JSON之Jackson（二）

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JSON之Jackson（一）

JSON之Jackson（二）

1.Jackson JsonParser

Jackson JsonParser比Jackson ObjectMapper更底层，这就使得Jackson JsonParser比ObjectMapper更快，但是也更加笨重。Jackson JsonParser的工作方式是：将JSON分成一个记号序列，让你迭代记号（Token）序列进行解析。Jackson Parser的Token有：

START_OBJECT
END_OBJECT
START_ARRAY
END_ARRAY
FIELD_NAME
VALUE_EMBEDDED_OBJECT
VALUE_FALSE
VALUE_TRUE
VALUE_NULL
VALUE_STRING
VALUE_NUMBER_INT
VALUE_NUMBER_FLOAT

这些Token在JsonToken类中，通过nextToken()获取。

getCurrentName()获取当前Field Name，而Value可以用getText()获取，也可以用getValueAsString()、getValueAsInt()等类似的方法获取。下面是一个简单的例子：

user.json文件：

{  "name": {    "first": "Joe",    "last": "Sixpack"  },  "gender": "MALE",  "verified": false,  "userImage": "Rm9vYm"}

示例代码：

public static void readFromFile(){String path = "F:\\Codes\\IDEA\\Tools\\data\\json\\user.json";//        String carJson = "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";JsonFactory jsonFactory = new JsonFactory();User user = new User();try {JsonParser parser = jsonFactory.createParser(new File(path));parser.nextToken();    //进入JSON的“{”while (parser.nextToken() != JsonToken.END_OBJECT){String fieldName = parser.getCurrentName();parser.nextToken();if(fieldName.equals("name")) {System.out.println(fieldName + ":");while (parser.nextToken() != JsonToken.END_OBJECT) {parser.nextToken();String field = parser.getCurrentName();System.out.println("\t" + field + ": " + parser.getValueAsString());}}else {System.out.println(fieldName + ": " + parser.getText());}}parser.close();}catch (Exception e){e.printStackTrace();}}

同Jackson ObjectMapper一样，你也可以解析String、Reader、InputStream、URL、byte数组、char数组。

String carJson =        "{ \"brand\" : \"Mercedes\", \"doors\" : 5 }";JsonFactory factory = new JsonFactory();JsonParser  parser  = factory.createParser(carJson);

2.Jackson Generator

Jackson Generator用于生成JSON。对于简单的变量这种数据类型，Jackson Generator和Jackson JsonParser一样从JsonFactory中创建。如：

// Write simple variables to filepublic static void writeSimpleToFile(){JsonFactory jsonFactory = new JsonFactory();OutputStream outputStream = null;//        Car car = new Car("BMW", 4, new Car.Owner("Gatsby", "Newton"), new String[]{"engine", "brake"});try {outputStream = new FileOutputStream("F:\\Codes\\IDEA\\Tools\\data\\json\\out\\generate_simple.json");JsonGenerator generator = jsonFactory.createGenerator(outputStream, JsonEncoding.UTF8);generator.writeStartObject();generator.writeStringField("brand", "Mercedes");generator.writeNumberField("doors", 5);generator.writeObjectFieldStart("owner");generator.writeStringField("first", "Gatsby");generator.writeStringField("last", "Newton");generator.writeEndObject();generator.writeArrayFieldStart("component");generator.writeString("engine");generator.writeString("brake");generator.writeEndArray();// Write a object.//            generator.writeObjectField("owner", new Car.Owner("Gatsby", "Newton"));//            generator.writeObjectField("component", new String[]{"engine", "brake"});generator.writeEndObject();generator.flush();generator.close();}catch (Exception e){e.printStackTrace();}}

但是对于复杂的数据类型，Jackson Generator必须从ObjectMapper.getJsonFactory()的JsonFactory中创建，否则会报一下错误：

java.lang.IllegalStateException: No ObjectCodec defined for the generator,can only serialize simple wrapper types (type passed edu.wzm.jackson.Car$Owner)

其中，ObjectMapper.getJsonFactory()方法被标记为deprecated，不过这并没有关系，这种情况在Java编程中经常遇到，这是可以用的。但是对于那种建议不要使用的方法，最好弃之不用。

// Write objects to file.public static void writeComplexToFile(){// If you use JsonFactory jsonFactory = new JsonFactory(), it shows the error:// java.lang.IllegalStateException: No ObjectCodec defined for the generator,// can only serialize simple wrapper types (type passed edu.wzm.jackson.Car$Owner)JsonFactory jsonFactory = new ObjectMapper().getJsonFactory();OutputStream outputStream = null;//        Car car = new Car("BMW", 4, new Car.Owner("Gatsby", "Newton"), new String[]{"engine", "brake"});try {outputStream = new FileOutputStream("F:\\Codes\\IDEA\\Tools\\data\\json\\out\\generate_complex.json");JsonGenerator generator = jsonFactory.createGenerator(outputStream, JsonEncoding.UTF8);generator.writeStartObject();generator.writeStringField("brand", "Mercedes");generator.writeNumberField("doors", 5);// Write a object.generator.writeObjectField("owner", new Car.Owner("Gatsby", "Newton"));generator.writeObjectField("component", new String[]{"engine", "brake"});generator.writeEndObject();generator.flush();generator.close();}catch (Exception e){e.printStackTrace();}}

例子的源代码托管在：https://github.com/GatsbyNewton/jackson

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