java之json字符串和对象之间的互转（jackson的使用）

墨蓝 2022-10-02 00:32 627阅读 0赞

上一篇博文我们使用json-lib 来对json字符串和java对象进行互转

[https://blog.csdn.net/liuying1802028915/article/details/92068465][https_blog.csdn.net_liuying1802028915_article_details_92068465]

这篇博文我们来使用jackson来对json字符串和java 对象进行互转

这里引用一下网上对jackson的介绍

[https://www.dubby.cn/detail.html?id=9068][https_www.dubby.cn_detail.html_id_9068]

Jackson作为一个知名，并且经典的Java平台的JSON库，有人说它是最好的JSON解析工具，或者只是一个简单的还行的JSON解析库，不管如何Jackson都是一个值得一学，值得一用的JSON处理库。

Jackson不仅支持流式处理json，还支持数据绑定（POJO和JSON之间的相互转化），甚至还拓展了很多其他第三方库所支持的数据格式（如：[Avro][], [BSON][], [CBOR][], [CSV][], [Smile][], [(Java) Properties][Java_ Properties], [Protobuf][], [XML][]和[YAML][]，[Guava][], [Joda][], [PCollections][]）。

**Jackson的三个核心模块：**

*  `jackson-core`:定义了低级的流式API，包括了JSON处理细节。
 *  `jackson-annotations`:包含了Jackson的注解。
 *  `jackson-databind`:实现了对象和JSON之间的转换，这个包依赖上面两个包。

以上是摘抄来的 ，出自[https://www.dubby.cn/detail.html?id=9068][https_www.dubby.cn_detail.html_id_9068]

前段时间在工作中我遇到了一个这样的问题，就是老大让我对我们项目中 jacason-databind.2.8.11.jar 2.8.11以下版本进行升级，升级到2.8.11，但是升级完之后，导致产品图表数据出不来，进过排查发现是后端转换的json数据跟之前的有差别，之前是用的是比2.8.11低的版本jar包，现在升级到2.8.11之后，该jar包对安全性做了提高，但是改了其中的一些内容，具体内容还没搞清楚，当时这个问题是老大解决的，所以这次就对jackson做个练习。

因为最近在研究json字符串和java对象的互转，这次就使用jackson 来进行 json的序列化和反序列化

一.首先新建一个maven项目，这里不再赘述

二。在项目的pom文件中假如对jackson-databind的依赖

<dependency>
          <groupId>com.fasterxml.jackson.core</groupId>
          <artifactId>jackson-databind</artifactId>
          <version>2.5.3</version>
        </dependency>

之后导入依赖包之后发现

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdXlpbmcxODAyMDI4OTE1_size_16_color_FFFFFF_t_70][]

jackson-databind 其实依赖了 jackson-core 和 jackson-annotation 这两个jar包，这也就证明了我在工作中为了升级jackson-databind包是相应的也将 jackson-core 和 jackson-annotation 相应升级

三。接下来我们来写代码

1.首先我们新建 com.taoj.entity.User 类

package com.taoj.entity;
    
    import java.util.Date;
    
    public class User {
        
        private String name;
        private Integer age;
        private Date birthday;
        private String email;
    
        public User(String name, Integer age, Date birthday, String email) {
            this.name = name;
            this.age = age;
            this.birthday = birthday;
            this.email = email;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Integer getAge() {
            return age;
        }
    
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public Date getBirthday() {
            return birthday;
        }
    
        public void setBirthday(Date birthday) {
            this.birthday = birthday;
        }
    
        public String getEmail() {
            return email;
        }
    
        public void setEmail(String email) {
            this.email = email;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    ", birthday=" + birthday +
                    ", email='" + email + '\'' +
                    '}';
        }
    }

这里写了一个User 实体类，包括有参构造函数，get set 方法 toString方法

下面写一个测试类

1》

将 java对象 转换成 json 字符串

/**
         * 将 java对象 转换成 json 字符串
         */
        public static void conventObjectToJson(){
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
            Date date = null;
            try {
                date = sdf.parse("1981-02-19");
            } catch (ParseException e) {
                e.printStackTrace();
            }
            User user = new User("梁朝伟", 25, date, "liangchaowei@qq.com");
    
            //新建ObjectMapper对象
            ObjectMapper objectMapper = new ObjectMapper();
            //使用ObjectMapper对象对 User对象进行转换
            String json = null;
            try {
                json = objectMapper.writeValueAsString(user);
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            }
            System.out.println(json);
        }

