检索大量数据时的Android OutOfMemoryError

Question

我有一个应用程序可以进行网络调用并检索 XML 数据。如果没有太多数据，下面的代码可以正常工作。

public class WebClient {

    private static final String TAG = WebClient.class.getSimpleName();
    private String result;


    private static String convertStreamToString(InputStream is) {
        /*
         * To convert the InputStream to String we use the
         * BufferedReader.readLine() method. We iterate until the BufferedReader
         * return null which means there's no more data to read. Each line will
         * appended to a StringBuilder and returned as String.
         */
        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
        StringBuilder sb = new StringBuilder();

        String line = null;
        try {
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return sb.toString();
    }


    public String connect(String url) {
        Log.e(TAG, "inside LoginWebClient.connect(url). url = " + url);

        result = null;

        HttpClient httpclient = new DefaultHttpClient();

        // Prepare a request object
        HttpGet httpget = new HttpGet(url);


        // Execute the request
        HttpResponse response;
        try { 
            response = httpclient.execute(httpget);
            // Examine the response status
            Log.i(TAG, response.getStatusLine().toString());





            // Get hold of the response entity
            HttpEntity entity = response.getEntity();
            // If the response does not enclose an entity, there is no need
            // to worry about connection release

            if (entity != null) {


                InputStream instream = entity.getContent();
                result = convertStreamToString(instream);
                //Log.i(TAG, result);



                // Closing the input stream will trigger connection release
                instream.close();

            }

                } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        Log.e(TAG, "inside WebClient.connect(url). result = " + result);

         return result;

    }//end of connect

.

如果应用进行网络调用并返回大量数据，我会收到 OutOfMemoryError。我用谷歌搜索了这个错误，它可能是由于 StringBuilder 的内存中保存了太多数据造成的。

作为我读过的解决方法，我可以将 Stream 直接读入 String 以避免 StringBuilder。

我将 com.google.common.io.CharStreams jar 文件导入到我的项目中并尝试了以下操作:

private static String convertStreamToStringForGetAlerts(InputStream is) {

        String stringFromStream = null;


        try {
            stringFromStream = CharStreams.toString(new InputStreamReader(is, "UTF-8"));
        } catch (UnsupportedEncodingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return stringFromStream;
    }

但我仍然收到以下错误。

06-05 09:44:40.809: E/AndroidRuntime(5851): FATAL EXCEPTION: AsyncTask #2
06-05 09:44:40.809: E/AndroidRuntime(5851): java.lang.RuntimeException: An error occured while executing doInBackground()
06-05 09:44:40.809: E/AndroidRuntime(5851):     at android.os.AsyncTask$3.done(AsyncTask.java:278)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.FutureTask.run(FutureTask.java:137)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.lang.Thread.run(Thread.java:856)
06-05 09:44:40.809: E/AndroidRuntime(5851): Caused by: java.lang.OutOfMemoryError
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.lang.AbstractStringBuilder.enlargeBuffer(AbstractStringBuilder.java:94)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.lang.AbstractStringBuilder.append0(AbstractStringBuilder.java:162)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.lang.StringBuilder.append(StringBuilder.java:311)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.lang.StringBuilder.append(StringBuilder.java:44)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.google.common.io.CharStreams.copy(CharStreams.java:204)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.google.common.io.CharStreams.toStringBuilder(CharStreams.java:245)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.google.common.io.CharStreams.toString(CharStreams.java:219)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.carefreegroup.rr3.carefreeoncall.WebClient.convertStreamToStringForGetAlerts(WebClient.java:151)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.carefreegroup.rr3.carefreeoncall.WebClient.connectForGetAlerts(WebClient.java:198)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.carefreegroup.rr3.carefreeoncall.WebService.getAlerts(WebService.java:1778)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.carefreegroup.rr3.carefreeoncall.ShowAlertsActivity$AsyncGetAlerts.doInBackground(ShowAlertsActivity.java:88)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at com.carefreegroup.rr3.carefreeoncall.ShowAlertsActivity$AsyncGetAlerts.doInBackground(ShowAlertsActivity.java:1)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at android.os.AsyncTask$2.call(AsyncTask.java:264)
06-05 09:44:40.809: E/AndroidRuntime(5851):     at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
06-05 09:44:40.809: E/AndroidRuntime(5851):     ... 4 more

有没有人知道如何在不导致此错误的情况下将此流读入字符串？

提前致谢。

Answer 1

As a workaround i have read i can read the Stream straight into a String to avoid the StringBuilder.

这没用。在幕后，CharStreams 方法最有可能使用 StringBuilder 或等效的...并且由于它不知道构建器需要多大，因此它将使用构建器用于扩展后备阵列的“双倍大小”策略。

将数据读入内存的最有效内存方式是预分配某种缓冲区(StringBuilder、byte[]、char[ ] 等)的大小足以容纳字符/字节，然后填充缓冲区。如果您需要将缓冲区内容转换为字符串，您仍然需要两倍的内存量。

但这只是避免了不可避免的缩放问题。最终，XML 将太大而无法保存在内存中。您真正需要做的是更改您的代码，不要试图将整个 XML 文档保存在内存中。您可以执行以下操作之一:

• 将其提供给事件驱动的 XML 解析器(例如 SAX)以提取感兴趣的信息，或者
• 一次写入一个本地文件，一个字节，一个字符，一行或一个缓冲区。