JSON之Jackson（二）

川长思鸟来 2022-07-29 11:53 188阅读 0赞

[JSON之Jackson（一）][JSON_Jackson]

[JSON之Jackson（二）][JSON_Jackson 1]

# 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：

1.  START\_OBJECT
2.  END\_OBJECT
3.  START\_ARRAY
4.  END\_ARRAY
5.  FIELD\_NAME
6.  VALUE\_EMBEDDED\_OBJECT
7.  VALUE\_FALSE
8.  VALUE\_TRUE
9.  VALUE\_NULL
10. VALUE\_STRING
11. VALUE\_NUMBER\_INT
12. 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 file
    public 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][https_github.com_GatsbyNewton_jackson]

[JSON_Jackson]: http://blog.csdn.net/u010376788/article/details/51169888
[JSON_Jackson 1]: http://blog.csdn.net/u010376788/article/details/51171853
[https_github.com_GatsbyNewton_jackson]: https://github.com/GatsbyNewton/jackson