java - Android 字符串 if 语句

Question

我的应用程序开头有一个 if 语句

    if (ready.equals("yes")){
...
}

后来在我的代码中我有

ready="yes";

但是 if 语句从未被调用，为什么？ 准备="is"；从后台线程调用，这是为什么吗？

    public void DownloadFromUrl(final String fileName) {  //this is the downloader method
            new Thread(new Runnable() {
                public void run() {
        try {

            URL url = new URL("https://xxxxxxx");
            File file = new File(PATH + fileName);

            long startTime = System.currentTimeMillis();
            Log.d("ImageManager", "download begining");
            Log.d("ImageManager", "download url:" + url);
            Log.d("ImageManager", "downloaded file name:" + fileName);
                    /* Open a connection to that URL. */
            URLConnection ucon = url.openConnection();

                    /*
                     * Define InputStreams to read from the URLConnection.
                     */
            InputStream is = ucon.getInputStream();
            BufferedInputStream bis = new BufferedInputStream(is);

                    /*
                     * Read bytes to the Buffer until there is nothing more to read(-1).
                     */
            ByteArrayBuffer baf = new ByteArrayBuffer(50);
            int current = 0;
            while ((current = bis.read()) != -1) {
                baf.append((byte) current);
            }

                    /* Convert the Bytes read to a String. */
            FileOutputStream fos = new FileOutputStream(file);
            fos.write(baf.toByteArray());
            fos.close();
            Log.e("Ready or not", ready);
            ready="yes";
            Log.d("ImageManager", "download ready in"
                    + ((System.currentTimeMillis() - startTime) / 1000)
                    + " sec");
            Log.e("Ready or not", ready);



        } catch (IOException e) {
            Log.d("ImageManager", "Error: " + e);
        }

    }
    }).start();

Answer 1

如果我错了，请纠正我。如果您说您的代码如下所示:

new Thread(new Runnable() { public void run()
                            {
                                // thread code
                                if (ready.equals("yes")) {
                                    // handler code
                                }
                                // more thread code
                            }).start();

// later on...
ready = "yes";

你问为什么ready = "yes"在if (ready.equals("yes"))之前不执行，那么这是因为不能保证多个线程按特定顺序执行。如果你想等到ready.equals("yes")在执行 if 之前语句，那么你必须使用 Object.wait()和Object.notifyAll()方法:

// this is a field
private Object waitOnThis = new Object();

new Thread(new Runnable() { public void run()
                            {
                                // thread code
                                waitOnThis.wait(); // blocks until notify / notifyAll is called on waitOnThis
                                // by this point ready.equals("yes")
                                if (ready.equals("yes")) {
                                    // handler code
                                }
                                // more thread code
                            }).start();

// later on...
ready = "yes";
waitOnThis.notifyAll(); // unblocks threads waiting on waitOnThis

祝你好运!

编辑:请务必将上面的每个代码 fragment 包装在 synchronized (waitOnThis) 中。阻止，否则你会得到 IllegalMonitorStateException .