java.lang.Object

org.dominokit.jacksonapt.stream.impl.DefaultJsonReader

All Implemented Interfaces:: JsonReader

public class DefaultJsonReader
extends java.lang.Object
implements JsonReader

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:

   
 <p>
   public List<Message> readJsonStream(InputStream in) {
     JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8"));
     try {
       return readMessagesArray(reader);
     } finally {
       reader.close();
     }
   }
 <p>
   public List<Message> readMessagesArray(JsonReader reader) {
     List<Message> messages = new ArrayList<Message>();
 <p>
     reader.beginArray();
     while (reader.hasNext()) {
       messages.add(readMessage(reader));
     }
     reader.endArray();
     return messages;
   }
 <p>
   public Message readMessage(JsonReader reader) {
     long id = -1;
     String text = null;
     User user = null;
     List<Double> geo = null;
 <p>
     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);
   }
 <p>
   public List<Double> readDoublesArray(JsonReader reader) {
     List<Double> doubles = new ArrayList<Double>();
 <p>
     reader.beginArray();
     while (reader.hasNext()) {
       doubles.add(reader.nextDouble());
     }
     reader.endArray();
     return doubles;
   }
 <p>
   public User readUser(JsonReader reader) {
     String username = null;
     int followersCount = -1;
 <p>
     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
Version:: $Id: $
Author:: Jesse Wilson

Constructor Summary

Constructors

Constructor Description

DefaultJsonReader(StringReader in)
Creates a new instance that reads a JSON-encoded stream from in.

Method Summary

Modifier and Type	Method	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.
`int`	`getColumnNumber()`	getColumnNumber
`java.lang.String`	`getInput()`	getInput
`int`	`getLineNumber()`	getLineNumber
`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.Number`	`nextNumber()`	Returns the `Number` value of the next token, consuming it.
`java.lang.String`	`nextString()`	Returns the `string` value of the next token, consuming it.
`java.lang.String`	`nextValue()`	Reads the next value recursively and returns it as a String.
`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

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

Constructor Details
- DefaultJsonReader
  
  public DefaultJsonReader(StringReader in)
  
  Creates a new instance that reads a JSON-encoded stream from in.
  
  Parameters:
  
  in - a StringReader object.
Method Details
- 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 ,.
  Specified by:
  
  setLenient in interface JsonReader
  
  Parameters:
  
  lenient - a boolean.
- isLenient
  
  public final boolean isLenient()
  
  Returns true if this parser is liberal in what it accepts.
  
  Returns:
  
  a boolean.
- beginArray
  
  public void beginArray()
  
  Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.
  
  Specified by:
  
  beginArray in interface JsonReader
- endArray
  
  public void endArray()
  
  Consumes the next token from the JSON stream and asserts that it is the end of the current array.
  
  Specified by:
  
  endArray in interface JsonReader
- beginObject
  
  public void beginObject()
  
  Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.
  
  Specified by:
  
  beginObject in interface JsonReader
- endObject
  
  public void endObject()
  
  Consumes the next token from the JSON stream and asserts that it is the end of the current object.
  
  Specified by:
  
  endObject in interface JsonReader
- hasNext
  
  public boolean hasNext()
  
  Returns true if the current array or object has another element.
  
  Specified by:
  
  hasNext in interface JsonReader
  
  Returns:
  
  a boolean.
- peek
  
  public JsonToken peek()
  
  Returns the type of the next token without consuming it.
  
  Specified by:
  
  peek in interface JsonReader
  
  Returns:
  
  a JsonToken object.
- nextName
  
  public java.lang.String nextName()
  
  Returns the next token, a property name, and consumes it.
  
  Specified by:
  
  nextName in interface JsonReader
  
  Returns:
  
  a String object.
- nextString
  
  public java.lang.String nextString()
  
  Returns the string value of the next token, consuming it. If the next token is a number, this method will return its string form.
  
  Specified by:
  
  nextString in interface JsonReader
  
  Returns:
  
  a String object.
- nextBoolean
  
  public boolean nextBoolean()
  
  Returns the boolean value of the next token, consuming it.
  
  Specified by:
  
  nextBoolean in interface JsonReader
  
  Returns:
  
  a boolean.
- nextNull
  
  public void nextNull()
  
  Consumes the next token from the JSON stream and asserts that it is a literal null.
  
  Specified by:
  
  nextNull in interface JsonReader
- nextDouble
  
  public double nextDouble()
  
  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).
  
  Specified by:
  
  nextDouble in interface JsonReader
  
  Returns:
  
  a double.
- nextLong
  
  public long nextLong()
  
  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.
  
  Specified by:
  
  nextLong in interface JsonReader
  
  Returns:
  
  a long.
- nextInt
  
  public int nextInt()
  
  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.
  
  Specified by:
  
  nextInt in interface JsonReader
  
  Returns:
  
  a int.
- close
  
  public void close()
  
  Closes this JSON reader and the underlying Reader.
  
  Specified by:
  
  close in interface JsonReader
- skipValue
  
  public void skipValue()
  
  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.
  
  Specified by:
  
  skipValue in interface JsonReader
- getLineNumber
  
  public int getLineNumber()
  
  getLineNumber
  
  Specified by:
  
  getLineNumber in interface JsonReader
  
  Returns:
  
  a int.
- getColumnNumber
  
  public int getColumnNumber()
  
  getColumnNumber
  
  Specified by:
  
  getColumnNumber in interface JsonReader
  
  Returns:
  
  a int.
- toString
  
  public java.lang.String toString()
  
  Overrides:
  
  toString in class java.lang.Object
- getInput
  
  public java.lang.String getInput()
  
  getInput
  
  Specified by:
  
  getInput in interface JsonReader
  
  Returns:
  
  a String object.
- nextValue
  
  public java.lang.String nextValue()
  
  Reads the next value recursively and returns it as a String. If it is an object or array, all nested elements are read.
  
  Specified by:
  
  nextValue in interface JsonReader
  
  Returns:
  
  a String object.
- nextNumber
  
  public java.lang.Number nextNumber()
  
  Returns the Number value of the next token, consuming it. This method will attempt to return the best matching number. For non-decimal number, if it fits into an int, an int is returned, else a long else a BigInteger. For decimal number, a double is returned. If the next token's numeric value cannot be exactly represented by a Java Number, this method throws.
  
  Specified by:
  
  nextNumber in interface JsonReader
  
  Returns:
  
  a Number object.

Class DefaultJsonReader

Parsing JSON

Example

Number Handling

Non-Execute Prefix

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Details

DefaultJsonReader

Method Details

setLenient

isLenient

beginArray

endArray

beginObject

endObject

hasNext

peek

nextName

nextString

nextBoolean

nextNull

nextDouble

nextLong

nextInt

close

skipValue

getLineNumber

getColumnNumber

toString

getInput

nextValue

nextNumber