Class JsonReader
- All Implemented Interfaces:
Closeable
,AutoCloseable
- Direct Known Subclasses:
JsonTreeReader
Parsing JSON
To create a recursive descent parser for your own JSON streams, first create an entry point method that creates aJsonReader
.
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 whenhasNext()
is false. Finally, read the array's closing bracket by callingendArray()
. - 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 whenhasNext()
is false. Finally, read the object's closing brace by callingendObject()
.
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:setStrictness(Strictness)
, the default isStrictness.LEGACY_STRICT
setNestingLimit(int)
, the default is 255
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 Summary
FieldsModifier and TypeFieldDescriptionprivate final char[]
Use a manual buffer to easily read and unread upcoming characters, and also so we can create strings without an intermediate StringBuilder.(package private) static final int
(package private) static final int
private final Reader
The input JSON.private int
private int
private int
private static final long
private int
private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private int[]
private String[]
(package private) int
private static final int
private static final int
private static final int
When this is returned, the string value is stored in peekedString.private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
When this is returned, the integer value is stored in peekedLong.private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private static final int
private long
A peeked value that was composed entirely of digits with an optional leading dash.private int
The number of characters in a peeked number literal.private String
A peeked string that should be parsed on the next double, long or string.private int
private int[]
The nesting stack.private int
private Strictness
-
Constructor Summary
ConstructorsConstructorDescriptionJsonReader
(Reader in) Creates a new instance that reads a JSON-encoded stream fromin
. -
Method Summary
Modifier and TypeMethodDescriptionvoid
Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.void
Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.private void
void
close()
Closes this JSON reader and the underlyingReader
.private void
Consumes the non-execute prefix if it exists.(package private) int
doPeek()
void
endArray()
Consumes the next token from the JSON stream and asserts that it is the end of the current array.void
Consumes the next token from the JSON stream and asserts that it is the end of the current object.private boolean
fillBuffer
(int minimum) Returns true oncelimit - pos >= minimum
.final int
Returns the nesting limit of this reader.getPath()
Returns a JSONPath in dot-notation to the next (or current) location in the JSON document.private String
getPath
(boolean usePreviousPath) Returns a JSONPath in dot-notation to the previous (or current) location in the JSON document.final Strictness
Returns the strictness of this reader.boolean
hasNext()
Returns true if the current array or object has another element.final boolean
Returns true if theStrictness
of this reader is equal toStrictness.LENIENT
.private boolean
isLiteral
(char c) (package private) String
boolean
Returns theboolean
value of the next token, consuming it.double
Returns thedouble
value of the next token, consuming it.int
nextInt()
Returns theint
value of the next token, consuming it.long
nextLong()
Returns thelong
value of the next token, consuming it.nextName()
Returns the next token, aproperty name
, and consumes it.private int
nextNonWhitespace
(boolean throwOnEof) Returns the next character in the stream that is neither whitespace nor a part of a comment.void
nextNull()
Consumes the next token from the JSON stream and asserts that it is a literal null.private String
nextQuotedValue
(char quote) Returns the string up to but not includingquote
, unescaping any character escape sequences encountered along the way.Returns thestring
value of the next token, consuming it.private String
Returns an unquoted value as a string.peek()
Returns the type of the next token without consuming it.private int
private int
private void
push
(int newTop) private char
Unescapes the character identified by the character or characters that immediately follow a backslash.final void
setLenient
(boolean lenient) Deprecated.final void
setNestingLimit
(int limit) Sets the nesting limit of this reader.final void
setStrictness
(Strictness strictness) Configures how liberal this parser is in what it accepts.private void
skipQuotedValue
(char quote) private boolean
private void
Advances the position until after the next newline character.private void
void
Skips the next value recursively.private MalformedJsonException
syntaxError
(String message) Throws a newMalformedJsonException
with the given message and information about the current location.toString()
private IllegalStateException
unexpectedTokenError
(String expected)
-
Field Details
-
MIN_INCOMPLETE_INTEGER
private static final long MIN_INCOMPLETE_INTEGER- See Also:
-
PEEKED_NONE
private static final int PEEKED_NONE- See Also:
-
PEEKED_BEGIN_OBJECT
private static final int PEEKED_BEGIN_OBJECT- See Also:
-
PEEKED_END_OBJECT
private static final int PEEKED_END_OBJECT- See Also:
-
PEEKED_BEGIN_ARRAY
private static final int PEEKED_BEGIN_ARRAY- See Also:
-
PEEKED_END_ARRAY
private static final int PEEKED_END_ARRAY- See Also:
-
PEEKED_TRUE
private static final int PEEKED_TRUE- See Also:
-
PEEKED_FALSE
private static final int PEEKED_FALSE- See Also:
-
PEEKED_NULL
private static final int PEEKED_NULL- See Also:
-
PEEKED_SINGLE_QUOTED
private static final int PEEKED_SINGLE_QUOTED- See Also:
-
PEEKED_DOUBLE_QUOTED
private static final int PEEKED_DOUBLE_QUOTED- See Also:
-
PEEKED_UNQUOTED
private static final int PEEKED_UNQUOTED- See Also:
-
PEEKED_BUFFERED
private static final int PEEKED_BUFFEREDWhen this is returned, the string value is stored in peekedString.- See Also:
-
PEEKED_SINGLE_QUOTED_NAME
private static final int PEEKED_SINGLE_QUOTED_NAME- See Also:
-
PEEKED_DOUBLE_QUOTED_NAME
private static final int PEEKED_DOUBLE_QUOTED_NAME- See Also:
-
PEEKED_UNQUOTED_NAME
private static final int PEEKED_UNQUOTED_NAME- See Also:
-
PEEKED_LONG
private static final int PEEKED_LONGWhen this is returned, the integer value is stored in peekedLong.- See Also:
-
PEEKED_NUMBER
