jackson序列化与反序列化的应用实践
源码地址:
https://github.com/zhouweixin/serializable
1 相关概念
- 序列化: 把对象转换为字节序列的过程称为对象的序列化
- 反序列化: 把字节序列恢复为对象的过程称为对象的反序列化
2 序列化的作用
- 用于把内存中的对象状态保存到一个文件中或者数据库中
- 用于网络传送对象
- 用于远程调用传输对象
3 准备序列化对象
准备了两个类, 教师类和学生类, 其中一个学生只有一个教师
这里省略了构造方法和setter, getter方法
Teacher.java
public class Teacher {
private String name;
private Integer age;
}
Student.java
package org.zwx;
public class Student {
private String name;
private Integer age;
private Sex sex;
private String fatherName;
private Date bornTime;
private Teacher teacher;
}
Sex.java
public enum Sex {
MALE("男"), FEMALE("女");
private String name;
Sex(String name) {
this.name = name;
}
public String getName() {
return name;
}
}
4 引入jackson依赖
本示例是基于gradle的, 从maven中心仓库中选择了2.11.2版本的jackson-databind
compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.11.2'
5 序列化与格式化输出
5.1 流程
- 首先需要有一个待序列化对象, 本例中的student对象
- 创建一个对象映射器, jackson包下的ObjectMapper
- 调用序列化函数, 本例中的writeValueAsString, 将对象转为字符串, 便于展示
5.2 代码
public void testSerializable() throws IOException {
Student student1 = new Student("小明", 18, Sex.MALE, "王富贵", new Date(), new Teacher("李老师", 40));
Student student2 = new Student("小花", 16, Sex.FEMALE, "钱很多", new Date(), new Teacher("赵老师", 38));
List<Student> students = new ArrayList<>();
students.add(student1);
students.add(student2);
ObjectMapper mapper = new ObjectMapper();
String s = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(students);
System.out.println(s);
}
5.3 结果
[ {
"name" : "小明",
"age" : 18,
"sex" : "MALE",
"fatherName" : "王富贵",
"bornTime" : 1599996926917,
"teacher" : {
"name" : "李老师",
"age" : 40
}
}, {
"name" : "小花",
"age" : 16,
"sex" : "FEMALE",
"fatherName" : "钱很多",
"bornTime" : 1599996926917,
"teacher" : {
"name" : "赵老师",
"age" : 38
}
} ]
5.4 分析
- 示例中调用了方法writerWithDefaultPrettyPrinter, 美化了json的格式
- 否则将打印
[{"name":"小明","age":18,"sex":"MALE","fatherName":"王富贵","bornTime":1599997061097,"teacher":{"name":"李老师","age":40}},{"name":"小花","age":16,"sex":"FEMALE","fatherName":"钱很多","bornTime":1599997061097,"teacher":{"name":"赵老师","age":38}}]
6 自定义序列化的名字
6.1 场景
假如需要将序列化的json由驼峰命名修改为下划线命名, 如fatherName修改为father_name
只需要在字段fatherName上用注解JsonProperty配置
6.2 示例代码
@JsonProperty("father_name")
private String fatherName;
@JsonProperty("born_time")
private Date bornTime;
6.3 示例结果
[ {
"name" : "小明",
"age" : 18,
"sex" : "MALE",
"teacher" : {
"name" : "李老师",
"age" : 40
},
"father_name" : "王富贵",
"born_time" : 1599997157609
}, {
"name" : "小花",
"age" : 16,
"sex" : "FEMALE",
"teacher" : {
"name" : "赵老师",
"age" : 38
},
"father_name" : "钱很多",
"born_time" : 1599997157610
} ]
7 自定义输出格式
7.1 bornTime格式设置
当前bornTime的格式为unix时间戮, 可读性非常差
现修改为yyyy-MM-dd HH:mm:ss
并设置时区为东八区
示例代码
@JsonProperty("born_time")
@JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
private Date bornTime;
结果
[ {
"name" : "小明",
"age" : 18,
"sex" : "MALE",
"teacher" : {
"name" : "李老师",
"age" : 40
},
"father_name" : "王富贵",
"born_time" : "2020-09-13 19:50:47"
}, {
"name" : "小花",
"age" : 16,
"sex" : "FEMALE",
"teacher" : {
"name" : "赵老师",
"age" : 38
},
