Answer 1

why the code above running in the UI thread is still right when the variables "mRequestMap" and "mThumbnailDownloadListener" and others are not defined in the UI thread but only defined in the AThread?

Any class instance (lets use "object" further) resides in the JVM heap (not "in threads" or anything else). When applying the new operator as

Type variableName = new Type();

a piece of memory is allocated on the heap and reference to the memory is stored as value of variableName . From now on, any object (eg a Thread object or an object that implements Runnable , like in your case) that has the reference ( variableName or its copy) to the Type object, can "operate" on it.

With that said, by posting the Runnable to the handler associated with the Looper of the UI-thread you tell the thread what job should be done with the objects that reside in the heap and can be referred by mRequestMap and mThumbnailDownloadListener .

For the reference: Java Memory Model .