如何从另一个线程返回字符串？

Question

I want to return a String "finalurl" from another thread, like this:

public String getImageURL(String url) {

    final String finalUrl = url;

    Thread t1 = new Thread(new Runnable() {
        public void run() {
            Document doc = null;
            try {
                doc = Jsoup.connect(finalUrl).get();
            } catch (IOException e) {
                e.printStackTrace();
            }
            Element masthead = doc.select("div.post-image").select("img[src~=(?i)\\.(png|jpe?g)]").first();
            finalUrl = masthead.absUrl("src");
        }
    });

    t1.start();
    return finalUrl;
}

But it does not work. How to make it correctly?

Answer 1

 return finalUrl;

This code will never return string, that you want.

You need to fix your logic by one of a lot techiques. The easiest way for you is using handler. Like this:

private static final int URL_MESSAGE = 1;

private final Handler handler = new Handler(Looper.getMainLooper()) {

    @Override
    public void handleMessage(Message message) {
        super.handleMessage(message);
        if (message.arg1 == URL_MESSAGE) {
            final Object obj = message.obj;

            if (obj instanceof String) {
                String url = (String) obj;
            }
        }
    }
};

private final Runnable runnable = new Runnable() {
    public void run() {
        Document doc = null;
        try {
            doc = Jsoup.connect(finalUrl).get();
        } catch (IOException e) {
            e.printStackTrace();
        }
        Element masthead = doc.select("div.post-image").select("img[src~=(?i)\\.(png|jpe?g)]").first();
        String finalUrl = masthead.absUrl("src");

        Message message = new Message();
        message.arg1 = URL_MESSAGE;
        message.obj = finalUrl;
        handler.sendMessage(message);
    }
};

@Override
protected void onDestroy() {
    super.onDestroy();
    handler.removeCallbacks(runnable);
}

private void execute() {
    handler.post(runnable);
}

Answer 2

For such need what you should do is rather:

1. Create a thread pool
2. Submit your task as a Callable to the thread pool
3. Get the result

Create your thread pool using Executors like this for example:

// Create a thread pool composed of only four threads in this case
private final ExecutorService executor = Executors.newFixedThreadPool(4);

Submit your task as a Callable

final Future<String> future = executor.submit(new Callable<String>() {
    @Override
    public String call() throws Exception {
        Document doc = Jsoup.connect(url).get();
        Element masthead = doc.select("div.post-image")
            .select("img[src~=(?i)\\.(png|jpe?g)]").first();
        return masthead.absUrl("src");
    }
});

Get the result

try {
    return future.get();
} catch (InterruptedException e) {
    Thread.currentThread().interrupt();
    throw new RuntimeException(e);
} catch (ExecutionException e) {
    throw new RuntimeException(e);
}

NB: The thread pool must not be created at each call, it must be created once for all in your class in order to reuse it at each call.

Answer 3

You haven't made it exactly clear what you are trying to do, but I'm pretty sure you wouldn't want your getImageURL(url) to return until it's got a url.

The right way to do that is to not use a thread .

public String getImageURL(String url) {
    Document doc = null;
    try {
        doc = Jsoup.connect(finalUrl).get();
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
    Element masthead = doc.select("div.post-image").select("img[src~=(?i)\\.(png|jpe?g)]").first();
    return masthead.absUrl("src");
}

If there's some reason why you think you need to use a thread here, then you should ask again in another question, and explain in more detail what you're trying to do and why you want to do it.

---

Error says: "06-18 21:22:12.701 7816-7816/pl.dariusz.app I/Choreographer: Skipped 39 frames! The application may be doing too much work on its main thread."

OK, that error doesn't literally mean that the application did too much work in one thread. It means that the GUI called one of your handler functions, and too much time elapsed before the handler returned.

If your event handler functions don't promptly return, it will prevent your application from promptly responding to whatever the user does next. That's what the error message really is trying to tell you.

You can't fix it by doing the "work" in some other thread if your handler still has to wait for the work to be done before it returns. It's the time that matters.

The way to fix that kind of problem is:

• Your handler puts the GUI in to some kind of state that tells the user, I'm waiting for whatever,

• Then your handler kicks off a background task that will wait for whatever, (Note: I'm not an Android programmer, but I'm pretty certain that there's a better way to kick off the task than creating a new, naked Thread object.)

• Then your handler returns,

• Your background task runs in some other thread doing something like Jsoup.connect(finalUrl).get();

• Meanwhile, if the user clicks on something, your application can still respond to the user input, even if it's only to pop up a message that says, "not now, I'm still waiting for whatever."

• Finally, your background task completes, and it posts and event to the application's event queue (I'm still not an Android programmer, so I don't actually know what method you call to do that) that will update the UI state to show that whatever finally has happened.