Class JsonReader

java.lang.Object
com.google.gson.stream.JsonReader
All Implemented Interfaces:
Closeable, AutoCloseable
Direct Known Subclasses:
JsonTreeReader

public class JsonReader extends Object implements 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
  • Field Details

    • MIN_INCOMPLETE_INTEGER

      private static final long MIN_INCOMPLETE_INTEGER
      See Also:
    • PEEKED_NONE

      private static final int PEEKED_NONE
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    • PEEKED_BEGIN_OBJECT

      private static final int PEEKED_BEGIN_OBJECT
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    • PEEKED_END_OBJECT

      private static final int PEEKED_END_OBJECT
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    • PEEKED_BEGIN_ARRAY

      private static final int PEEKED_BEGIN_ARRAY
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    • PEEKED_END_ARRAY

      private static final int PEEKED_END_ARRAY
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    • PEEKED_TRUE

      private static final int PEEKED_TRUE
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    • PEEKED_FALSE

      private static final int PEEKED_FALSE
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    • PEEKED_NULL

      private static final int PEEKED_NULL
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    • PEEKED_SINGLE_QUOTED

      private static final int PEEKED_SINGLE_QUOTED
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    • PEEKED_DOUBLE_QUOTED

      private static final int PEEKED_DOUBLE_QUOTED
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    • PEEKED_UNQUOTED

      private static final int PEEKED_UNQUOTED
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    • PEEKED_BUFFERED

      private static final int PEEKED_BUFFERED
      When this is returned, the string value is stored in peekedString.
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    • PEEKED_SINGLE_QUOTED_NAME

      private static final int PEEKED_SINGLE_QUOTED_NAME
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    • PEEKED_DOUBLE_QUOTED_NAME

      private static final int PEEKED_DOUBLE_QUOTED_NAME
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    • PEEKED_UNQUOTED_NAME

      private static final int PEEKED_UNQUOTED_NAME
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    • PEEKED_LONG

      private static final int PEEKED_LONG
      When this is returned, the integer value is stored in peekedLong.
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    • PEEKED_NUMBER

      private static final int PEEKED_NUMBER
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    • PEEKED_EOF

      private static final int PEEKED_EOF
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    • NUMBER_CHAR_NONE

      private static final int NUMBER_CHAR_NONE
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    • NUMBER_CHAR_SIGN

      private static final int NUMBER_CHAR_SIGN
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    • NUMBER_CHAR_DIGIT

      private static final int NUMBER_CHAR_DIGIT
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    • NUMBER_CHAR_DECIMAL

      private static final int NUMBER_CHAR_DECIMAL
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    • NUMBER_CHAR_FRACTION_DIGIT

      private static final int NUMBER_CHAR_FRACTION_DIGIT
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    • NUMBER_CHAR_EXP_E

      private static final int NUMBER_CHAR_EXP_E
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    • NUMBER_CHAR_EXP_SIGN

      private static final int NUMBER_CHAR_EXP_SIGN
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    • NUMBER_CHAR_EXP_DIGIT

      private static final int NUMBER_CHAR_EXP_DIGIT
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    • in

      private final Reader in
      The input JSON.
    • lenient

      private boolean lenient
      True to accept non-spec compliant JSON
    • buffer

      private final char[] buffer
      Use a manual buffer to easily read and unread upcoming characters, and also so we can create strings without an intermediate StringBuilder. We decode literals directly out of this buffer, so it must be at least as long as the longest token that can be reported as a number.
    • pos

      private int pos
    • limit

      private int limit
    • lineNumber

      private int lineNumber
    • lineStart

      private int lineStart
    • peeked

      int peeked
    • peekedLong

      private long peekedLong
      A peeked value that was composed entirely of digits with an optional leading dash. Positive values may not have a leading 0.
    • peekedNumberLength

      private int peekedNumberLength
      The number of characters in a peeked number literal. Increment 'pos' by this after reading a number.
    • peekedString

      private String peekedString
      A peeked string that should be parsed on the next double, long or string. This is populated before a numeric value is parsed and used if that parsing fails.
    • stack

      private int[] stack
    • stackSize

      private int stackSize
    • pathNames

      private String[] pathNames
    • pathIndices

      private int[] pathIndices
  • Constructor Details

    • JsonReader

      public JsonReader(Reader in)
      Creates a new instance that reads a JSON-encoded stream from in.
  • Method Details

    • 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
    • peek

      public JsonToken peek() throws IOException
      Returns the type of the next token without consuming it.
      Throws:
      IOException
    • doPeek

      int doPeek() throws IOException
      Throws:
      IOException
    • peekKeyword

      private int peekKeyword() throws IOException
      Throws:
      IOException
    • peekNumber

      private int peekNumber() throws IOException
      Throws:
      IOException
    • isLiteral

      private boolean isLiteral(char c) throws IOException
      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
    • nextQuotedValue

      private String nextQuotedValue(char quote) throws IOException
      Returns the string up to but not including quote, unescaping any character escape sequences encountered along the way. The opening quote should have already been read. This consumes the closing quote, but does not include it in the returned string.
      Parameters:
      quote - either ' or ".
      Throws:
      NumberFormatException - if any unicode escape sequences are malformed.
      IOException
    • nextUnquotedValue

      private String nextUnquotedValue() throws IOException
      Returns an unquoted value as a string.
      Throws:
      IOException
    • skipQuotedValue

      private void skipQuotedValue(char quote) throws IOException
      Throws:
      IOException
    • skipUnquotedValue

      private void skipUnquotedValue() throws IOException
      Throws:
      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 AutoCloseable
      Specified by:
      close in interface Closeable
      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
    • push

      private void push(int newTop)
    • fillBuffer

      private boolean fillBuffer(int minimum) throws IOException
      Returns true once limit - pos >= minimum. If the data is exhausted before that many characters are available, this returns false.
      Throws:
      IOException
    • nextNonWhitespace

      private int nextNonWhitespace(boolean throwOnEof) throws IOException
      Returns the next character in the stream that is neither whitespace nor a part of a comment. When this returns, the returned character is always at buffer[pos-1]; this means the caller can always push back the returned character by decrementing pos.
      Throws:
      IOException
    • checkLenient

      private void checkLenient() throws IOException
      Throws:
      IOException
    • skipToEndOfLine

      private void skipToEndOfLine() throws IOException
      Advances the position until after the next newline character. If the line is terminated by "\r\n", the '\n' must be consumed as whitespace by the caller.
      Throws:
      IOException
    • skipTo

      private boolean skipTo(String toFind) throws IOException
      Parameters:
      toFind - a string to search for. Must not contain a newline.
      Throws:
      IOException
    • toString

      public String toString()
      Overrides:
      toString in class Object
    • locationString

      String locationString()
    • getPath

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

      private char readEscapeCharacter() throws IOException
      Unescapes the character identified by the character or characters that immediately follow a backslash. The backslash '\' should have already been read. This supports both unicode escapes "u000A" and two-character escapes "\n".
      Throws:
      NumberFormatException - if any unicode escape sequences are malformed.
      IOException
    • syntaxError

      private IOException syntaxError(String message) throws IOException
      Throws a new IO exception with the given message and a context snippet with this reader's content.
      Throws:
      IOException
    • consumeNonExecutePrefix

      private void consumeNonExecutePrefix() throws IOException
      Consumes the non-execute prefix if it exists.
      Throws:
      IOException