private static final int PEEKED_NUMBER- See Also:
-
PEEKED_EOF
private static final int PEEKED_EOF- See Also:
-
NUMBER_CHAR_NONE
private static final int NUMBER_CHAR_NONE- See Also:
-
NUMBER_CHAR_SIGN
private static final int NUMBER_CHAR_SIGN- See Also:
-
NUMBER_CHAR_DIGIT
private static final int NUMBER_CHAR_DIGIT- See Also:
-
NUMBER_CHAR_DECIMAL
private static final int NUMBER_CHAR_DECIMAL- See Also:
-
NUMBER_CHAR_FRACTION_DIGIT
private static final int NUMBER_CHAR_FRACTION_DIGIT- See Also:
-
NUMBER_CHAR_EXP_E
private static final int NUMBER_CHAR_EXP_E- See Also:
-
NUMBER_CHAR_EXP_SIGN
private static final int NUMBER_CHAR_EXP_SIGN- See Also:
-
NUMBER_CHAR_EXP_DIGIT
private static final int NUMBER_CHAR_EXP_DIGIT- See Also:
-
in
The input JSON. -
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[] bufferUse 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 peekedLongA peeked value that was composed entirely of digits with an optional leading dash. Positive values may not have a leading 0. -
peekedNumberLength
private int peekedNumberLengthThe number of characters in a peeked number literal. Increment 'pos' by this after reading a number. -
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[] stackThe nesting stack. Using a manual array rather than an ArrayList saves 20%. -
stackSize
private int stackSize -
pathNames
-
pathIndices
private int[] pathIndices
-
-
Constructor Details
-
JsonReader
Creates a new instance that reads a JSON-encoded stream fromin
.
-
-
Method Details
-
setLenient
Deprecated.Please usesetStrictness(Strictness)
instead.JsonReader.setLenient(true)
should be replaced byJsonReader.setStrictness(Strictness.LENIENT)
andJsonReader.setLenient(false)
should be replaced byJsonReader.setStrictness(Strictness.LEGACY_STRICT)
.
However, if you usedsetLenient(false)
before, you might preferStrictness.STRICT
now instead.Sets the strictness of this reader.- Parameters:
lenient
- whether this reader should be lenient. If true, the strictness is set toStrictness.LENIENT
. If false, the strictness is set toStrictness.LEGACY_STRICT
.- See Also:
-
isLenient
public final boolean isLenient()Returns true if theStrictness
of this reader is equal toStrictness.LENIENT
.- See Also:
-
setStrictness
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
andnull
to have any capitalization, for examplefAlSe
orNULL
- JsonReader supports the escape sequence
\'
, representing a'
(single-quote) - JsonReader supports the escape sequence
\LF
(withLF
being the Unicode characterU+000A
), resulting in aLF
within the read JSON string - JsonReader allows unescaped control characters (
U+0000
throughU+001F
)
- JsonReader allows the literals
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
orinfinities
represented byNaN
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,
.
- Streams that start with the non-execute prefix,
- Parameters:
strictness
- the new strictness value of this reader. May not benull
.- Since:
- 2.11.0
- See Also:
-
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 recursiveTypeAdapter
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
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
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
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
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
Returns true if the current array or object has another element.- Throws:
IOException
-
peek
Returns the type of the next token without consuming it.- Throws:
IOException
-
doPeek
- Throws:
IOException
-
peekKeyword
- Throws:
IOException
-
peekNumber
- Throws:
IOException
-
isLiteral
- Throws:
IOException
-
nextName
Returns the next token, aproperty name
, and consumes it.- Throws:
IllegalStateException
- if the next token is not a property name.IOException
-
nextString
Returns thestring
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
Returns theboolean
value of the next token, consuming it.- Throws:
IllegalStateException
- if the next token is not a boolean.IOException
-
nextNull
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
Returns thedouble
value of the next token, consuming it. If the next token is a string, this method will attempt to parse it as a double usingDouble.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 notlenient
.IOException
-
nextLong
Returns thelong
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 Javalong
, 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
Returns the string up to but not includingquote
, 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
Returns an unquoted value as a string.- Throws:
IOException
-
skipQuotedValue
- Throws:
IOException
-
skipUnquotedValue
- Throws:
IOException
-
nextInt
Returns theint
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 Javaint
, 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
Closes this JSON reader and the underlyingReader
.Using the JSON reader after it has been closed will throw an
IllegalStateException
in most cases.- Specified by:
close
in interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Throws:
IOException
-
skipValue
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
- Throws:
MalformedJsonException
-
fillBuffer
Returns true oncelimit - pos >= minimum
. If the data is exhausted before that many characters are available, this returns false.- Throws:
IOException
-
nextNonWhitespace
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 atbuffer[pos-1]
; this means the caller can always push back the returned character by decrementingpos
.- Throws:
IOException
-
checkLenient
- Throws:
MalformedJsonException
-
skipToEndOfLine
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
- Parameters:
toFind
- a string to search for. Must not contain a newline.- Throws:
IOException
-
toString
-
locationString
String locationString() -
getPath
-
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
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.
- For JSON arrays the path points to the index of the previous element.
-
readEscapeCharacter
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 malformedIOException
-
syntaxError
Throws a newMalformedJsonException
with the given message and information about the current location.- Throws:
MalformedJsonException
-
unexpectedTokenError
- Throws:
IOException
-
consumeNonExecutePrefix
Consumes the non-execute prefix if it exists.- Throws:
IOException
-
setStrictness(Strictness)
instead.