com.google.gson.stream

Class JsonReader

  • All Implemented Interfaces:
    Closeable, AutoCloseable


    public class JsonReaderextends Objectimplements Closeable
    Reads a JSON (RFC 7159) 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

      All Methods Instance Methods Concrete Methods 
      Modifier and TypeMethod and Description
      voidbeginArray()
      Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.
      voidbeginObject()
      Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.
      voidclose()
      Closes this JSON reader and the underlying Reader.
      voidendArray()
      Consumes the next token from the JSON stream and asserts that it is the end of the current array.
      voidendObject()
      Consumes the next token from the JSON stream and asserts that it is the end of the current object.
      StringgetPath()
      Returns a JsonPath to the current location in the JSON value.
      booleanhasNext()
      Returns true if the current array or object has another element.
      booleanisLenient()
      Returns true if this parser is liberal in what it accepts.
      booleannextBoolean()
      Returns the boolean value of the next token, consuming it.
      doublenextDouble()
      Returns the double value of the next token, consuming it.
      intnextInt()
      Returns the int value of the next token, consuming it.
      longnextLong()
      Returns the long value of the next token, consuming it.
      StringnextName()
      Returns the next token, a property name, and consumes it.
      voidnextNull()
      Consumes the next token from the JSON stream and asserts that it is a literal null.
      StringnextString()
      Returns the string value of the next token, consuming it.
      JsonTokenpeek()
      Returns the type of the next token without consuming it.
      voidsetLenient(boolean lenient)
      Configure this parser to be liberal in what it accepts.
      voidskipValue()
      Skips the next value recursively.
      StringtoString() 
    • 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 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
      • 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
      • 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
      • 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
      • 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
      • getPath

        public String getPath()
        Returns a JsonPath to the current location in the JSON value.

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