public class JsonReader extends Object implements 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.
| Constructor and Description |
|---|
JsonReader(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.
|
void |
close()
Closes this JSON reader and the underlying
Reader. |
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.
|
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.
|
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. |
String |
nextName()
Returns the next token, a
property name, and consumes it. |
void |
nextNull()
Consumes the next token from the JSON stream and asserts that it is a literal
null.
|
String |
nextString()
Returns the
string value
of the next token, consuming it. |
JsonToken |
peek()
Returns the type of the next token without consuming it.
|
void |
setLenient(boolean lenient)
Configure this parser to be liberal in what it accepts.
|
void |
skipValue()
Skips the next value recursively.
|
String |
toString() |
public JsonReader(Reader in)
in.public final boolean isLenient()
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 void beginArray()
throws IOException
IOExceptionpublic void endArray()
throws IOException
IOExceptionpublic void beginObject()
throws IOException
IOExceptionpublic void endObject()
throws IOException
IOExceptionpublic boolean hasNext()
throws IOException
IOExceptionpublic JsonToken peek() throws IOException
IOExceptionpublic String nextName() throws IOException
property name, and consumes it.IOException - if the next token in the stream is not a property name.public String nextString() throws IOException
string value
of the next token, consuming it. If the next token is a number, this method
will return its string form.IllegalStateException - if the next token is not a string or if this
reader is closed.IOExceptionpublic boolean nextBoolean()
throws IOException
boolean
value of the next token, consuming it.IllegalStateException - if the next token is not a boolean or if this
reader is closed.IOExceptionpublic void nextNull()
throws IOException
IllegalStateException - if the next token is not null or if this reader
is closed.IOExceptionpublic double nextDouble()
throws 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).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.IOExceptionpublic long nextLong()
throws 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.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.IOExceptionpublic int nextInt()
throws 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.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.IOExceptionpublic void close()
throws IOException
Reader.close in interface Closeableclose in interface AutoCloseableIOExceptionpublic void skipValue()
throws IOException
IOException