Android Guides | Samples

Android.Util.JsonReader Class

Reads a JSON () encoded value as a stream of tokens.

See Also: JsonReader

Syntax

[Android.Runtime.Register("android/util/JsonReader", DoNotGenerateAcw=true)]
public sealed class JsonReader : Object, ICloseable, IDisposable

Remarks

Reads a JSON () 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 JsonReader.BeginArray to consume the array's opening bracket. Then create a while loop that accumulates values, terminating when JsonReader.HasNext is false. Finally, read the array's closing bracket by calling JsonReader.EndArray.
  • Within object handling methods, first call JsonReader.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 JsonReader.HasNext is false. Finally, read the object's closing brace by calling JsonReader.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 JsonReader.SkipValue to recursively skip the value's nested tokens, which may otherwise conflict.

If a value may be null, you should first check using JsonReader.Peek. Null literals can be consumed using either JsonReader.NextNull or JsonReader.SkipValue.

Example

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

java Example

[
   {
     "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
     }
   }
 ]}
This code implements the parser for the above structure:

java Example

public List readJsonStream(InputStream in) throws IOException {
     JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8"));
     try {
       return readMessagesArray(reader);
      finally {
       reader.close();
     }
   }

   public List readMessagesArray(JsonReader reader) throws IOException {
     List messages = new ArrayList();

     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 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") && 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 readDoublesArray(JsonReader reader) throws IOException {
     List doubles = new ArrayList();

     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);
   }}

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 JsonReader.NextInt or JsonReader.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.

[Android Documentation]

Requirements

Namespace: Android.Util
Assembly: Mono.Android (in Mono.Android.dll)
Assembly Versions: 0.0.0.0
Since: Added in API level 11

The members of Android.Util.JsonReader are listed below.

See Also: Object

Public Constructors

Creates a new instance that reads a JSON-encoded stream from in.

Public Properties

[read-only]
HasNextBoolean. Returns true if the current array or object has another element.
LenientBoolean. Returns true if this parser is liberal in what it accepts.

Protected Properties

[read-only]
override
ThresholdClassIntPtr. This API supports the Mono for Android infrastructure and is not intended to be used directly from your code.
[read-only]
override
ThresholdTypeType. This API supports the Mono for Android infrastructure and is not intended to be used directly from your code.

Public Methods

BeginArray()
Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.
BeginArrayAsync() : Task
BeginObject()
Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.
BeginObjectAsync() : Task
Close()
Closes this JSON reader and the underlying Reader.
EndArray()
Consumes the next token from the JSON stream and asserts that it is the end of the current array.
EndArrayAsync() : Task
EndObject()
Consumes the next token from the JSON stream and asserts that it is the end of the current array.
EndObjectAsync() : Task
NextBoolean() : Boolean
Returns the JsonToken.Boolean value of the next token, consuming it.
NextBooleanAsync() : System.Threading.Tasks.Task<bool>
NextDouble() : Double
Returns the JsonToken.Number value of the next token, consuming it.
NextDoubleAsync() : System.Threading.Tasks.Task<double>
NextInt() : Int32
Returns the JsonToken.Number value of the next token, consuming it.
NextIntAsync() : System.Threading.Tasks.Task<int>
NextLong() : Int64
Returns the JsonToken.Number value of the next token, consuming it.
NextLongAsync() : System.Threading.Tasks.Task<long>
NextName() : String
Returns the next token, a JsonToken.Name, and consumes it.
NextNameAsync() : System.Threading.Tasks.Task<string>
NextNull()
Consumes the next token from the JSON stream and asserts that it is a literal null.
NextNullAsync() : Task
NextString() : String
Returns the JsonToken.String value of the next token, consuming it.
NextStringAsync() : System.Threading.Tasks.Task<string>
Peek() : JsonToken
Returns the type of the next token without consuming it.
PeekAsync() : System.Threading.Tasks.Task<JsonToken>
SkipValue()
Skips the next value recursively.
SkipValueAsync() : Task