- java.lang.Object
-
- com.google.gson.stream.JsonReader
-
- All Implemented Interfaces:
Closeable
,AutoCloseable
public class JsonReader extends Object implements Closeable
Reads a JSON (RFC 7159) 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 eithernextNull()
orskipValue()
.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 } } ]
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 eithernextInt()
ornextString()
. This behavior is intended to prevent lossy numeric conversions: double is JavaScript's only numeric type and very large values like9007199254740993
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 whenlenient 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
- Author:
- Jesse Wilson
-
-
Constructor Summary
Constructors Constructor Description JsonReader(Reader in)
Creates a new instance that reads a JSON-encoded stream fromin
.
-
Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method 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 underlyingReader
.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 in dot-notation to the next (or current) location in the JSON document: 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.String
getPreviousPath()
Returns a JSONPath in dot-notation to the previous (or current) location in the JSON document: 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.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 theboolean
value of the next token, consuming it.double
nextDouble()
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.String
nextName()
Returns the next token, aproperty 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 thestring
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()
-
-
-
Constructor Detail
-
JsonReader
public JsonReader(Reader in)
Creates a new instance that reads a JSON-encoded stream fromin
.
-
-
Method Detail
-
setLenient
public final void setLenient(boolean lenient)
Configure this parser to 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.
- 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,
.
Note: Even in strict mode there are slight derivations from the JSON specification:
- JsonReader allows the literals
true
,false
andnull
to have any capitalization, for examplefAlSe
- JsonReader supports the escape sequence
\'
, representing a'
- JsonReader supports the escape sequence
\LF
(withLF
being the Unicode character U+000A), resulting in aLF
within the read JSON string - JsonReader allows unescaped control characters (U+0000 through U+001F)
- 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
public void beginArray() throws IOException
Consumes the next token from the JSON stream and asserts that it is the beginning of a new array.- Throws:
IOException
-
endArray
public void endArray() throws IOException
Consumes the next token from the JSON stream and asserts that it is the end of the current array.- Throws:
IOException
-
beginObject
public void beginObject() throws IOException
Consumes the next token from the JSON stream and asserts that it is the beginning of a new object.- Throws:
IOException
-
endObject
public void endObject() throws IOException
Consumes the next token from the JSON stream and asserts that it is the end of the current object.- Throws:
IOException
-
hasNext
public boolean hasNext() throws IOException
Returns true if the current array or object has another element.- Throws:
IOException
-
peek
public JsonToken peek() throws IOException
Returns the type of the next token without consuming it.- Throws:
IOException
-
nextName
public String nextName() throws IOException
Returns the next token, aproperty name
, and consumes it.- Throws:
IOException
- if the next token in the stream is not a property name.
-
nextString
public String nextString() throws IOException
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
public boolean nextBoolean() throws IOException
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
public void nextNull() throws IOException
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
public double nextDouble() throws IOException
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.MalformedJsonException
- if the next literal value is NaN or Infinity and this reader is notlenient
.IOException
-
nextLong
public long nextLong() throws IOException
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
-
nextInt
public int nextInt() throws IOException
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
public void close() throws IOException
Closes this JSON reader and the underlyingReader
.- Specified by:
close
in interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Throws:
IOException
-
skipValue
public void skipValue() throws IOException
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
-
getPreviousPath
public String getPreviousPath()
Returns a JSONPath in dot-notation to the previous (or current) location in the JSON document:- 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.
-
getPath
public String getPath()
Returns a JSONPath in dot-notation to the next (or current) location in the JSON document:- 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.
-
-