输出为：

\{"name":"梁朝伟","age":25,"birthday":351360000000,"email":"liangchaowei@qq.com"\}

2》

将 List 转换成 json 字符串

/**
         * 将 List 转换成 json 字符串
         */
        public static void conventListToJson(){
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
            Date date1 = null;
            Date date2 = null;
            try {
                date1 = sdf.parse("1987-10-01");
                date2 = sdf.parse("1989-06-06");
            } catch (ParseException e) {
                e.printStackTrace();
            }
            User user1 = new User("周润发", 23, date1, "zhourunfa@qq.com");
            User user2 = new User("刘德华", 25, date2, "Liudehua@qq.com");
            List<User> list = new ArrayList<User>();
            list.add(user1);
            list.add(user2);
    
            ObjectMapper objectMapper = new ObjectMapper();
            String listStr = null;
            try {
                listStr = objectMapper.writeValueAsString(list);
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            }
            System.out.println(listStr);
        }

输出为：\[\{"name":"周润发","age":23,"birthday":560016000000,"email":"zhourunfa@qq.com"\},\{"name":"刘德华","age":25,"birthday":613062000000,"email":"Liudehua@qq.com"\}\]

3》

将 json字符串 转换为 java对象

/**
         * 将 json字符串 转换为 java对象
         */
        public static void conventJsonToObject(){
            String json = "{\"name\":\"周润发\",\"age\":23,\"birthday\":560016000000,\"email\":\"zhourunfa@qq.com\"}";
            ObjectMapper objectMapper = new ObjectMapper();
    
            User user = null;
            try {
                user = objectMapper.readValue(json, User.class);
            } catch (IOException e) {
                e.printStackTrace();
            }
            System.out.println(user);
        }

当运行这个方法时 发现报错了

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdXlpbmcxODAyMDI4OTE1_size_16_color_FFFFFF_t_70 1][]

错误如下：

null  
com.fasterxml.jackson.databind.JsonMappingException: No suitable constructor found for type \[simple type, class com.taoj.entity.User\]: can not instantiate from JSON object (missing default constructor or creator, or perhaps need to add/enable type information?)

他说没有默认的构造函数，的确User对象，我只添加了有参构造函数，默认的无参构造函数就没了，现在给User对象添加无参构造函数

添加后的User.java 如下

package com.taoj.entity;
    
    import java.util.Date;
    
    public class User {
    
        private String name;
        private Integer age;
        private Date birthday;
        private String email;
    
        public User() {
        }
    
        public User(String name, Integer age, Date birthday, String email) {
            this.name = name;
            this.age = age;
            this.birthday = birthday;
            this.email = email;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public Integer getAge() {
            return age;
        }
    
        public void setAge(Integer age) {
            this.age = age;
        }
    
        public Date getBirthday() {
            return birthday;
        }
    
        public void setBirthday(Date birthday) {
            this.birthday = birthday;
        }
    
        public String getEmail() {
            return email;
        }
    
        public void setEmail(String email) {
            this.email = email;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    ", birthday=" + birthday +
                    ", email='" + email + '\'' +
                    '}';
        }
    }

此时运行刚刚那个方式时输出为：

User\{name='周润发', age=23, birthday=Thu Oct 01 00:00:00 CST 1987, email='zhourunfa@qq.com'\}

4》

将json 字符串 转换为 java对象

/**
         * 将json 字符串 转换为 java对象
         */
        public static void conventJsonToList(){
            String json = "[{\"name\":\"周润发\",\"age\":23,\"birthday\":560016000000,\"email\":\"zhourunfa@qq.com\"},{\"name\":\"刘德华\",\"age\":25,\"birthday\":613062000000,\"email\":\"Liudehua@qq.com\"}]";
            ObjectMapper objectMapper = new ObjectMapper();
            List<User> list = null;
            try {
                list = objectMapper.readValue(json, new TypeReference<List<User>>(){});
            } catch (IOException e) {
                e.printStackTrace();
            }
            for(User user : list){
                System.out.println(user);
            }
        }

以上是如何使用ObjectMapper对象实现对json字符串和java互转的四个方法，这里仅仅是一个练习。记录一下，防止以后忘记。

当然关于jackson-databind的学习还得继续，最好是通读一下源码这样才能真正的了解jackson的核心，加油

不积跬步，无以至千里

不积小流，无以成江海

这里引入一下 [https://blog.csdn.net/u011054333/article/details/80504154][https_blog.csdn.net_u011054333_article_details_80504154] 博文中的一个内容 ，就是

objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE); 和

objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);的用法

public static void conventObjectToJsonWithNull(){
            User user = new User("黎明", 0, null, "上海");
            ObjectMapper objectMapper = new ObjectMapper();
    //        objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
    //        objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
            try {
                //将对象转换成字符串
                String json = objectMapper.writeValueAsString(user);
                System.out.println(json);
    
