Class JsonReader

  • All Implemented Interfaces:
    java.io.Closeable, java.lang.AutoCloseable

    public class JsonReader
    extends IOContext
    implements java.io.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
    • Field Detail

      • NON_EXECUTE_PREFIX

        private static final char[] NON_EXECUTE_PREFIX
        The only non-execute prefix this parser permits
      • MIN_INCOMPLETE_INTEGER

        private static final long MIN_INCOMPLETE_INTEGER
        See Also:
        Constant Field Values
      • PEEKED_BUFFERED

        private static final int PEEKED_BUFFERED
        When this is returned, the string value is stored in peekedString.
        See Also:
        Constant Field Values
      • PEEKED_SINGLE_QUOTED_NAME

        private static final int PEEKED_SINGLE_QUOTED_NAME
        See Also:
        Constant Field Values
      • PEEKED_DOUBLE_QUOTED_NAME

        private static final int PEEKED_DOUBLE_QUOTED_NAME
        See Also:
        Constant Field Values
      • PEEKED_LONG

        private static final int PEEKED_LONG
        When this is returned, the integer value is stored in peekedLong.
        See Also:
        Constant Field Values
      • NUMBER_CHAR_FRACTION_DIGIT

        private static final int NUMBER_CHAR_FRACTION_DIGIT
        See Also:
        Constant Field Values
      • in

        private final java.io.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

        private 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 java.lang.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 java.lang.String[] pathNames
      • pathIndices

        private int[] pathIndices
    • Constructor Detail

      • JsonReader

        public JsonReader​(java.io.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 java.io.IOException
        Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.
        Throws:
        java.io.IOException
      • endArray

        public void endArray()
                      throws java.io.IOException
        Consumes the next token from the JSON stream and asserts that it is the end of the current array.
        Throws:
        java.io.IOException
      • beginObject

        public void beginObject()
                         throws java.io.IOException
        Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.
        Throws:
        java.io.IOException
      • endObject

        public void endObject()
                       throws java.io.IOException
        Consumes the next token from the JSON stream and asserts that it is the end of the current object.
        Throws:
        java.io.IOException
      • hasNext

        public boolean hasNext()
                        throws java.io.IOException
        Returns true if the current array or object has another element.
        Throws:
        java.io.IOException
      • peek

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

        private int doPeek()
                    throws java.io.IOException
        Throws:
        java.io.IOException
      • peekKeyword

        private int peekKeyword()
                         throws java.io.IOException
        Throws:
        java.io.IOException
      • peekNumber

        private int peekNumber()
                        throws java.io.IOException
        Throws:
        java.io.IOException
      • isLiteral

        private boolean isLiteral​(char c)
                           throws java.io.IOException
        Throws:
        java.io.IOException
      • nextName

        public java.lang.String nextName()
                                  throws java.io.IOException
        Returns the next token, a property name, and consumes it.
        Throws:
        java.io.IOException - if the next token in the stream is not a property name.
      • nextString

        public java.lang.String nextString()
                                    throws java.io.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:
        java.lang.IllegalStateException - if the next token is not a string or if this reader is closed.
        java.io.IOException
      • nextBoolean

        public boolean nextBoolean()
                            throws java.io.IOException
        Returns the boolean value of the next token, consuming it.
        Throws:
        java.lang.IllegalStateException - if the next token is not a boolean or if this reader is closed.
        java.io.IOException
      • nextNull

        public void nextNull()
                      throws java.io.IOException
        Consumes the next token from the JSON stream and asserts that it is a literal null.
        Throws:
        java.lang.IllegalStateException - if the next token is not null or if this reader is closed.
        java.io.IOException
      • nextDouble

        public double nextDouble()
                          throws java.io.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:
        java.lang.IllegalStateException - if the next token is not a literal value.
        java.lang.NumberFormatException - if the next literal value cannot be parsed as a double, or is non-finite.
        java.io.IOException
      • nextLong

        public long nextLong()
                      throws java.io.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:
        java.lang.IllegalStateException - if the next token is not a literal value.
        java.lang.NumberFormatException - if the next literal value cannot be parsed as a number, or exactly represented as a long.
        java.io.IOException
      • nextQuotedValue

        private java.lang.String nextQuotedValue​(char quote)
                                          throws java.io.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:
        java.lang.NumberFormatException - if any unicode escape sequences are malformed.
        java.io.IOException
      • nextUnquotedValue

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

        private void skipQuotedValue​(char quote)
                              throws java.io.IOException
        Throws:
        java.io.IOException
      • skipUnquotedValue

        private void skipUnquotedValue()
                                throws java.io.IOException
        Throws:
        java.io.IOException
      • nextInt

        public int nextInt()
                    throws java.io.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:
        java.lang.IllegalStateException - if the next token is not a literal value.
        java.lang.NumberFormatException - if the next literal value cannot be parsed as a number, or exactly represented as an int.
        java.io.IOException
      • close

        public void close()
                   throws java.io.IOException
        Closes this JSON reader and the underlying Reader.
        Specified by:
        close in interface java.lang.AutoCloseable
        Specified by:
        close in interface java.io.Closeable
        Throws:
        java.io.IOException
      • skipValue

        public void skipValue()
                       throws java.io.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:
        java.io.IOException
      • push

        private void push​(int newTop)
      • fillBuffer

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

        private int getLineNumber()
      • getColumnNumber

        private int getColumnNumber()
      • nextNonWhitespace

        private int nextNonWhitespace​(boolean throwOnEof)
                               throws java.io.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:
        java.io.IOException
      • checkLenient

        private void checkLenient()
                           throws java.io.IOException
        Throws:
        java.io.IOException
      • skipToEndOfLine

        private void skipToEndOfLine()
                              throws java.io.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:
        java.io.IOException
      • skipTo

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

        public java.lang.String toString()
        Overrides:
        toString in class java.lang.Object
      • getPath

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

        private char readEscapeCharacter()
                                  throws java.io.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:
        java.lang.NumberFormatException - if any unicode escape sequences are malformed.
        java.io.IOException
      • syntaxError

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

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