public class JsonReader
extends java.lang.Object
implements java.io.Closeable
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.
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()
.
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()
.
[
{
"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);
}
[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.
<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.
Modifier and Type | Field and Description |
---|---|
private char[] |
buffer
Use a manual buffer to easily read and unread upcoming characters, and
also so we can create strings without an intermediate StringBuilder.
|
private java.io.Reader |
in
The input JSON.
|
private boolean |
lenient
True to accept non-spec compliant JSON
|
private int |
limit |
private int |
lineNumber |
private int |
lineStart |
private static long |
MIN_INCOMPLETE_INTEGER |
private static char[] |
NON_EXECUTE_PREFIX
The only non-execute prefix this parser permits
|
private static int |
NUMBER_CHAR_DECIMAL |
private static int |
NUMBER_CHAR_DIGIT |
private static int |
NUMBER_CHAR_EXP_DIGIT |
private static int |
NUMBER_CHAR_EXP_E |
private static int |
NUMBER_CHAR_EXP_SIGN |
private static int |
NUMBER_CHAR_FRACTION_DIGIT |
private static int |
NUMBER_CHAR_NONE |
private static int |
NUMBER_CHAR_SIGN |
private int[] |
pathIndices |
private java.lang.String[] |
pathNames |
(package private) int |
peeked |
private static int |
PEEKED_BEGIN_ARRAY |
private static int |
PEEKED_BEGIN_OBJECT |
private static int |
PEEKED_BUFFERED
When this is returned, the string value is stored in peekedString.
|
private static int |
PEEKED_DOUBLE_QUOTED |
private static int |
PEEKED_DOUBLE_QUOTED_NAME |
private static int |
PEEKED_END_ARRAY |
private static int |
PEEKED_END_OBJECT |
private static int |
PEEKED_EOF |
private static int |
PEEKED_FALSE |
private static int |
PEEKED_LONG
When this is returned, the integer value is stored in peekedLong.
|
private static int |
PEEKED_NONE |
private static int |
PEEKED_NULL |
private static int |
PEEKED_NUMBER |
private static int |
PEEKED_SINGLE_QUOTED |
private static int |
PEEKED_SINGLE_QUOTED_NAME |
private static int |
PEEKED_TRUE |
private static int |
PEEKED_UNQUOTED |
private static int |
PEEKED_UNQUOTED_NAME |
private long |
peekedLong
A peeked value that was composed entirely of digits with an optional
leading dash.
|
private int |
peekedNumberLength
The number of characters in a peeked number literal.
|
private java.lang.String |
peekedString
A peeked string that should be parsed on the next double, long or string.
|
private int |
pos |
private int[] |
stack |
private int |
stackSize |
Constructor and Description |
---|
JsonReader(java.io.Reader in)
Creates a new instance that reads a JSON-encoded stream from
in . |
Modifier and Type | Method and Description |
---|---|
void |
beginArray()
Consumes the next token from the JSON stream and asserts that it is the
beginning of a new array.
|
void |
beginObject()
Consumes the next token from the JSON stream and asserts that it is the
beginning of a new object.
|
private void |
checkLenient() |
void |
close()
Closes this JSON reader and the underlying
Reader . |
private void |
consumeNonExecutePrefix()
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 |
endObject()
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 once
limit - pos >= minimum . |
java.lang.String |
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.
|
boolean |
isLenient()
Returns true if this parser is liberal in what it accepts.
|
private boolean |
isLiteral(char c) |
(package private) java.lang.String |
locationString() |
boolean |
nextBoolean()
Returns the
boolean value of the next token,
consuming it. |
double |
nextDouble()
Returns the
double value of the next token,
consuming it. |
int |
nextInt()
Returns the
int value of the next token,
consuming it. |
long |
nextLong()
Returns the
long value of the next token,
consuming it. |
java.lang.String |
nextName()
Returns the next token, a
property 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 java.lang.String |
nextQuotedValue(char quote)
Returns the string up to but not including
quote , unescaping any
character escape sequences encountered along the way. |
java.lang.String |
nextString()
Returns the
string value of the next token,
consuming it. |
private java.lang.String |
nextUnquotedValue()
Returns an unquoted value as a string.
|
JsonToken |
peek()
Returns the type of the next token without consuming it.
|
private int |
peekKeyword() |
private int |
peekNumber() |
private void |
push(int newTop) |
private char |
readEscapeCharacter()
Unescapes the character identified by the character or characters that
immediately follow a backslash.
|
void |
setLenient(boolean lenient)
Configure this parser to be liberal in what it accepts.
|
private void |
skipQuotedValue(char quote) |
private boolean |
skipTo(java.lang.String toFind) |
private void |
skipToEndOfLine()
Advances the position until after the next newline character.
|
private void |
skipUnquotedValue() |
void |
skipValue()
Skips the next value recursively.
|
private java.io.IOException |
syntaxError(java.lang.String message)
Throws a new IO exception with the given message and a context snippet
with this reader's content.
