对象与json字符串相互转化

在java编程中，json字符串和对象的相互转化十分常用，下面我们就对象如何转化为json字符串以及json字符串如何转化为对象进行简要介绍，以便在代码中能方便使用。

1、依赖

---

本次介绍的方法依赖jackson，这是非常通用的json工具。

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.8</version>
</dependency>

2、实体

---

下面我们编写一个稍微复杂的实体，以便进行测试。

package com.yefengyu.utils;

import java.util.Date;


public class Car
{
    private Integer id;

    private String brand;

    private Date date;

    public Car()
    {
    }

    public Car(Integer id, String brand, Date date)
    {
        this.id = id;
        this.brand = brand;
        this.date = date;
    }

    public Integer getId()
    {
        return id;
    }

    public void setId(Integer id)
    {
        this.id = id;
    }

    public String getBrand()
    {
        return brand;
    }

    public void setBrand(String brand)
    {
        this.brand = brand;
    }

    public Date getDate()
    {
        return date;
    }

    public void setDate(Date date)
    {
        this.date = date;
    }

    @Override
    public String toString()
    {
        return "Car{" +
               "id=" + id +
               ", brand='" + brand + '\'' +
               ", date=" + date +
               '}';
    }
}

上面编写了一个Car类，下面再编写一个Person类。

package com.yefengyu.utils;

public class Person
{
    private Integer id;

    private String name;

    private String email;

    private Car car;

    public Person()
    {
    }

    public Person(Integer id, String name, String email, Car car)
    {
        this.id = id;
        this.name = name;
        this.email = email;
        this.car = car;
    }

    public Integer getId()
    {
        return id;
    }

    public void setId(Integer id)
    {
        this.id = id;
    }

    public String getName()
    {
        return name;
    }

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

    public String getEmail()
    {
        return email;
    }

    public void setEmail(String email)
    {
        this.email = email;
    }

    public Car getCar()
    {
        return car;
    }

    public void setCar(Car car)
    {
        this.car = car;
    }

    @Override
    public String toString()
    {
        return "Person{" +
               "id=" + id +
               ", name='" + name + '\'' +
               ", email='" + email + '\'' +
               ", car=" + car +
               '}';
    }
}

注意的是：必须有无参构造函数。若无则在字符串转为对象的时候：

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.yefengyu.utils.Person` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
 at [Source: (String)"{"id":1,"name":"yefengyu","email":"[e-mail]","car":{"id":1,"brand":"Rolls-Royce","date":"2019-06-22 13:25:30"}}"; line: 1, column: 2]

3、工具

---

下面的代码是一个工具类，实现了Json字符串到对象的相互转化过程。

package com.yefengyu.utils;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;
import java.text.SimpleDateFormat;

//使用final关键字防止继承
public final class JsonSerializer
{
    private static final ObjectMapper MAPPER;

    static
    {
        MAPPER = new ObjectMapper();
        MAPPER.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));//日期格式化
     }

    private JsonSerializer()
    {
        //防止new对象
     }

    //对象转化为json字符串
     public static String toJson(Object object)
    {
        try
        {
            return MAPPER.writeValueAsString(object);
        }
        catch (JsonProcessingException e)
        {
            e.printStackTrace();
        }
        return null;
    }

    //对象转化为json字符串，并格式化输出
     public static String toPrettyJson(Object object)
    {
        try
        {
            return MAPPER.writerWithDefaultPrettyPrinter().writeValueAsString(object);
        }
        catch (JsonProcessingException e)
        {
            e.printStackTrace();
        }
        return null;
    }

    //字符串转为单个对象
     public static <T> T fromJson(String json, Class<T> clazz)
    {
        if (json == null || json.trim().isEmpty() || clazz == null)
        {
            return null;
        }

        try
        {
            return MAPPER.readValue(json, clazz);
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
        return null;
    }

    //字符串转为对象，比如对象列表
     public static <T> T fromJson(String json, TypeReference<T> typeReference)
    {
        if (json == null || json.trim().isEmpty() || typeReference == null)
        {
            return null;
        }

        try
        {
            return MAPPER.readValue(json, typeReference);
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
        return null;
    }
}