"father_name" : "钱很多",
"born_time" : "2020-09-13 19:50:47"
} ]
7.2 sex设置为中文
只需要为Sex添加一个方法getOrdinal, 并添加注解JsonValue即可
示例代码
@JsonValue
public String getOrdinal() {
return name;
}
示例结果
[ {
"name" : "小明",
"age" : 18,
"sex" : "男",
"teacher" : {
"name" : "李老师",
"age" : 40
},
"father_name" : "王富贵",
"born_time" : "2020-09-13 19:57:47"
}, {
"name" : "小花",
"age" : 16,
"sex" : "女",
"teacher" : {
"name" : "赵老师",
"age" : 38
},
"father_name" : "钱很多",
"born_time" : "2020-09-13 19:57:47"
} ]
7.3 sex设置为序号
有些场景喜欢用0和1等序号设置男女, 即枚举的序号: 0表示男, 1表示女
此时需要修改Set的getOrdinal方法
- 修改返回值类型为int
- 调用父类的getOrdinal方法
示例代码
@JsonValue
public int getOrdinal() {
return super.ordinal();
}
示例结果
[ {
"name" : "小明",
"age" : 18,
"sex" : 0,
"teacher" : {
"name" : "李老师",
"age" : 40
},
"father_name" : "王富贵",
"born_time" : "2020-09-13 20:01:44"
}, {
"name" : "小花",
"age" : 16,
"sex" : 1,
"teacher" : {
"name" : "赵老师",
"age" : 38
},
"father_name" : "钱很多",
"born_time" : "2020-09-13 20:01:44"
} ]
8 拍平嵌套类型
场景
如前面提到的结果所示, teacher的两个属性并不在student的第一层,
有时可能会更深的层次, 使用起来不太友好
如何用teacher_name和teacher_age两个属性代替teacher呢?
- 在Student的teacher属性上添加注解JsonUnwrapped, 意为不包裹
- 在Teacher的属性上利用注解JsonProperty重命名
示例代码
Student.java
@JsonUnwrapped
private Teacher teacher;
Teacher.java
@JsonProperty("teacher_name")
private String name;
@JsonProperty("teacher_age")
private Integer age;
示例结果
[ {
"name" : "小明",
"age" : 18,
"sex" : 0,
"teacher_name" : "李老师",
"teacher_age" : 40,
"father_name" : "王富贵",
"born_time" : "2020-09-13 20:21:53"
}, {
"name" : "小花",
"age" : 16,
"sex" : 1,
"teacher_name" : "赵老师",
"teacher_age" : 38,
"father_name" : "钱很多",
"born_time" : "2020-09-13 20:21:53"
} ]
9 自定义序列化器
9.1 场景
假如需要将年龄调整为理论学龄, 即将年龄减去7, 得到理论学龄, 如何操作呢?
- 创建自定义年龄序列化器AgeSerializer, 继承StdSerializer<>
- 创建AgeSerializer的构造方法
- 重写serialize函数
- 利用注解修指定Student属性age的序列化器AgeSerializer
9.2 示例代码
AgeSerializer.java
public class AgeSerializer extends StdSerializer<Integer> {
protected AgeSerializer() {
super(Integer.class);
}
@Override
public void serialize(Integer value, JsonGenerator gen, SerializerProvider provider) throws IOException {
gen.writeNumber(value - 7);
}
}
Student.java
@JsonSerialize(using = AgeSerializer.class)
private Integer age;
9.3 示例结果
[ {
"name" : "小明",
"age" : 11,
"sex" : 0,
"teacher_name" : "李老师",
"teacher_age" : 40,
"father_name" : "王富贵",
"born_time" : "2020-09-13 20:31:59"
}, {
"name" : "小花",
"age" : 9,
"sex" : 1,
"teacher_name" : "赵老师",
"teacher_age" : 38,
"father_name" : "钱很多",
"born_time" : "2020-09-13 20:31:59"
} ]
10 反序列化
10.1 流程
- 首先需要有序列化好的数据, 可以是string, byte[], 文件二进制等
- 创建一个对象映射器, jackson包下的ObjectMapper
- 调用反序列化函数, 本例中的readValue, 将字符串转为对象
10.2 反序列化对象数据
示例代码
public void testDeserializable() throws JsonProcessingException {
String s = "{"name":"小明","age":11,"sex":0,"teacher_name":"李老师","teacher_age":40,"father_name":"王富贵","born_time":"2020-09-13 20:46:10"}";
ObjectMapper mapper = new ObjectMapper();
Student student = mapper.readValue(s, Student.class);
System.out.println(student);
}
示例结果