|
java.lang.String |
toString() |
private static final char[] NON_EXECUTE_PREFIX
private static final long MIN_INCOMPLETE_INTEGER
private static final int PEEKED_NONE
private static final int PEEKED_BEGIN_OBJECT
private static final int PEEKED_END_OBJECT
private static final int PEEKED_BEGIN_ARRAY
private static final int PEEKED_END_ARRAY
private static final int PEEKED_TRUE
private static final int PEEKED_FALSE
private static final int PEEKED_NULL
private static final int PEEKED_SINGLE_QUOTED
private static final int PEEKED_DOUBLE_QUOTED
private static final int PEEKED_UNQUOTED
private static final int PEEKED_BUFFERED
private static final int PEEKED_SINGLE_QUOTED_NAME
private static final int PEEKED_DOUBLE_QUOTED_NAME
private static final int PEEKED_UNQUOTED_NAME
private static final int PEEKED_LONG
private static final int PEEKED_NUMBER
private static final int PEEKED_EOF
private static final int NUMBER_CHAR_NONE
private static final int NUMBER_CHAR_SIGN
private static final int NUMBER_CHAR_DIGIT
private static final int NUMBER_CHAR_DECIMAL
private static final int NUMBER_CHAR_FRACTION_DIGIT
private static final int NUMBER_CHAR_EXP_E
private static final int NUMBER_CHAR_EXP_SIGN
private static final int NUMBER_CHAR_EXP_DIGIT
private final java.io.Reader in
private boolean lenient
private final char[] buffer
private int pos
private int limit
private int lineNumber
private int lineStart
int peeked
private long peekedLong
private int peekedNumberLength
private java.lang.String peekedString
private int[] stack
private int stackSize
private java.lang.String[] pathNames
private int[] pathIndices
public JsonReader(java.io.Reader in)
in
.public final void setLenient(boolean lenient)
")]}'\n"
.
NaNs
or infinities
.
//
or #
and
ending with a newline character.
/*
and ending with
*
/
. Such comments may not be nested.
'single quoted'
.
'single quoted'
.
;
instead of ,
.
=
or =>
instead of
:
.
;
instead of ,
.
public final boolean isLenient()
public void beginArray() throws java.io.IOException
java.io.IOException
public void endArray() throws java.io.IOException
java.io.IOException
public void beginObject() throws java.io.IOException
java.io.IOException
public void endObject() throws java.io.IOException
java.io.IOException
public boolean hasNext() throws java.io.IOException
java.io.IOException
public JsonToken peek() throws java.io.IOException
java.io.IOException
int doPeek() throws java.io.IOException
java.io.IOException
private int peekKeyword() throws java.io.IOException
java.io.IOException
private int peekNumber() throws java.io.IOException
java.io.IOException
private boolean isLiteral(char c) throws java.io.IOException
java.io.IOException
public java.lang.String nextName() throws java.io.IOException
property name
, and
consumes it.java.io.IOException
- if the next token in the stream is not a property
name.public java.lang.String nextString() throws java.io.IOException
string
value of the next token,
consuming it. If the next token is a number, this method will return its
string form.java.lang.IllegalStateException
- if the next token is not a string or if
this reader is closed.java.io.IOException
public boolean nextBoolean() throws java.io.IOException
boolean
value of the next token,
consuming it.java.lang.IllegalStateException
- if the next token is not a boolean or if
this reader is closed.java.io.IOException
public void nextNull() throws java.io.IOException
java.lang.IllegalStateException
- if the next token is not null or if this
reader is closed.java.io.IOException
public double nextDouble() throws java.io.IOException
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)
.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
public long nextLong() throws java.io.IOException
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.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
private java.lang.String nextQuotedValue(char quote) throws java.io.IOException
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.quote
- either ' or ".java.lang.NumberFormatException
- if any unicode escape sequences are
malformed.java.io.IOException
private java.lang.String nextUnquotedValue() throws java.io.IOException
java.io.IOException
private void skipQuotedValue(char quote) throws java.io.IOException
java.io.IOException
private void skipUnquotedValue() throws java.io.IOException
java.io.IOException
public int nextInt() throws java.io.IOException
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.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
public void close() throws java.io.IOException
Reader
.close
in interface java.io.Closeable
close
in interface java.lang.AutoCloseable
java.io.IOException
public void skipValue() throws java.io.IOException
java.io.IOException
private void push(int newTop)
private boolean fillBuffer(int minimum) throws java.io.IOException
limit - pos >= minimum
. If the data is
exhausted before that many characters are available, this returns
false.java.io.IOException
private int nextNonWhitespace(boolean throwOnEof) throws java.io.IOException
buffer[pos-1]
; this means the caller can always push back the
returned character by decrementing pos
.java.io.IOException
private void checkLenient() throws java.io.IOException
java.io.IOException
private void skipToEndOfLine() throws java.io.IOException
java.io.IOException
private boolean skipTo(java.lang.String toFind) throws java.io.IOException
toFind
- a string to search for. Must not contain a newline.java.io.IOException
public java.lang.String toString()
toString
in class java.lang.Object
java.lang.String locationString()
public java.lang.String getPath()
private char readEscapeCharacter() throws java.io.IOException
java.lang.NumberFormatException
- if any unicode escape sequences are
malformed.java.io.IOException
private java.io.IOException syntaxError(java.lang.String message) throws java.io.IOException
java.io.IOException
private void consumeNonExecutePrefix() throws java.io.IOException
java.io.IOException