作者 | zhouweixin

来源 | urlify.cn/iEbiAz

66套java从入门到精通实战课程分享

1 相关概念

1. 序列化: 把对象转换为字节序列的过程称为对象的序列化
2. 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

1. 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
2. 用于网络传送对象
3. 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

1. 首先需要有一个待序列化对象, 本例中的student对象
2. 创建一个对象映射器, jackson包下的ObjectMapper
3. 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 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

1. 示例中调用了方法writerWithDefaultPrettyPrinter, 美化了json的格式
2. 否则将打印
3. [{"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

@JsonValuepublic 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

1. 修改返回值类型为int
2. 调用父类的getOrdinal方法

示例代码

@JsonValuepublic 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?

1. 在Student的teacher属性上添加注解JsonUnwrapped, 意为不包裹
2. 在Teacher的属性上利用注解JsonProperty重命名

示例代码

Student.java

@JsonUnwrappedprivate 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?

1. 创建自定义年龄序列化器AgeSerializer, 继承StdSerializer<>
   1. 创建AgeSerializer的构造方法
   2. 重写serialize函数
2. 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 {    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

1. 首先需要有序列化好的数据, 可以是string, byte[], 文件二进制等
2. 创建一个对象映射器, jackson包下的ObjectMapper
3. 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

1. 为了便于打印对象数据, 重写了Student和Teacher的toString方法
2. 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

1. 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

1. 创建自定义反序列化器AgeDeserializer, 继承StdDeserializer<>
2. 重写deserialize方法
3. 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 {    @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:falseInclude.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

1. 当属性为默认值, 即零值时, 不序列化
2. 常见的零值:
   1. int: 0
   2. bool: false,
   3. 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

@JsonIgnoreprivate String name;

@JsonIgnoreprivate String name;