                //将字符串转换成对象
                User user2 = objectMapper.readValue(json, User.class);
                System.out.println(user2);
    
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

如上方法 把

objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE); 和

objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES); 注释掉了

输出结果为：

\{"name":"黎明","age":0,"birthday":null,"email":"上海"\}  
User\{name='黎明', age=0, birthday=null, email='上海'\}

第一行为json ,故可以转换成相应的java对象

换一种方法

public static void conventObjectToJsonWithNull(){
            User user = new User("黎明", 0, null, "上海");
            ObjectMapper objectMapper = new ObjectMapper();
    //        objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
            objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
            try {
                //将对象转换成字符串
                String json = objectMapper.writeValueAsString(user);
                System.out.println(json);
    
                //将字符串转换成对象
                User user2 = objectMapper.readValue(json, User.class);
                System.out.println(user2);
    
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

如上 只把

objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE);注释掉了

输出结果为：

\{"name":"黎明","age":0,"birthday":null,"email":"上海"\}  
User\{name='黎明', age=0, birthday=null, email='上海'\}

看来是没有影响

在看下面

public static void conventObjectToJsonWithNull(){
            User user = new User("黎明", 0, null, "上海");
            ObjectMapper objectMapper = new ObjectMapper();
            objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
            objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
            try {
                //将对象转换成字符串
                String json = objectMapper.writeValueAsString(user);
                System.out.println(json);
    
                //将字符串转换成对象
                User user2 = objectMapper.readValue(json, User.class);
                System.out.println(user2);
    
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

两句都放开 输出结果为：

\{"User":\{"name":"黎明","age":0,"birthday":null,"email":"上海"\}\}  
User\{name='null', age=null, birthday=null, email='null'\}

没有将字符串转换成相应的对象 对象的值为空了

下面

public static void conventObjectToJsonWithNull(){
            User user = new User("黎明", 0, null, "上海");
            ObjectMapper objectMapper = new ObjectMapper();
            objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
    //        objectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
            try {
                //将对象转换成字符串
                String json = objectMapper.writeValueAsString(user);
                System.out.println(json);
    
                //将字符串转换成对象
                User user2 = objectMapper.readValue(json, User.class);
                System.out.println(user2);
    
            } catch (JsonProcessingException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

如上只放开

objectMapper.enable(SerializationFeature.WRAP_ROOT_VALUE);

输出结果为：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdXlpbmcxODAyMDI4OTE1_size_16_color_FFFFFF_t_70 2][]

输出报错了，这里简单认识了这两个属性，记录一下，详细的话可以看这篇

[https://blog.csdn.net/u011054333/article/details/80504154][https_blog.csdn.net_u011054333_article_details_80504154]

[https_blog.csdn.net_liuying1802028915_article_details_92068465]: https://blog.csdn.net/liuying1802028915/article/details/92068465
[https_www.dubby.cn_detail.html_id_9068]: https://www.dubby.cn/detail.html?id=9068
[Avro]: https://github.com/FasterXML/jackson-dataformat-avro
[BSON]: https://github.com/michel-kraemer/bson4jackson
[CBOR]: https://github.com/FasterXML/jackson-dataformat-cbor
[CSV]: https://github.com/FasterXML/jackson-dataformat-csv
[Smile]: https://github.com/FasterXML/jackson-dataformat-smile
[Java_ Properties]: https://github.com/FasterXML/jackson-dataformat-properties
[Protobuf]: https://github.com/FasterXML/jackson-dataformat-protobuf
[XML]: https://github.com/FasterXML/jackson-dataformat-xml
[YAML]: https://github.com/FasterXML/jackson-dataformat-yaml
[Guava]: https://github.com/FasterXML/jackson-datatype-guava
[Joda]: https://github.com/FasterXML/jackson-datatype-joda
[PCollections]: https://github.com/FasterXML/jackson-datatypes-collections
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdXlpbmcxODAyMDI4OTE1_size_16_color_FFFFFF_t_70]: /images/20220113/4d49369897c447cab2ae311b4d7f1ce0.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdXlpbmcxODAyMDI4OTE1_size_16_color_FFFFFF_t_70 1]: /images/20220113/1b3574c63b2345529f321b338e96ff89.png
[https_blog.csdn.net_u011054333_article_details_80504154]: https://blog.csdn.net/u011054333/article/details/80504154
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpdXlpbmcxODAyMDI4OTE1_size_16_color_FFFFFF_t_70 2]: /images/20220113/f33b7b30440543539e55159315a6266b.png