Android APIs
public final class

JsonReader

extends Object
implements Closeable
java.lang.Object
   ↳ android.util.JsonReader

Class Overview

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:
 [
   {
     "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:
   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 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(Reader in)
Creates a new instance that reads a JSON-encoded stream from in.
Public Methods
void beginArray()
Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.
void beginObject()
Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.
void close()
Closes this JSON reader and the underlying Reader.
void endArray()
Consumes the next token from the JSON stream and asserts that it is the end of the current array.
void endObject()
Consumes the next token from the JSON stream and asserts that it is the end of the current array.
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.
void 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.
void setLenient(boolean lenient)
Configure this parser to be be liberal in what it accepts.
void skipValue()
Skips the next value recursively.
String toString()
Returns a string containing a concise, human-readable description of this object.
[Expand]
Inherited Methods
From class java.lang.Object
From interface java.io.Closeable
From interface java.lang.AutoCloseable

Public Constructors

public JsonReader (Reader in)

Added in API level 11

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

Parameters
in Reader

Public Methods

public void beginArray ()

Added in API level 11

Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.

Throws
IOException

public void beginObject ()

Added in API level 11

Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.

Throws
IOException

public void close ()

Added in API level 11

Closes this JSON reader and the underlying Reader.

Throws
IOException

public void endArray ()

Added in API level 11

Consumes the next token from the JSON stream and asserts that it is the end of the current array.

Throws
IOException

public void endObject ()

Added in API level 11

Consumes the next token from the JSON stream and asserts that it is the end of the current array.

Throws
IOException

public boolean hasNext ()

Added in API level 11

Returns true if the current array or object has another element.

Returns
boolean
Throws
IOException

public boolean isLenient ()

Added in API level 11

Returns true if this parser is liberal in what it accepts.

Returns
boolean

public boolean nextBoolean ()

Added in API level 11

Returns the boolean value of the next token, consuming it.

Returns
boolean
Throws
IllegalStateException if the next token is not a boolean or if this reader is closed.
IOException

public double nextDouble ()

Added in API level 11

Returns the double value of the next token, consuming it. If the next token is a string, this method will attempt to parse it as a double using parseDouble(String).

Returns
double
Throws
IllegalStateException if the next token is not a literal value.
IOException

public int nextInt ()

Added in API level 11

Returns the int value of the next token, consuming it. If the next token is a string, this method will attempt to parse it as an int. If the next token's numeric value cannot be exactly represented by a Java int, this method throws.

Returns
int
Throws
IllegalStateException if the next token is not a literal value.
NumberFormatException if the next literal value cannot be parsed as a number, or exactly represented as an int.
IOException

public long nextLong ()

Added in API level 11

Returns the long value of the next token, consuming it. If the next token is a string, this method will attempt to parse it as a long. If the next token's numeric value cannot be exactly represented by a Java long, this method throws.

Returns
long
Throws
IllegalStateException if the next token is not a literal value.
NumberFormatException if the next literal value cannot be parsed as a number, or exactly represented as a long.
IOException

public String nextName ()

Added in API level 11

Returns the next token, a property name, and consumes it.

Returns
String
Throws
IOException if the next token in the stream is not a property name.

public void nextNull ()

Added in API level 11

Consumes the next token from the JSON stream and asserts that it is a literal null.

Throws
IllegalStateException if the next token is not null or if this reader is closed.
IOException

public String nextString ()

Added in API level 11

Returns the string value of the next token, consuming it. If the next token is a number, this method will return its string form.

Returns
String
Throws
IllegalStateException if the next token is not a string or if this reader is closed.
IOException

public JsonToken peek ()

Added in API level 11

Returns the type of the next token without consuming it.

Returns
JsonToken
Throws
IOException

public void setLenient (boolean lenient)

Added in API level 11

Configure this parser to be be liberal in what it accepts. By default, this parser is strict and only accepts JSON as specified by RFC 4627. Setting the parser to lenient causes it to ignore the following syntax errors:

  • End of line comments starting with // or # and ending with a newline character.
  • C-style comments starting with /* and ending with */. Such comments may not be nested.
  • Names that are unquoted or 'single quoted'.
  • Strings that are unquoted or 'single quoted'.
  • Array elements separated by ; instead of ,.
  • Unnecessary array separators. These are interpreted as if null was the omitted value.
  • Names and values separated by = or => instead of :.
  • Name/value pairs separated by ; instead of ,.

Parameters
lenient boolean

public void skipValue ()

Added in API level 11

Skips the next value recursively. If it is an object or array, all nested elements are skipped. This method is intended for use when the JSON token stream contains unrecognized or unhandled values.

Throws
IOException

public String toString ()

Added in API level 11

Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression:

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

See Writing a useful toString method if you intend implementing your own toString method.

Returns
String a printable representation of this object.