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 ?",
     "geo": null,
     "user": {
       "name": "_newb",
       "followers_count": 41
   	}
   },
   {
     "id": 912345678902,
     "text": "@_newb just use JSONReader!",
     "geo": [50.454722, -104.606667],
     "user": {
       "name": "jesse",
       "followers_count": 2
   	}
   }
 ]
 

This code implements the parser for the above structure:

   public Vector readJsonStream(InputStream in) throws IOException {
     JSONReader reader = new JSONReader(new InputStreamReader(in, "UTF-8"));
     try {
       return readMessagesArray(reader);
     } finally {
       reader.close();
     }
   }
 
   public Vector readMessagesArray(JSONReader reader) throws IOException {
     Vector messages = new Vector();
 
     reader.beginArray();
     while (reader.hasNext()) {
       messages.addElement(readMessage(reader));
     }
     reader.endArray();
     return messages;
   }
 
   public Message readMessage(JSONReader reader) throws IOException {
     long id = -1;
     String text = null;
     User user = null;
     Vector 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 Vector readDoublesArray(JSONReader reader) throws IOException {
     Vector doubles = new Vector();
 
     reader.beginArray();
     while (reader.hasNext()) {
       doubles.addElement(new Double(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.
	JSONReader(String 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.
int	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 representation of the object.

[Expand] Inherited Methods

From class java.lang.Object boolean equals(Object obj) Indicates whether some other object is "equal to" this one. final Class<?> getClass() Returns the runtime class of an object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeout, int nanos) Causes current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed. final void wait(long timeout) Causes current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed. final void wait() Causes current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.