Java转JSON

• 从Java对象转换
• 从JsonNode Tree
• 构建JSON Stream

从java对象

ObjectMapper mapper = new ObjectMapper();String result = mapper.writeValueAsString(object);

从JsonNode

// Create the node factory that gives us nodes.JsonNodeFactory factory = new JsonNodeFactory(false);// create a json factory to write the treenode as json. for the example// we just write to consoleJsonFactory jsonFactory = new JsonFactory();JsonGenerator generator = jsonFactory.createGenerator(System.out);ObjectMapper mapper = new ObjectMapper();// the root node - albumJsonNode album = factory.objectNode();mapper.writeTree(generator, album);

从JSON Stream

JsonFactory factory = new JsonFactory();JsonGenerator generator = factory.createGenerator(new FileWriter(new File("albums.json")));// start writing with {generator.writeStartObject();generator.writeFieldName("title");generator.writeString("Free Music Archive - Albums");generator.writeFieldName("dataset");// start an arraygenerator.writeStartArray();generator.writeStartObject();generator.writeStringField("album_title", "A.B.A.Y.A.M");generator.writeEndObject();generator.writeEndArray();generator.writeEndObject();generator.close();

JSON转Java

• Streaming - 使用JsonParser解析JSON stream. 它提供JSON elements作为tokens. 使用JsonGenerator产生JSON字符串
• Tree Traversing - JSON可以被读进一个JsonNode中， 然后可以通过遍历来找到对应的属性
• Data Binding - 从JSON字符串构建Java对象

通过Streaming

// Get a list of albums from free music archive. limit the results to 5        String url = "http://freemusicarchive.org/api/get/albums.json?api_key=60BLHNQCAOUFPIBZ&limit=5";        // get an instance of the json parser from the json factory        JsonFactory factory = new JsonFactory();        JsonParser parser = factory.createParser(new URL(url));         // continue parsing the token till the end of input is reached        while (!parser.isClosed()) {            // get the token            JsonToken token = parser.nextToken();            // if its the last token then we are done            if (token == null)                break;            // we want to look for a field that says dataset             if (JsonToken.FIELD_NAME.equals(token) && "dataset".equals(parser.getCurrentName())) {                // we are entering the datasets now. The first token should be                // start of array                token = parser.nextToken();                if (!JsonToken.START_ARRAY.equals(token)) {                    // bail out                    break;                }                // each element of the array is an album so the next token                // should be {                token = parser.nextToken();                if (!JsonToken.START_OBJECT.equals(token)) {                    break;                }                // we are now looking for a field that says "album_title". We                // continue looking till we find all such fields. This is                // probably not a best way to parse this json, but this will                // suffice for this example.                while (true) {                    token = parser.nextToken();                    if (token == null)                        break;                    if (JsonToken.FIELD_NAME.equals(token) && "album_title".equals(parser.getCurrentName())) {                        token = parser.nextToken();                        System.out.println(parser.getText());                    }                 }             }         }     }

通过JsonNode

Jackson通过com.fasterxml.jackson.databind.JsonNode提供一个树形的node， 类似XML DOM树中的node.

// create an ObjectMapper instance.        ObjectMapper mapper = new ObjectMapper();        // use the ObjectMapper to read the json string and create a tree        JsonNode node = mapper.readTree(genreJson);         // lets see what type the node is        System.out.println(node.getNodeType()); // prints OBJECT        // is it a container        System.out.println(node.isContainerNode()); // prints true        // lets find out what fields it has        Iterator
    
      fieldNames = node.fieldNames();        while (fieldNames.hasNext()) {            String fieldName = fieldNames.next();            System.out.println(fieldName);// prints title, message, errors,                                            // total,                                            // total_pages, page, limit, dataset        }
    

通过Data Binding

String url = "http://freemusicarchive.org/api/get/albums.json?api_key=60BLHNQCAOUFPIBZ&limit=2";        ObjectMapper mapper = new ObjectMapper();        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);        Albums albums = mapper.readValue(new URL(url), Albums.class);

对于简单类型的List和Map， Jackson可以直接转换

Map
    
      scoreByName = mapper.readValue(jsonSource, Map.class);List
     
       names = mapper.readValue(jsonSource, List.class);
     
    

但是对于POJO类型的List和Map需要指定具体类型

Map
    
      results = mapper.readValue(jsonSource,   new TypeReference
     
    

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