2021-09-26

• 什么是JSON
• • 对象格式
• Java解析JSON
• • Gson
  • fastjson
  • Jackson

JSON：Javascript Object Notation JS对象简谱，是一种轻量级的数据交换格式。

一个对象，由一个大括号表示，
括号中描述对象的属性和函数，通过键值对来描述对象的属性与函数
格式：
键和值之间用冒号连接，多个键值对用逗号分隔。
键值对的键使用引号包裹
键值对中的值可以使用JS中的任意一种数据类型

1. 用Java描述一个人

class Person{
	private String name;
	private String age;
	getter/setter/构造
}
Person p = new Person("小明",18);

2. 用js来描述一个人

var p = new Object();
p.name = '小明';
p.age = 18;

3. 用XML描述一个人

	小明
	18

4. 用JSON描述一个人

{
	"name":"小明",
	"age":18
}

对象

public class Person{
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

• 将对象转换为JSON字符串

1.引入gson.jar
2.调用Gson的toJSON(obj)

举例：

public class GsonTest {
    public static void main(String[] args) {
        Person p = new Person("小明",18);
        String s = new Gson().toJson(p);
        System.out.println(s);
    }
}

结果：

{"name":"小明","age":18}

• string转为对象
  举例：

public class GsonTest {
    public static void main(String[] args) {
        String s = "{"age":18,"name":"小明"}";
        Person person = new Gson().fromJson(s, Person.class);
        System.out.println(person);
    }
}

结果：

com.dmlll.Person@2a17b7b6

• 将对象转换为JSON字符串

1.引入fastjson.jar
2.使用fastjson的JSON.toJSONString(obj)

举例

public class FastTest {
    public static void main(String[] args) {
        Person p = new Person("小明",18);
        String string = JSON.toJSONString(p);
        System.out.println(string);
    }
}

结果：

{}

此时打印不出json字符串，原因在于FastJSON是根据对象类的getter、setter方法获得JSON，此时应该为Person类添加getter、setter方法。

public class Person{
    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

继续运行FastTest，结果：

{"age":18,"name":"小明"}

• string转为对象

public class FastTest {
    public static void main(String[] args) {
        String s = "{"name":"小明","age":18}";
        Person person = JSON.parseObject(s, Person.class);
        System.out.println(person);
    }
}

结果：

com.dmlll.Person@6f1fba17

• 将对象转换为JSON字符串

1.引入jackson-databind.jar、jackson-core.jar、jackson-annotations.jar
2.使用fastjson的JSON.toJSONString(obj)

public class JackTest {
    public static void main(String[] args) throws JsonProcessingException {
        Person p = new Person("小明",18);
        ObjectMapper objectMapper = new ObjectMapper();
        String string = objectMapper.writevalueAsString(p);
        System.out.println(string);
    }
}

结果：

{"name":"小明","age":18}

• string转为对象

public class JackTest {
    public static void main(String[] args) throws IOException {
        String s = "{"name":"小明","age":18}";
        ObjectMapper objectMapper = new ObjectMapper();
        Person person = objectMapper.readValue(s, Person.class);
        System.out.println(person);
    }
}

结果：

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.dmlll.Person` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (String)"{"name":"小明","age":18}"; line: 1, column: 2]
	at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
	at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452)
	at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerbase.deserializeFromObjectUsingNonDefault(BeanDeserializerbase.java:1297)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3004)
	at com.dmlll.JackTest.main(JackTest.java:11)

使用jackson将string转换成Person（反序列化）需要使用到无参构造

public class Person {
    private String name;
    private int age;

    public Person() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

结果：

com.dmlll.Person@290d210d