下面(摘录之后)的一段简单的 midlet 代码(Moo 类)死锁(至少我认为它在阅读这篇关于线程的帖子后死锁了 here)。

我转载了帖子的相关摘录:

    String url = ...
    Connection conn = null;

    try {
        conn = Connector.open( url );
        // do something here
    }
    catch( IOException e ){
        // error
    }

The root of the problem is the blocking nature of the open() call. On some platforms, the system does the actual connection under the covers, on the equivalent of a separate thread. The calling thread blocks until the connection thread makes the connection. At the same time, the security subsystem may require the user to confirm the connection, and the connection thread blocks until the event thread gets confirmation from the user. Deadlock occurs because the event thread is already waiting for the connection thread.

public class Moo extends MIDlet {

    protected void destroyApp(boolean arg0) throws MIDletStateChangeException {
        // TODO Auto-generated method stub

    }

    protected void pauseApp() {
    }

    protected void startApp() throws MIDletStateChangeException {
        Display display = Display.getDisplay(this);
        MyCanvas myCanvas = new MyCanvas();
        display.setCurrent(myCanvas);
        myCanvas.repaint();

    }

    class MyCanvas extends Canvas {

        protected void paint(Graphics graphics) {
                try {
                        Image bgImage = Image.createImage(getWidth(), getHeight());

                        HttpConnection httpConnection = (HttpConnection) Connector
                                        .open("http://stackoverflow.com/content/img/so/logo.png");
                        Image image = Image.createImage(httpConnection
                                        .openInputStream());
                        bgImage.getGraphics().drawImage(image, 0, 0, 0);
                        httpConnection.close();

                        graphics.drawImage(bgImage, 0, 0, 0);
                } catch (IOException e) {
                        e.printStackTrace();
                }
        }

    }

}

Can someone please tell me how the system thread invocation is done here (event and notification threads) and the sequence of events leading to the deadlock. I am not clear as to what are the thread involved here that lead to deadlock.

1. Is there any documentation on j2me thread model?
2. Where can I get the sources for j2me system classes (I want to check out the implementation of Connection classes)?

EDIT : In the above code I get the logic. But the below code should at least work right? This one also deadlocks where I am doing the network connection in a separate thread.

public class Foo extends MIDlet {

    protected void destroyApp(boolean arg0) throws MIDletStateChangeException {
        // TODO Auto-generated method stub
    }

    protected void pauseApp() {
        // TODO Auto-generated method stub
    }

    protected void startApp() throws MIDletStateChangeException {
        Display display = Display.getDisplay(this);
        MyCanvas myCanvas = new MyCanvas();
        display.setCurrent(myCanvas);
        myCanvas.repaint();
    }

    class MyCanvas extends Canvas {
        protected void paint(Graphics graphics) {
            try {
                Image bgImage = Image.createImage(getWidth(), getHeight());

                FetchImage fetchImage = new FetchImage();
                Thread thread = new Thread(fetchImage);
                thread.start();

                thread.join();

                bgImage.getGraphics().drawImage(fetchImage.image, 0, 0, 0);

                graphics.drawImage(bgImage, 0, 0, 0);
            } catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    public class FetchImage implements Runnable {
        public Image image;

        public void run() {
            HttpConnection httpConnection;
            try {
                httpConnection = (HttpConnection) Connector
                        .open("http://10.4.71.200/stage/images/front/car.png");
                image = Image.createImage(httpConnection.openInputStream());
                httpConnection.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

    }
}

最佳答案

Where can I get the sources for j2me system classes (I want to check out the implementation of Connection classes)?

你不能。它实际上取决于供应商。诺基亚处理这种情况的方式可能与摩托罗拉不同。

您必须吸取的教训是，不要在系统回调中进行昂贵的计算，因为这可能会使系统无响应。因此，将耗时的操作放在单独的线程中，并始终尽可能早地从回调中返回。

即使您在第二个示例中创建了一个单独的线程，您在 paint() 中等待它完成，它最终导致根本没有线程!

你可以做的一件事是

class MyCanvas extends Canvas {

    Image image;
    boolean imageFetchFailed;

    protected void paint(Graphics g) {
        if (image == null) {
            fetchImage();
            g.drawString("Fetching...", getWidth() >> 1, getHeight() >> 1, Graphics.HCENTER | Graphics.TOP)

        } else if (imageFetchFailed) {
            g.drawString("Failed to fetch image", getWidth() >> 1, getHeight() >> 1, Graphics.HCENTER | Graphics.TOP)
        } else {
            g.drawImage(image, 0, 0, 0);
        }
    }


    private void fetchImage() {
        new Thread(new Runnable() {
            public void run() {
                HttpConnection httpConnection = null;
                try {
                    httpConnection = (HttpConnection) Connector
                            .open("http://10.4.71.200/stage/images/front/car.png");
                    image = Image.createImage(httpConnection.openInputStream());
                } catch (IOException e) {
                    e.printStackTrace();
                    imageFetchFailed = true;
                }

                if (httpConnection != null) {
                    try {
                        httpConnection.close();
                    } catch (IOException ignored) {
                    }
                }

                // Following will trigger a paint call 
                // and this time image wont be null and will get painted on screen
                repaint();    
            }
        }).start();
    }
}

关于java - j2me 网络、线程和死锁，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/867660/