4、测试

---

1、单个对象转化为json字符串

Car car = new Car(1,"Rolls-Royce",new Date());
Person person = new Person(1, "yefengyu", "[e-mail]", car);
System.out.println(JsonSerializer.toJson(person));
System.out.println(JsonSerializer.toPrettyJson(person));

结果为：

{"id":1,"name":"yefengyu","email":"[e-mail]","car":{"id":1,"brand":"Rolls-Royce","date":"2019-06-22 13:25:30"}}
{ "id" : 1, "name" : "yefengyu", "email" : "[e-mail]", "car" : { "id" : 1, "brand" : "Rolls-Royce", "date" : "2019-06-22 13:25:30"
 } }

2、集合对象转化为json字符串

Car car = new Car(1,"Rolls-Royce",new Date());

List<Person> personList = new ArrayList<>();

Person p1 = new Person(1, "yefengyu", "[e-mail]",car);
Person p2 = new Person(2, "yefengyu", "[e-mail]",car);
Person p3 = new Person(3, "yefengyu", "[e-mail]",car);

personList.add(p1);
personList.add(p2);
personList.add(p3);

System.out.println(JsonSerializer.toJson(personList));
System.out.println(JsonSerializer.toPrettyJson(personList));

结果为：

[{"id":1,"name":"yefengyu","email":"[e-mail]","car":{"id":1,"brand":"Rolls-Royce","date":"2019-06-22 13:33:10"}},{"id":2,"name":"yefengyu","email":"[e-mail]","car":{"id":1,"brand":"Rolls-Royce","date":"2019-06-22 13:33:10"}},{"id":3,"name":"yefengyu","email":"[e-mail]","car":{"id":1,"brand":"Rolls-Royce","date":"2019-06-22 13:33:10"}}]
[ { "id" : 1, "name" : "yefengyu", "email" : "[e-mail]", "car" : { "id" : 1, "brand" : "Rolls-Royce", "date" : "2019-06-22 13:33:10"
 } }, { "id" : 2, "name" : "yefengyu", "email" : "[e-mail]", "car" : { "id" : 1, "brand" : "Rolls-Royce", "date" : "2019-06-22 13:33:10"
 } }, { "id" : 3, "name" : "yefengyu", "email" : "[e-mail]", "car" : { "id" : 1, "brand" : "Rolls-Royce", "date" : "2019-06-22 13:33:10"
 } } ]

3、字符串转为单个对象

String json = "{\"id\":1,\"name\":\"yefengyu\",\"email\":\"[e-mail]\",\"car\":{\"id\":1,\"brand\":\"Rolls-Royce\",\"date\":\"2019-06-22 13:25:30\"}}";
System.out.println(JsonSerializer.fromJson(json, Person.class));

结果如下：

Person{id=1, name='yefengyu', email='[e-mail]', car=Car{id=1, brand='Rolls-Royce', date=Sat Jun 22 13:25:30 CST 2019}}

4、字符串转为对象集合

String list = "[{\"id\":1,\"name\":\"yefengyu\",\"email\":\"[e-mail]\",\"car\":{\"id\":1,\"brand\":\"Rolls-Royce\",\"date\":\"2019-06-22 13:33:10\"}},{\"id\":2,\"name\":\"yefengyu\",\"email\":\"[e-mail]\",\"car\":{\"id\":1,\"brand\":\"Rolls-Royce\",\"date\":\"2019-06-22 13:33:10\"}},{\"id\":3,\"name\":\"yefengyu\",\"email\":\"[e-mail]\",\"car\":{\"id\":1,\"brand\":\"Rolls-Royce\",\"date\":\"2019-06-22 13:33:10\"}}]";

List<Person> persons = JsonSerializer.fromJson(list, new TypeReference<List<Person>>()
{
});

for (Person person : persons)
{
    System.out.println(person);
}

结果如下：

Person{id=1, name='yefengyu', email='[e-mail]', car=Car{id=1, brand='Rolls-Royce', date=Sat Jun 22 13:33:10 CST 2019}}
Person{id=2, name='yefengyu', email='[e-mail]', car=Car{id=1, brand='Rolls-Royce', date=Sat Jun 22 13:33:10 CST 2019}}
Person{id=3, name='yefengyu', email='[e-mail]', car=Car{id=1, brand='Rolls-Royce', date=Sat Jun 22 13:33:10 CST 2019}}