Class JsonReader
java.lang.Object
com.carrotsearch.ant.tasks.junit4.gson.stream.IOContext
com.carrotsearch.ant.tasks.junit4.gson.stream.JsonReader
- All Implemented Interfaces:
Closeable
,AutoCloseable
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 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()
.
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 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.private final Reader
The input JSON.private boolean
True to accept non-spec compliant JSONprivate int
private int
private int
private static final long
private static final char[]
The only non-execute prefix this parser permitsprivate 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[]
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[]
private int
-
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.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
.private int
private int
getPath()
Returns a JsonPath to the current location in the JSON value.boolean
hasNext()
Returns true if the current array or object has another element.final boolean
Returns true if this parser is liberal in what it accepts.private boolean
isLiteral
(char c) 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) Configure this parser to be be liberal 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 IOException
syntaxError
(String message) Throws a new IO exception with the given message and a context snippet with this reader's content.toString()
Methods inherited from class com.carrotsearch.ant.tasks.junit4.gson.stream.IOContext
inContext, lookupInContext, registerInContext
-
Field Details
-
NON_EXECUTE_PREFIX
private static final char[] NON_EXECUTE_PREFIXThe only non-execute prefix this parser permits -
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. -
lenient
private boolean lenientTrue to accept non-spec compliant JSON -
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
private 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[] stack -
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
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
orinfinities
. - 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,
-
isLenient
public final boolean isLenient()Returns true if this parser is liberal in what it accepts. -
beginArray
Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.- Throws:
IOException
-
endArray
Consumes the next token from the JSON stream and asserts that it is the end of the current array.- Throws:
IOException
-
beginObject
Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.- Throws:
IOException
-
endObject
Consumes the next token from the JSON stream and asserts that it is the end of the current object.- Throws:
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:
IOException
- if the next token in the stream is not a property name.
-
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 or if this reader is closed.IOException
-
nextBoolean
Returns theboolean
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
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
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 not a literal value.NumberFormatException
- if the next literal value cannot be parsed as a double, or is non-finite.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 not a literal value.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:
NumberFormatException
- if any unicode escape sequences are malformed.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 not a literal value.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
.- Specified by:
close
in interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Throws:
IOException
-
skipValue
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
Returns true oncelimit - pos >= minimum
. If the data is exhausted before that many characters are available, this returns false.- Throws:
IOException
-
getLineNumber
private int getLineNumber() -
getColumnNumber
private int getColumnNumber() -
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:
IOException
-
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
-
getPath
Returns a JsonPath to the current location in the JSON value. -
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:
NumberFormatException
- if any unicode escape sequences are malformed.IOException
-
syntaxError
Throws a new IO exception with the given message and a context snippet with this reader's content.- Throws:
IOException
-
consumeNonExecutePrefix
Consumes the non-execute prefix if it exists.- Throws:
IOException
-