Question

我想知道是否有正确的＆＃34;使用Jackson解析JSON文件的方法，其中JSON文件包含一个巨大的属性而不将整个流加载到内存中。我需要保持低内存，因为它是一个Android应用程序。我不是在问这里如何Android: Parsing large JSON file而是一个属性真的很大而其他属性并不重要。

例如，让我说我有以下内容：

{
    "filename": "afilename.jpg",
    "data": "**Huge data here, about 20Mb base64 string**",
    "mime": "mimeType",
    "otherProperties": "..."
}

如果需要，可以将数据属性提取到新文件（通过输出流或其他含义）但我无法使用Jackson实现此目的。开放使用其他库我只是认为杰克逊是理想的感谢它的流媒体API。

由于

Answer 1

最后我设法恢复我的大量数据，其中in是json文件的输入流，我想解析数据，out是我要写入数据的文件：

public boolean extrationContenuDocument(FileInputStream in, FileOutputStream out, FileInfo info) 
throws JsonParseException, IOException {

    SerializedString keyDocContent = new SerializedString("data");
    boolean isDone = false;

    JsonParser jp = this.jsonFactory.createJsonParser(in);

    // Let's move our inputstream cursor until the 'data' property is found
    while (!jp.nextFieldName(keyDocContent)) {
        Log.v("Traitement JSON", "Searching for 'data' property ...");
    }

    // Found it? Ok, move the inputstream cursor until the begining of it's
    // content
    JsonToken current = jp.nextToken();

    // if the current token is not String value it means u didn't found the
    // 'data' property or it's content is not a correct => stop
    if (current == JsonToken.VALUE_STRING) {
        Log.v("Traitement JSON", "Property 'data' found");

        // Here it gets a little tricky cause if the file is not big enough
        // all the content of the 'data' property could be read directly
        // insted of using this
        if (info.getSize() > TAILLE_MIN_PETIT_FICHER) {
            Log.v("Traitement JSON", "the content of 'data' is too big to be read directly -> using buffered reading");

            // JsonParser uses a buffer to read, there is some data that
            // could have been read by it, i need to fetch it
            ByteArrayOutputStream debutDocStream = new ByteArrayOutputStream();
            int premierePartieRead = jp.releaseBuffered(debutDocStream);
            byte[] debutDoc = debutDocStream.toByteArray();

            // Write the head of the content of the 'data' property, this is
            // actually what as read from the inputstream by the JsonParser
            // when did jp.nextToken()
            Log.v("Traitement JSON", "Write the head");
            out.write(debutDoc);

            // Now we need to write the rest until we find the tail of the
            // content of the 'data' property
            Log.v("Traitement JSON", "Write the middle");

            // So i prepare a buffer to continue reading the inputstream
            byte[] buffer = new byte[TAILLE_BUFFER_GROS_FICHER];

            // The escape char that determines where to stop reading will be "
            byte endChar = (byte) '"';

            // Fetch me some bytes from the inputstream
            int bytesRead = in.read(buffer);
            int bytesBeforeEndChar = 0;

            int deuxiemePartieRead = 0;
            boolean isDocContentFin = false;

            // Are we at the end of the 'data' property? Keep writing the
            // content of the 'data' property if it's not the case
            while ((bytesRead > 0) && !isDocContentFin) {
                bytesBeforeEndChar = 0;

                // Since am using a buffer the escape char could be in the
                // middle of it, gotta look if it is
                for (byte b : buffer) {
                    if (b != endChar) {
                        bytesBeforeEndChar++;
                    } else {
                        isDocContentFin = true;
                        break;
                    }
                }

                if (bytesRead > bytesBeforeEndChar) {
                    Log.v("Traitement JSON", "Write the tail");
                    out.write(buffer, 0, bytesBeforeEndChar);
                    deuxiemePartieRead += bytesBeforeEndChar;
                } else {
                    out.write(buffer, 0, bytesRead);
                    deuxiemePartieRead += bytesRead;
                }

                bytesRead = in.read(buffer);
            }

            Log.v("Traitement JSON", "Bytes read: " + (premierePartieRead + deuxiemePartieRead) + " (" + premierePartieRead + " head,"
                    + deuxiemePartieRead + " tail)");
            isDone = true;
        } else {
            Log.v("Traitement JSON", "File is small enough to be read directly");
            String contenuFichier = jp.getText();
            out.write(contenuFichier.getBytes());
            isDone = true;
        }
    } else {
        throw new JsonParseException("The property " + keyDocContent.getValue() + " couldn't be found in the Json Stream.", null);
    }
    jp.close();

    return isDone;
}

它不漂亮，但却像魅力一样！ @staxman让我知道你的想法。

编辑：

现在这是一项已实施的功能，请参阅：https://github.com/FasterXML/jackson-core/issues/14 和JsonParser.readBinaryValue()

Answer 2

编辑：对于这个问题，这不是一个好的答案 - 如果子树是要绑定的对象，它会起作用，但是当问题是单个大的Base64编码的字符串时不行。< / p>

如果我正确理解了这个问题，是的，如果你的输入包含一系列JSON对象或数组，你可以逐步读取文件但仍然是数据绑定。

如果是这样，您可以使用JsonParser将流提前指向第一个对象（其START_OBJECT标记），然后在JsonParser（JsonParser.readValueAs()）中使用数据绑定方法或ObjectMapper（ObjectMapper.readValue(JsonParser, type)）。

类似的东西：

ObjectMapper mapper = new ObjectMapper();
JsonParser jp = mapper.getJsonFactory().createJsonParser(new File("file.json"));
while (jp.nextToken() != null) {
   MyPojo pojo = jp.readValueAs(MyPojo.class);
   // do something
}

（注意：根据JSON的确切结构，您可能需要跳过一些元素 - 在调用readValueAs（）时，解析器必须已经收到启动JSON Object以绑定的START_ELEMENT。）

或者，甚至更简单，您可以在readValues中使用方法ObjectReader：

ObjectReader r = mapper.reader(MyPojo.class);
MappingIterator<MyPojo> it = r.readValues(new File("file.json"));
while (it.hasNextValue()) {
   MyPojo pojo. = it.nextValue();
  // do something with it
}

在这两种情况下，Jackson数据绑定器只读取生成单个Object（MyPojo或您拥有的任何类型）所需的JSON令牌。 JsonParser本身只需要足够的内存来包含单个JSON令牌的信息。

Android：使用内存使用率较低的大属性解析JSON文件

2 个答案:

编辑：