Android 使用 JSON 从服务器下载大量数据

Question

我目前希望通过 PHP 捕获打印数据从远程服务器传输数据，然后将其存储到设备的 SQLite 数据库中。

问题是有时我需要从一个表中接收超过 6000 行，将所有数据存储在设备的 RAM 内存（变量）中并不有效，所以我想知道是否有人请告诉我如何将传入的数据保存到设备的内存（sd、资产或资源）中，并在处理到 SQLite 数据库后将其删除。我目前用来接收数据的方法是这样的：

public JSONArray requestTable(List<NameValuePair> nameValuePairs) throws     ClientProtocolException, IOException, JSONException, UnknownHostException, ConnectTimeoutException
{
  //NameValuePairs contains the Query so it can be catched by php's $_POST

       HttpClient httpclient = new DefaultHttpClient();
   HttpPost httppost = new HttpPost(http://something.php);
   HttpParams httpparams = new BasicHttpParams();

   HttpConnectionParams.setConnectionTimeout(httpparams, msTimeout);
   JSONArray tableData = null;
       httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs, HTTP.UTF_8));

   HttpResponse httpresponse = httpclient.execute(httppost);

       Log.i("How are you?", httpresponse.getStatusLine().toString());

       HttpEntity entity = httpresponse.getEntity();


    if (entity != null)
    {
        InputStream instream = entity.getContent();
        String result= convertStreamToString(instream);
            if(result.charAt(0) == '<')
                Log.e("php error", result);
            else
                tableData = new JSONArray(result);

        instream.close();
        httpresponse = null;
        return tableData;
    }

    else
        return null;



private static String convertStreamToString(InputStream is) {

    BufferedReader reader = new BufferedReader(new InputStreamReader(is), 16 * 1024);
    StringBuilder sb = new StringBuilder();

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

    return sb.toString();
}

通过使用这种方法，我得到了这个错误：

01-22 17:43:54.098: INFO/dalvikvm-heap(380): Grow heap (frag case) to 18.874MB for 2834992-byte allocation
01-22 17:44:06.368: INFO/dalvikvm-heap(380): Forcing collection of SoftReferences for 4252484-byte allocation

01-22 17:44:08.490: ERROR/dalvikvm-heap(380): Out of memory on a 4252484-byte allocation.
01-22 17:44:08.490: INFO/dalvikvm(380): "Thread-8" prio=5 tid=7 RUNNABLE
01-22 17:44:08.490: INFO/dalvikvm(380):   | group="main" sCount=0 dsCount=0 s=N obj=0x44f182d8 self=0x118bf8
01-22 17:44:08.490: INFO/dalvikvm(380):   | sysTid=389 nice=0 sched=0/0 cgrp=default handle=2525744
01-22 17:44:08.498: INFO/dalvikvm(380):   | schedstat=( 328938307540 280414788056 42400 )
01-22 17:44:08.498: INFO/dalvikvm(380):   at java.lang.AbstractStringBuilder.enlargeBuffer(AbstractStringBuilder.java:~97)
01-22 17:44:08.498: INFO/dalvikvm(380):   at java.lang.AbstractStringBuilder.append0(AbstractStringBuilder.java:136)
01-22 17:44:08.498: INFO/dalvikvm(380):   at  java.lang.StringBuilder.append(StringBuilder.java:272)
01-22 17:44:08.498: INFO/dalvikvm(380):   at java.io.BufferedReader.readLine(BufferedReader.java:452)
01-22 17:44:08.498: INFO/dalvikvm(380):   at srdroid.cata.Server.convertStreamToString(Server.java:628)  
01-22 17:44:08.498: INFO/dalvikvm(380):   at srdroid.cata.Server.requestTable(Server.java:226)
01-22 17:44:08.498: INFO/dalvikvm(380):   at      srdroid.cata.Server$2.run(Server.java:418)
01-22 17:44:08.498: INFO/dalvikvm(380):   at java.lang.Thread.run(Thread.java:1096)

抱歉发了这么长的帖子，任何帮助将不胜感激！

Answer 1

查看 Google 的 GSON Library，尤其是 JsonReader 类。

您可以使用它逐个元素地“提取” JSON 数据，而不是将整个文档读入对象结构（正如您所说的那样，这对内存有很大影响）。然后，您可以一次读取一行 JSON 数据并将其写入数据库。

JsonReader 实际上是 API 级别 11 的 Android 的一部分。如果您要针对早期版本的 Android 进行编译，则可以使用 GSON stream jar。