Jackson2 – 将 Java 对象转换为 JSON，并将 JSON 字符串转换为对象

原文： https://howtodoinjava.com/jackson/jackson-2-convert-json-to-from-java-object/

在本教程中，我们将学习使用 Jackson2 库将 JSON 转换为 Java 对象 – 和将 Java 对象转换为 JSON。

Table of Contents
Jackson2 maven dependency
Convert Java object to JSON
Convert JSON to Java object

1.Jackson2 Maven 依赖

要将 Jackson2 库包含在您的项目中，请在pom.xml中包含以下依赖项。

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.6.4</version>
</dependency>

您可以从 maven 站点找到可下载的 jar 文件。

2.将 Java 对象转换为 JSON 的示例

在简单的 pojo 对象之间进行转换时，Jackson 非常直接。 通常只涉及两个步骤，

• 创建com.fasterxml.jackson.databind.ObjectMapper的实例
• 使用objectMapper.writeValueAsString()方法将 pojo 转换为 JSON

//Simple POJO
Employee emp = new Employee(1, "Lokesh Gupta", 34, "India");
//Object mapper instance
ObjectMapper mapper = new ObjectMapper();
//Convert POJO to JSON
String json = mapper.writeValueAsString(emp);

2.1 将 Java 对象转换为 JSON 的 Java 程序

package com.howtodoinjava.jackson2.example;
import java.io.IOException;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.howtodoinjava.jackson2.example.pojo.Employee;
public class FromJavaPojoToJSON 
{
    public static void main(String[] args) 
    {
        Employee emp = new Employee(1, "Lokesh Gupta", 34, "India");
        ObjectMapper mapper = new ObjectMapper();
        try 
        {
            String json = mapper.writeValueAsString(emp);
            System.out.println(json);
            //Use pretty print for printing the output
            String beutifulJson = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(emp);
            System.out.println(beutifulJson);
        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Output:
{"id":1,"name":"Lokesh Gupta","age":34,"location":"India"}
{
  "id" : 1,
  "name" : "Lokesh Gupta",
  "age" : 34,
  "location" : "India"
}

这里使用的 POJO 对象是：

package com.howtodoinjava.jackson2.example.pojo;
public class Employee
{
    private Integer id;
    private String name;
    private Integer age;
    private String location;
    public Employee(Integer id, String name, Integer age, String location) {
        super();
        this.id = id;
        this.name = name;
        this.age = age;
        this.location = location;
    }
    //Setters and Getters will be added here
    @Override
    public String toString() {
        return "Employee [id=" + id + ", name=" + name + ", age=" + age
                + ", location=" + location + "]";
    }
}

3.将 JSON 转换为 Java 对象的示例

就像上面的示例一样，从 JSON 到 POJO 的转换也仅涉及两个步骤，

• 创建com.fasterxml.jackson.databind.ObjectMapper的实例
• 使用objectMapper.readValue()方法将 JSON 转换为 POJO

例如：

//JSON input
String json = "{\"id\":1,\"name\":\"Lokesh Gupta\",\"age\":34,\"location\":\"India\"}";
//Object mapper instance
ObjectMapper mapper = new ObjectMapper();
//Convert JSON to POJO
Employee emp = mapper.readValue(json, Employee.class);

我们可以在 Java 中读取 json 文件，并将其传递给 jackson 库。

3.1 将 JSON 转换为 Java 对象的 Java 程序

package com.howtodoinjava.jackson2.example;
import java.io.IOException;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.howtodoinjava.jackson2.example.pojo.Employee;
public class FromJSONToPOJO {
    public static void main(String[] args) {
        String json = "{\"id\":1,\"name\":\"Lokesh Gupta\",\"age\":34,\"location\":\"India\"}";
        ObjectMapper mapper = new ObjectMapper();
        try 
        {
            Employee emp = mapper.readValue(json, Employee.class);
            System.out.println(emp);
        } 
        catch (JsonGenerationException e) 
        {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

程序输出。

Employee [id=1, name=Lokesh Gupta, age=34, location=India]

请不要忘记在 POJO 类中添加默认的构造器，否则会出现此错误：

com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type [simple type, class com.howtodoinjava.jackson2.example.pojo.Employee]: can not instantiate from JSON object (missing default constructor or creator, or perhaps need to add/enable type information?)
 at [Source: {"id":1,"name":"Lokesh Gupta","age":34,"location":"India"}; line: 1, column: 2]
    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:148)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1106)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:296)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:133)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3736)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2726)
    at com.howtodoinjava.jackson2.example.FromJSONToPOJO.main(FromJSONToPOJO.java:18)

以上就是使用 Jackson2 库将 Java 对象转换为 JSON 并将 JSON 字符串转换为 Java 对象的所有示例。

让我知道您是否对如何将 json 对象转换为 java 对象有任何疑问。

学习愉快！