com.google.gson.stream

Class JsonReader

java.lang.Object

com.google.gson.stream.JsonReader

All Implemented Interfaces:

Closeable, AutoCloseable

public class JsonReader
extends Object
implements 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 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 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"));
     try {
       return readMessagesArray(reader);
     } finally {
       reader.close();
     }
   }

   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

Constructors

Constructor and Description
JsonReader(Reader in) Creates a new instance that reads a JSON-encoded stream from in.

Method Summary

Methods

Modifier and Type	Method and Description
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 object.
String	getPath() Returns a JsonPath to the current location in the JSON value.
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()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

JsonReader

public JsonReader(Reader in)

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

Method Detail

setLenient

public final 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:

• Streams that start with the non-execute prefix, ")]}'\n".
• Streams that include multiple top-level values. With strict parsing, each stream must contain exactly one top-level value.
• Top-level values of any type. With strict parsing, the top-level value must be an object or an array.
• Numbers may be NaNs or infinities.
• 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 ,.

isLenient

public final boolean isLenient()

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

beginArray

public void beginArray()
                throws IOException

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

Throws:

IOException

endArray

public void endArray()
              throws IOException

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

Throws:

IOException

beginObject

public void beginObject()
                 throws IOException

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

Throws:

IOException

endObject

public void endObject()
               throws IOException

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

Throws:

IOException

hasNext

public boolean hasNext()
                throws IOException

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

Throws:

IOException

peek

public JsonToken peek()
               throws IOException

Returns the type of the next token without consuming it.

Throws:

IOException

nextName

public String nextName()
                throws IOException

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

Throws:

IOException - if the next token in the stream is not a property name.

nextString

public String nextString()
                  throws 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:

IllegalStateException - if the next token is not a string or if this reader is closed.

IOException

nextBoolean

public boolean nextBoolean()
                    throws IOException

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

Throws:

IllegalStateException - if the next token is not a boolean or if this reader is closed.

IOException

nextNull

public void nextNull()
              throws IOException

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

nextDouble

public double nextDouble()
                  throws 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 using Double.parseDouble(String).

Throws:

IllegalStateException - if the next token is not a literal value.

NumberFormatException - if the next literal value cannot be parsed as a double, or is non-finite.

IOException

nextLong

public long nextLong()
              throws 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:

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

nextInt

public int nextInt()
            throws 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:

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

close

public void close()
           throws IOException

Closes this JSON reader and the underlying Reader.

Specified by:

close in interface Closeable

Specified by:

close in interface AutoCloseable

Throws:

IOException

skipValue

public void skipValue()
               throws 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:

IOException

toString

public String toString()

Overrides:

toString in class Object

getPath

public String getPath()

Returns a JsonPath to the current location in the JSON value.