Student{name='小明', age=11, sex=MALE, fatherName='王富贵', bornTime=Sun Sep 13 20:46:10 CST 2020, teacher=Teacher{name='李老师', age=40}}
分析
- 为了便于打印对象数据, 重写了Student和Teacher的toString方法
- 从数据中可以看出, age的结果是错误的, 原因在于之前自定义的序列化器将年龄减小了7, 10.4节将会通过自定义反序列化器来解决此问题
10.3 反序列化对象数组数据
示例代码
public void testDeserializableStudents() throws JsonProcessingException {
String s = "[{"name":"小明","age":11,"sex":0,"teacher_name":"李老师","teacher_age":40,"father_name":"王富贵","born_time":"2020-09-13 20:51:31"},{"name":"小花","age":9,"sex":1,"teacher_name":"赵老师","teacher_age":38,"father_name":"钱很多","born_time":"2020-09-13 20:51:31"}]";
ObjectMapper mapper = new ObjectMapper();
Student[] students = mapper.readValue(s, Student[].class);
for (Student student : students) {
System.out.println(student);
}
}
示例结果
Student{name='小明', age=11, sex=MALE, fatherName='王富贵', bornTime=Sun Sep 13 20:51:31 CST 2020, teacher=Teacher{name='李老师', age=40}}
Student{name='小花', age=9, sex=FEMALE, fatherName='钱很多', bornTime=Sun Sep 13 20:51:31 CST 2020, teacher=Teacher{name='赵老师', age=38}}
分析
- readValue的第二个参数需要传类型, 这里推荐用数组, 不推荐用List, 具体原因笔者目前也没花时间去研究
10.4 自定义反序列化器
从10.2节及10.3的现象中可以看出来, 仅仅自定义的序列化器会导致序列化的过程是正常的, 反序列化的过程仍然是默认逻辑, 有时候会导致意想不到的结果
遇到此场景, 可以考虑自定义反序列化器
- 创建自定义反序列化器AgeDeserializer, 继承StdDeserializer<>
- 重写deserialize方法
- 在Student的age属性上添加注解JsonDeserialize, 并指定反序列化器AgeDeserializer
示例代码
AgeDeserializer.java
public class AgeDeserializer extends JsonDeserializer<Integer> {
@Override
public Integer deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
return p.getIntValue() + 7;
}
}
Student.java
@JsonSerialize(using = AgeSerializer.class)
@JsonDeserialize(using = AgeDeserializer.class)
private Integer age;
示例结果
Student{name='小明', age=18, sex=MALE, fatherName='王富贵', bornTime=Sun Sep 13 20:51:31 CST 2020, teacher=Teacher{name='李老师', age=40}}
Student{name='小花', age=16, sex=FEMALE, fatherName='钱很多', bornTime=Sun Sep 13 20:51:31 CST 2020, teacher=Teacher{name='赵老师', age=38}}
11 注解JsonInclude
该注解使用在实体类上, 格式@JsonInclude(value = JsonInclude.Include.NON_DEFAULT)
其中, Include有7种参数, 功能对比如下
| 参数 | 功能 | 备注 |
|---|---|---|
| Include.ALWAYS | 属性总是序列化(需要有get方法) | 默认值 |
| Include.NON_DEFAULT | 属性为默认值不序列化 | 如: int:0, bool:false |
| Include.NON_EMPTY | 属性为空("")或null不序列化 | |
| Include.NON_NULL | 属性为null不序列化 | |
| Include.CUSTOM | ||
| Include.USE_DEFAULTS | ||
| Include.NON_ABSENT |
代码示例
Student.java
@JsonInclude(value = JsonInclude.Include.NON_DEFAULT)
public class Student {
public void testNonDefault() throws IOException {
Student student = new Student("", 0, null, null, null, null);
ObjectMapper mapper = new ObjectMapper();
String s = mapper.writeValueAsString(student);
System.out.println(s);
}
示例输出
{
"name" : "",
"age" : -7
}
分析
- 当属性为默认值, 即零值时, 不序列化
- 常见的零值:
- int: 0
- bool: false,
- String: null
12 注解JsonIgnoreProperties
该注解为类注解, 配置忽略序列化和反序列化的字段名
如下所示, 忽略字段name和age
@JsonIgnoreProperties(value = {"name", "age"})
@JsonInclude(value = JsonInclude.Include.NON_DEFAULT)
public class Student {
13 注解JsonIgnore
该注解为属性注解, 表示忽略当前属性, 如下所示, 表示忽略name字段
@JsonIgnore
private String name;