如何解析JSON输入流

Question

我使用java来调用返回JSON对象的url:

url = new URL("my URl");
urlInputStream = url.openConnection().getInputStream();

如何将响应转换为字符串形式并解析它？

Answer 1

我建议您必须使用Reader来转换您的InputStream.

BufferedReader streamReader = new BufferedReader(new InputStreamReader(in, "UTF-8")); 
StringBuilder responseStrBuilder = new StringBuilder();

String inputStr;
while ((inputStr = streamReader.readLine()) != null)
    responseStrBuilder.append(inputStr);
new JSONObject(responseStrBuilder.toString());

我试过in.toString()但它返回:

getClass().getName() + '@' + Integer.toHexString(hashCode())

(就像文档说它从Object派生到toString)

Answer 2

所有当前的答案都假设可以将整个JSON拉入内存,其中InputStream的优点是您可以一点一点地读取输入.如果你想避免一次阅读整个Json文件,那么我建议使用Jackson库(这是我个人的最爱,但我确信像Gson这样的其他人也有类似的功能).

使用Jackson,您可以使用JsonParser一次读取一个部分.下面是我编写的代码示例,它包含了在Iterator中读取JsonObjects数组的内容.如果您只想查看Jackson的示例,请查看initJsonParser,initFirstElement和initNextObject方法.

public class JsonObjectIterator implements Iterator<Map<String, Object>>, Closeable {
    private static final Logger LOG = LoggerFactory.getLogger(JsonObjectIterator.class);

    private final InputStream inputStream;
    private JsonParser jsonParser;
    private boolean isInitialized;

    private Map<String, Object> nextObject;

    public JsonObjectIterator(final InputStream inputStream) {
        this.inputStream = inputStream;
        this.isInitialized = false;
        this.nextObject = null;
    }

    private void init() {
        this.initJsonParser();
        this.initFirstElement();
        this.isInitialized = true;
    }

    private void initJsonParser() {
        final ObjectMapper objectMapper = new ObjectMapper();
        final JsonFactory jsonFactory = objectMapper.getFactory();

        try {
            this.jsonParser = jsonFactory.createParser(inputStream);
        } catch (final IOException e) {
            LOG.error("There was a problem setting up the JsonParser: " + e.getMessage(), e);
            throw new RuntimeException("There was a problem setting up the JsonParser: " + e.getMessage(), e);
        }
    }

    private void initFirstElement() {
        try {
            // Check that the first element is the start of an array
            final JsonToken arrayStartToken = this.jsonParser.nextToken();
            if (arrayStartToken != JsonToken.START_ARRAY) {
                throw new IllegalStateException("The first element of the Json structure was expected to be a start array token, but it was: " + arrayStartToken);
            }

            // Initialize the first object
            this.initNextObject();
        } catch (final Exception e) {
            LOG.error("There was a problem initializing the first element of the Json Structure: " + e.getMessage(), e);
            throw new RuntimeException("There was a problem initializing the first element of the Json Structure: " + e.getMessage(), e);
        }

    }

    private void initNextObject() {
        try {
            final JsonToken nextToken = this.jsonParser.nextToken();

            // Check for the end of the array which will mean we're done
            if (nextToken == JsonToken.END_ARRAY) {
                this.nextObject = null;
                return;
            }

            // Make sure the next token is the start of an object
            if (nextToken != JsonToken.START_OBJECT) {
                throw new IllegalStateException("The next token of Json structure was expected to be a start object token, but it was: " + nextToken);
            }

            // Get the next product and make sure it's not null
            this.nextObject = this.jsonParser.readValueAs(new TypeReference<Map<String, Object>>() { });
            if (this.nextObject == null) {
                throw new IllegalStateException("The next parsed object of the Json structure was null");
            }
        } catch (final Exception e) {
            LOG.error("There was a problem initializing the next Object: " + e.getMessage(), e);
            throw new RuntimeException("There was a problem initializing the next Object: " + e.getMessage(), e);
        }
    }

    @Override
    public boolean hasNext() {
        if (!this.isInitialized) {
            this.init();
        }

        return this.nextObject != null;
    }

    @Override
    public Map<String, Object> next() {
        // This method will return the current object and initialize the next object so hasNext will always have knowledge of the current state

        // Makes sure we're initialized first
        if (!this.isInitialized) {
            this.init();
        }

        // Store the current next object for return
        final Map<String, Object> currentNextObject = this.nextObject;

        // Initialize the next object
        this.initNextObject();

        return currentNextObject;
    }

    @Override
    public void close() throws IOException {
        IOUtils.closeQuietly(this.jsonParser);
        IOUtils.closeQuietly(this.inputStream);
    }

}

如果您不关心内存使用情况,那么读取整个文件并将其解析为其他答案中提到的一个大Json肯定会更容易.

Answer 3

对于那些指出你不能像这样使用InputStream的toString方法的事实,请参阅/sf/answers/381161301/:

我的正确答案是:

import org.json.JSONObject;

public static String convertStreamToString(java.io.InputStream is) {
    java.util.Scanner s = new java.util.Scanner(is).useDelimiter("\\A");
    return s.hasNext() ? s.next() : "";
}

...

JSONObject json = new JSONObject(convertStreamToString(url.openStream());

Answer 4

使用jackson将json输入流转换为地图或对象http://jackson.codehaus.org/

还有一些其他有用的json库,你可以google:json java

Answer 5

如果您喜欢使用Jackson Databind（Spring默认情况下为其使用HttpMessageConverters），那么您可以使用ObjectMapper.readTree(InputStream) API。例如，

ObjectMapper mapper = new ObjectMapper();
JsonNode json = mapper.readTree(myInputStream);

Answer 6

使用图书馆。

• 格森
• 杰克逊
• 或许多其他 JSON 库之一。