JsonReader

Kotlin |Java

class JsonReader : Closeable

kotlin.Any
↳	android.util.JsonReader

Reads a JSON (RFC 4627) encoded value as a stream of tokens. This stream includes both literal values (strings, numbers, booleans, and nulls) as well as the begin and end delimiters of objects and arrays. The tokens are traversed in depth-first order, the same order that they appear in the JSON document. Within JSON objects, name/value pairs are represented by a single token.

Parsing JSON

To create a recursive descent parser for your own JSON streams, first create an entry point method that creates a JsonReader.

Next, create handler methods for each structure in your JSON text. You'll need a method for each object type and for each array type.

Within array handling methods, first call beginArray to consume the array's opening bracket. Then create a while loop that accumulates values, terminating when hasNext is false. Finally, read the array's closing bracket by calling endArray.
Within object handling methods, first call beginObject to consume the object's opening brace. Then create a while loop that assigns values to local variables based on their name. This loop should terminate when hasNext is false. Finally, read the object's closing brace by calling endObject.

When a nested object or array is encountered, delegate to the corresponding handler method.

When an unknown name is encountered, strict parsers should fail with an exception. Lenient parsers should call skipValue() to recursively skip the value's nested tokens, which may otherwise conflict.

If a value may be null, you should first check using peek(). Null literals can be consumed using either nextNull() or skipValue().

Example

Suppose we'd like to parse a stream of messages such as the following:

<code>[
    {
      "id": 912345678901,
      "text": "How do I read JSON on Android?",
      "geo": null,
      "user": {
        "name": "android_newb",
        "followers_count": 41
       }
    },
    {
      "id": 912345678902,
      "text": "@android_newb just use android.util.JsonReader!",
      "geo": [50.454722, -104.606667],
      "user": {
        "name": "jesse",
        "followers_count": 2
      }
    }
  ]</code>

This code implements the parser for the above structure:

<code>public List&lt;Message&gt; readJsonStream(InputStream in) throws IOException {
      JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8"));
      try {
        return readMessagesArray(reader);
      } finally {
        reader.close();
      }
    }
 
    public List&lt;Message&gt; readMessagesArray(JsonReader reader) throws IOException {
      List&lt;Message&gt; messages = new ArrayList&lt;Message&gt;();
 
      reader.beginArray();
      while (reader.hasNext()) {
        messages.add(readMessage(reader));
      }
      reader.endArray();
      return messages;
    }
 
    public Message readMessage(JsonReader reader) throws IOException {
      long id = -1;
      String text = null;
      User user = null;
      List&lt;Double&gt; geo = null;
 
      reader.beginObject();
      while (reader.hasNext()) {
        String name = reader.nextName();
        if (name.equals("id")) {
          id = reader.nextLong();
        } else if (name.equals("text")) {
          text = reader.nextString();
        } else if (name.equals("geo") &amp;&amp; reader.peek() != JsonToken.NULL) {
          geo = readDoublesArray(reader);
        } else if (name.equals("user")) {
          user = readUser(reader);
        } else {
          reader.skipValue();
        }
      }
      reader.endObject();
      return new Message(id, text, user, geo);
    }
 
    public List&lt;Double&gt; readDoublesArray(JsonReader reader) throws IOException {
      List&lt;Double&gt; doubles = new ArrayList&lt;Double&gt;();
 
      reader.beginArray();
      while (reader.hasNext()) {
        doubles.add(reader.nextDouble());
      }
      reader.endArray();
      return doubles;
    }
 
    public User readUser(JsonReader reader) throws IOException {
      String username = null;
      int followersCount = -1;
 
      reader.beginObject();
      while (reader.hasNext()) {
        String name = reader.nextName();
        if (name.equals("name")) {
          username = reader.nextString();
        } else if (name.equals("followers_count")) {
          followersCount = reader.nextInt();
        } else {
          reader.skipValue();
        }
      }
      reader.endObject();
      return new User(username, followersCount);
    }</code>

Number Handling

This reader permits numeric values to be read as strings and string values to be read as numbers. For example, both elements of the JSON array [1, "1"] may be read using either nextInt or nextString. This behavior is intended to prevent lossy numeric conversions: double is JavaScript's only numeric type and very large values like 9007199254740993 cannot be represented exactly on that platform. To minimize precision loss, extremely large values should be written and read as strings in JSON.

Each JsonReader may be used to read a single JSON stream. Instances of this class are not thread safe.

Summary

Public constructors
`JsonReader(in: Reader!)` Creates a new instance that reads a JSON-encoded stream from `in`.

Public methods
Unit	`beginArray()` Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.
Unit	`beginObject()` Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.
Unit	`close()` Closes this JSON reader and the underlying `Reader`.
Unit	`endArray()` Consumes the next token from the JSON stream and asserts that it is the end of the current array.
Unit	`endObject()` Consumes the next token from the JSON stream and asserts that it is the end of the current object.
Boolean	`hasNext()` Returns true if the current array or object has another element.
Boolean	`isLenient()` Returns true if this parser is liberal in what it accepts.
Boolean	`nextBoolean()` Returns the `boolean` value of the next token, consuming it.
Double	`nextDouble()` Returns the `double` value of the next token, consuming it.
Int	`nextInt()` Returns the `int` value of the next token, consuming it.
Long	`nextLong()` Returns the `long` value of the next token, consuming it.
String!	`nextName()` Returns the next token, a `property name`, and consumes it.
Unit	`nextNull()` Consumes the next token from the JSON stream and asserts that it is a literal null.
String!	`nextString()` Returns the `string` value of the next token, consuming it.
JsonToken!	`peek()` Returns the type of the next token without consuming it.
Unit	`setLenient(lenient: Boolean)` Configure this parser to be be liberal in what it accepts.
Unit	`skipValue()` Skips the next value recursively.
String	`toString()`

Exceptions
`java.lang.Exception`	if this resource cannot be closed
`java.io.IOException`	if an I/O error occurs

Exceptions
`java.lang.IllegalStateException`	if the next token is not a literal value.
`java.lang.NumberFormatException`	if the next literal value cannot be parsed as a number, or exactly represented as an int.

JsonReader

Parsing JSON

Example

Number Handling

Summary

Public constructors

JsonReader

Public methods

beginArray

beginObject

close

endArray

endObject

hasNext

isLenient

nextBoolean

nextDouble

nextInt

nextLong

nextName

nextNull

nextString

peek

setLenient

skipValue

toString