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 8259) 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().

Configuration

The behavior of this reader can be customized with the following methods: The default configuration of JsonReader instances used internally by the Gson class differs, and can be adjusted with the various GsonBuilder methods.

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<>();

   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<>();

   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
      See Also:
    • 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.
      See Also:
    • 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.
    • strictness

      private Strictness strictness
    • DEFAULT_NESTING_LIMIT

      static final int DEFAULT_NESTING_LIMIT
      See Also:
    • nestingLimit

      private int nestingLimit
    • BUFFER_SIZE

      static final int BUFFER_SIZE
      See Also:
    • 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
      The nesting stack. Using a manual array rather than an ArrayList saves 20%.
    • 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

      @Deprecated public final void setLenient(boolean lenient)
      Deprecated.
      Please use setStrictness(Strictness) instead. JsonReader.setLenient(true) should be replaced by JsonReader.setStrictness(Strictness.LENIENT) and JsonReader.setLenient(false) should be replaced by JsonReader.setStrictness(Strictness.LEGACY_STRICT).
      However, if you used setLenient(false) before, you might prefer Strictness.STRICT now instead.
      Sets the strictness of this reader.
      Parameters:
      lenient - whether this reader should be lenient. If true, the strictness is set to Strictness.LENIENT. If false, the strictness is set to Strictness.LEGACY_STRICT.
      See Also:
    • isLenient

      public final boolean isLenient()
      Returns true if the Strictness of this reader is equal to Strictness.LENIENT.
      See Also:
    • setStrictness

      public final void setStrictness(Strictness strictness)
      Configures how liberal this parser is in what it accepts.

      In strict mode, the parser only accepts JSON in accordance with RFC 8259. In legacy strict mode (the default), only JSON in accordance with the RFC 8259 is accepted, with a few exceptions denoted below for backwards compatibility reasons. In lenient mode, all sort of non-spec compliant JSON is accepted (see below).

      Strictness.STRICT
      In strict mode, only input compliant with RFC 8259 is accepted.
      Strictness.LEGACY_STRICT
      In legacy strict mode, the following departures from RFC 8259 are accepted:
      • JsonReader allows the literals true, false and null to have any capitalization, for example fAlSe or NULL
      • JsonReader supports the escape sequence \', representing a ' (single-quote)
      • JsonReader supports the escape sequence \LF (with LF being the Unicode character U+000A), resulting in a LF within the read JSON string
      • JsonReader allows unescaped control characters (U+0000 through U+001F)
      Strictness.LENIENT
      In lenient mode, all input that is accepted in legacy strict mode is accepted in addition to the following departures from RFC 8259:
      • Streams that start with the non-execute prefix, ")]'\n"}
      • Streams that include multiple top-level values. With legacy strict or strict parsing, each stream must contain exactly one top-level value.
      • Numbers may be NaNs or infinities represented by NaN and (-)Infinity respectively.
      • 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 ,.
      Parameters:
      strictness - the new strictness value of this reader. May not be null.
      Since:
      2.11.0
      See Also:
    • getStrictness

      public final Strictness getStrictness()
      Returns the strictness of this reader.
      Since:
      2.11.0
      See Also:
    • setNestingLimit

      public final void setNestingLimit(int limit)
      Sets the nesting limit of this reader.

      The nesting limit defines how many JSON arrays or objects may be open at the same time. For example a nesting limit of 0 means no arrays or objects may be opened at all, a nesting limit of 1 means one array or object may be open at the same time, and so on. So a nesting limit of 3 allows reading the JSON data [{"a":[true]}], but for a nesting limit of 2 it would fail at the inner [true].

      The nesting limit can help to protect against a StackOverflowError when recursive TypeAdapter implementations process deeply nested JSON data.

      The default nesting limit is 255.

      Throws:
      IllegalArgumentException - if the nesting limit is negative.
      Since:
      2.12.0
      See Also:
    • getNestingLimit

      public final int getNestingLimit()
      Returns the nesting limit of this reader.
      Since:
      2.12.0
      See Also:
    • 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:
      IllegalStateException - if the next token is not the beginning of an array.
      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:
      IllegalStateException - if the next token is not the end of an array.
      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:
      IllegalStateException - if the next token is not the beginning of an object.
      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:
      IllegalStateException - if the next token is not the end of an object.
      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:
      IllegalStateException - if the next token is not a property name.
      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.
      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.
      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 a JSON null.
      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 neither a number nor a string.
      NumberFormatException - if the next literal value cannot be parsed as a double.
      MalformedJsonException - if the next literal value is NaN or Infinity and this reader is not lenient.
      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 neither a number nor a string.
      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:
      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 neither a number nor a string.
      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.

      Using the JSON reader after it has been closed will throw an IllegalStateException in most cases.

      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. This method is intended for use when the JSON token stream contains unrecognized or unhandled values.

      The behavior depends on the type of the next JSON token:

      • Start of a JSON array or object: It and all of its nested values are skipped.
      • Primitive value (for example a JSON number): The primitive value is skipped.
      • Property name: Only the name but not the value of the property is skipped. skipValue() has to be called again to skip the property value as well.
      • End of a JSON array or object: Only this end token is skipped.
      • End of JSON document: Skipping has no effect, the next token continues to be the end of the document.
      Throws:
      IOException
    • push

      private void push(int newTop) throws MalformedJsonException
      Throws:
      MalformedJsonException
    • 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 MalformedJsonException
      Throws:
      MalformedJsonException
    • 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

      private String getPath(boolean usePreviousPath)
    • getPath

      public String getPath()
      Returns a JSONPath in dot-notation to the next (or current) location in the JSON document. That means:
      • For JSON arrays the path points to the index of the next element (even if there are no further elements).
      • For JSON objects the path points to the last property, or to the current property if its name has already been consumed.

      This method can be useful to add additional context to exception messages before a value is consumed, for example when the peeked token is unexpected.

    • getPreviousPath

      public String getPreviousPath()
      Returns a JSONPath in dot-notation to the previous (or current) location in the JSON document. That means:
      • For JSON arrays the path points to the index of the previous element.
        If no element has been consumed yet it uses the index 0 (even if there are no elements).
      • For JSON objects the path points to the last property, or to the current property if its name has already been consumed.

      This method can be useful to add additional context to exception messages after a value has been consumed.

    • 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:
      MalformedJsonException - if the escape sequence is malformed
      IOException
    • syntaxError

      private MalformedJsonException syntaxError(String message) throws MalformedJsonException
      Throws a new MalformedJsonException with the given message and information about the current location.
      Throws:
      MalformedJsonException
    • unexpectedTokenError

      private IllegalStateException unexpectedTokenError(String expected) throws IOException
      Throws:
      IOException
    • consumeNonExecutePrefix

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