com.google.gson.stream
Class JsonReader

java.lang.Object
  extended by com.google.gson.stream.JsonReader
All Implemented Interfaces:
java.io.Closeable

public final class JsonReader
extends java.lang.Object
implements java.io.Closeable

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 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.

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 a JSON stream in Java?",
     "geo": null,
     "user": {
       "name": "json_newb",
       "followers_count": 41
      }
   },
   {
     "id": 912345678902,
     "text": "@json_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 List<Message> readJsonStream(InputStream in) throws IOException {
     JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8"));
     return readMessagesArray(reader);
   }

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

     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<Double> 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<Double> readDoublesArray(JsonReader reader) throws IOException {
     List<Double> doubles = new ArrayList<Double>();

     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.

Non-Execute Prefix

Web servers that serve private data using JSON may be vulnerable to
Cross-site request forgery attacks. In such an attack, a malicious site gains access to a private JSON file by executing it with an HTML <script> tag.

Prefixing JSON files with ")]}'\n" makes them non-executable by <script> tags, disarming the attack. Since the prefix is malformed JSON, strict parsing fails when it is encountered. This class permits the non-execute prefix when lenient parsing is enabled.

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

Since:
1.6
Author:
Jesse Wilson

Constructor Summary
JsonReader(java.io.Reader in)
          Creates a new instance that reads a JSON-encoded stream from in.
 
Method Summary
 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.
 java.lang.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.
 java.lang.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.
 java.lang.String toString()
           
 
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

JsonReader

public JsonReader(java.io.Reader in)
Creates a new instance that reads a JSON-encoded stream from in.

Method Detail

setLenient

public void setLenient(boolean lenient)
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:


isLenient

public boolean isLenient()
Returns true if this parser is liberal in what it accepts.


beginArray

public void beginArray()
                throws java.io.IOException
Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.

Throws:
java.io.IOException

endArray

public void endArray()
              throws java.io.IOException
Consumes the next token from the JSON stream and asserts that it is the end of the current array.

Throws:
java.io.IOException

beginObject

public void beginObject()
                 throws java.io.IOException
Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.

Throws:
java.io.IOException

endObject

public void endObject()
               throws java.io.IOException
Consumes the next token from the JSON stream and asserts that it is the end of the current array.

Throws:
java.io.IOException

hasNext

public boolean hasNext()
                throws java.io.IOException
Returns true if the current array or object has another element.

Throws:
java.io.IOException

peek

public JsonToken peek()
               throws java.io.IOException
Returns the type of the next token without consuming it.

Throws:
java.io.IOException

nextName

public java.lang.String nextName()
                          throws java.io.IOException
Returns the next token, a property name, and consumes it.

Throws:
java.io.IOException - if the next token in the stream is not a property name.

nextString

public java.lang.String nextString()
                            throws java.io.IOException
Returns the string value of the next token, consuming it. If the next token is a number, this method will return its string form.

Throws:
java.lang.IllegalStateException - if the next token is not a string or if this reader is closed.
java.io.IOException

nextBoolean

public boolean nextBoolean()
                    throws java.io.IOException
Returns the boolean value of the next token, consuming it.

Throws:
java.lang.IllegalStateException - if the next token is not a boolean or if this reader is closed.
java.io.IOException

nextNull

public void nextNull()
              throws java.io.IOException
Consumes the next token from the JSON stream and asserts that it is a literal null.

Throws:
java.lang.IllegalStateException - if the next token is not null or if this reader is closed.
java.io.IOException

nextDouble

public double nextDouble()
                  throws java.io.IOException
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.

Throws:
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 double, or is non-finite.
java.io.IOException

nextLong

public long nextLong()
              throws java.io.IOException
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.

Throws:
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 a long.
java.io.IOException

nextInt

public int nextInt()
            throws java.io.IOException
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.

Throws:
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.
java.io.IOException

close

public void close()
           throws java.io.IOException
Closes this JSON reader and the underlying Reader.

Specified by:
close in interface java.io.Closeable
Throws:
java.io.IOException

skipValue

public void skipValue()
               throws java.io.IOException
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:
java.io.IOException

toString

public java.lang.String toString()
Overrides:
toString in class java.lang.Object


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