Examples of com.google.gson.stream.JsonReader

• com.google.gson.stream.JsonReader
  etf.org/rfc/rfc4627.txt">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 a {@code 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.

  • Within array handling methods, first call {@link #beginArray} to consume the array's opening bracket. Then create awhile loop that accumulates values, terminating when {@link #hasNext}is false. Finally, read the array's closing bracket by calling {@link #endArray}.
  • Within object handling methods, first call {@link #beginObject} to consume the object's opening brace. Then create awhile loop that assigns values to local variables based on their name. This loop should terminate when {@link #hasNext} is false. Finally,read the object's closing brace by calling {@link #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 {@link #skipValue()} to recursivelyskip the value's nested tokens, which may otherwise conflict.

  If a value may be null, you should first check using {@link #peek()}. Null literals can be consumed using either {@link #nextNull()} or {@link #skipValue()}.

  Example

  Suppose we'd like to parse a stream of messages such as the following:

    {@code [}{ "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:

      {@code}public List readJsonStream(InputStream in) throws IOException  JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8")); try { return readMessagesArray(reader); } finally { reader.close(); } } public List readMessagesArray(JsonReader reader) throws IOException { List 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 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 readDoublesArray(JsonReader reader) throws IOException { List 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 {@code [1, "1"]} may be read using either {@link #nextInt} or {@link #nextString}. This behavior is intended to prevent lossy numeric conversions: double is JavaScript's only numeric type and very large values like {@code 9007199254740993} cannot be represented exactly on that platform. To minimizeprecision 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 {@code