Source of problem

Today I'm learning Volley's official documents. Sending a Simple Request When you see the request lifecycle diagram below.

The meaning of the blue box is clear: request gets response deliver y from cache/network to main thread.
(Ps - begin:
Personality Youth: You mean it.
Ben Diao Silk Mason: Trace to the end, the source code shows the true meaning!
Ps - end)

Create RequestQueue

According to the official document, there are two ways:

• Convenient Creation Method

// Instantiate the RequestQueue.
RequestQueue queue = Volley.newRequestQueue(this);

• Explicit Creation

RequestQueue mRequestQueue;

// Instantiate the cache
Cache cache = new DiskBasedCache(getCacheDir(), 1024 * 1024); // 1MB cap

// Set up the network to use HttpURLConnection as the HTTP client.
Network network = new BasicNetwork(new HurlStack());

// Instantiate the RequestQueue with the cache and network.
mRequestQueue = new RequestQueue(cache, network);

Looking at the source code discovery convenient creation method is actually the set of logic of explicit creation method, but the Volley framework has done something for us. Look at the code:

/**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @param context A {@link Context} to use for creating the cache dir.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context) {
        return newRequestQueue(context, null);
    }

Continue to deepen:

/**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @param context A {@link Context} to use for creating the cache dir.
     * @param stack An {@link HttpStack} to use for the network, or null for default.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);

        String userAgent = "volley/0";
        try {
            String packageName = context.getPackageName();
            PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
            userAgent = packageName + "/" + info.versionCode;
        } catch (NameNotFoundException e) {
        }

        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                // Prior to Gingerbread, HttpUrlConnection was unreliable.
                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);
        // look here !!!
        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    }

See, it's the same as explicit creation.
Okay, then I'll explore the issues that started this article and keep track of the construction method of RequestQueue.

/**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize) {
        this(cache, network, threadPoolSize,
                new ExecutorDelivery(new Handler(Looper.getMainLooper())));
    }

Seeing this, I finally understand that it was Hadler who instantiated a main thread.
Take another look at the internal implementation of Executor Delivery:

/**
     * Creates a new response delivery interface.
     * @param handler {@link Handler} to post responses on
     */
    public ExecutorDelivery(final Handler handler) {
        // Make an Executor that just wraps the handler.
        mResponsePoster = new Executor() {
            @Override
            public void execute(Runnable command) {
                handler.post(command);
            }
        };
    }

RequestQueue starts work after calling start():

/**
     * Starts the dispatchers in this queue.
     */
    public void start() {
        stop();  // Make sure any currently running dispatchers are stopped.
        // Create the cache dispatcher and start it.
        mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
        mCacheDispatcher.start();

        // Create network dispatchers (and corresponding threads) up to the pool size.
        for (int i = 0; i < mDispatchers.length; i++) {
            NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
                    mCache, mDelivery);
            mDispatchers[i] = networkDispatcher;
            networkDispatcher.start();
        }
    }

Cache Dispatcher and Network Dispatcher are solid worker thread s that deliver responses to main thread s through mDelivery.

mDelivery.postResponse(request, response);

OK! This is the end of the article.