用好Jackson，操作Json节省一半时间

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mb629082748804a 2022-10-06 14:17:33

前言

目前解析Json的工具包有，Gson，FastJson，Jackson，Json-lib。综合来看，Jackson的性能较优，稳定性也比较高，而且spring-boot-starter-web默认会引入Jackson包。因此介绍一下Jackson的使用。

Jackson目前有2个版本
1.x版本包名为org.codehaus.jackson
2.x版本包名为com.fasterxml.jackson

使用

在pom中加入如下依赖即可。

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.9.2</version>
</dependency>

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.2</version>
</dependency>

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>2.9.2</version>
</dependency>

前文说过，当使用spring-boot-starter-web模块时，会默认引入Jackson包，不必在pom中再次引入上面依赖

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

序列化

将java对象序列化成json,@Data注解是用lombok插件来自动生成get和set方法

@Data
public class Student {

    /** 名字 */
    private String name;
    /** 年龄 */
    private Integer age;
    /** 头像 */
    private String profileImageUrl;
}

将常用的方法封装成一个常用的工具类，如下所示，有2个好处

ObjectMapper 类只会生成一个，节省空间
可以定义统一的配置（后面细说）

@Slf4j
public class JsonUtil {

    private static ObjectMapper objectMapper = new ObjectMapper();

    /** 将对象转为string */
    public static <T> String obj2String(T obj){

        if(obj == null){
            return null;
        }
        try {
            return obj instanceof String ? (String)obj :  objectMapper.writeValueAsString(obj);
        } catch (Exception e) {
            log.warn("Parse Object to String error",e);
            return null;
        }
    }

}

先简单介绍一下封装的工具类

1. 将对象转为json

public static <T> String obj2String(T obj){

    if(obj == null){
        return null;
    }
    try {
        return obj instanceof String ? (String)obj :  objectMapper.writeValueAsString(obj);
    } catch (Exception e) {
        log.warn("Parse Object to String error",e);
        return null;
    }
}

各种类型均可

@Test
public void obj2String() throws Exception {

    Student student = new Student();
    student.setAge(10);
    student.setName("name");
    student.setProfileImageUrl("link");
    String result = JsonUtil.obj2String(student);
    // {"name":"name","age":10,"profileImageUrl":"link"}
    System.out.println(result);

    Map<String, List<Integer>> map = new HashMap<>();
    map.put("a", Arrays.asList(1, 2, 3));
    map.put("b", Arrays.asList(1, 2, 3));
    String result1 = JsonUtil.obj2String(map);
    // {"a":[1,2,3],"b":[1,2,3]}
    System.out.println(result1);
}

2. 将对象转为json，并格式化的输出

public static <T> String obj2StringPretty(T obj){
    if(obj == null){
        return null;
    }
    try {
        return obj instanceof String ? (String)obj :  objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(obj);
    } catch (Exception e) {
        log.warn("Parse Object to String error",e);
        return null;
    }
}

@Test
public void obj2StringPretty() throws Exception {
    Student student = new Student();
    student.setAge(10);
    student.setName("name");
    student.setProfileImageUrl("link");
    String result = JsonUtil.obj2StringPretty(student);

    /*
    {
        "name" : "name",
        "age" : 10,
        "profileImageUrl" : "link"
    }
    */
    System.out.println(result);

    Map<String, List<Integer>> map = new HashMap<>();
    map.put("a", Arrays.asList(1, 2, 3));
    map.put("b", Arrays.asList(1, 2, 3));
    String result1 = JsonUtil.obj2StringPretty(map);

    /*
    {
        "a" : [ 1, 2, 3 ],
        "b" : [ 1, 2, 3 ]
    }
    */
    System.out.println(result1);
}

反序列化

将json转为java对象

方式1

public static <T> T string2Obj(String str, Class<T> clazz){
    if(StringUtils.isEmpty(str) || clazz == null){
        return null;
    }

    try {
        return clazz.equals(String.class)? (T)str : objectMapper.readValue(str, clazz);
    } catch (Exception e) {
        log.warn("Parse String to Object error",e);
        return null;
    }
}

基本类型的转换

@Test
public void string2Obj() throws Exception {
    String str = "{\"name\":\"name\",\"age\":10,\"profileImageUrl\":\"link\"}";
    Student student = JsonUtil.string2Obj(str, Student.class);
    // Student(name=name, age=10, profileImageUrl=link)
    System.out.println(student);
}

各种复杂类型的转换，示例1

@Test
public void string2Obj1() throws Exception {

    Student student1 = new Student();
    student1.setAge(10);
    student1.setName("name1");
    student1.setProfileImageUrl("link1");

    Student student2 = new Student();
    student2.setAge(20);
    student2.setName("name2");
    student2.setProfileImageUrl("link2");

    List<Student> studentList = new ArrayList<>();
    studentList.add(student1);
    studentList.add(student2);
    String result = JsonUtil.obj2String(studentList);
    // [{"name":"name1","age":10,"profileImageUrl":"link1"},{"name":"name2","age":20,"profileImageUrl":"link2"}]
    System.out.println(result);

    List<Student> finalList = JsonUtil.string2Obj(result, List.class);
    // [{name=name1, age=10, profileImageUrl=link1}, {name=name2, age=20, profileImageUrl=link2}]
    System.out.println(finalList);
}

复杂类型的转换，示例2

@Test
public void string2Obj2() throws Exception {

    Map<String, List<Integer>> testMap = new HashMap<>();
    testMap.put("1", Arrays.asList(1, 2, 3));
    testMap.put("2", Arrays.asList(2, 3, 4));

    String result = JsonUtil.obj2String(testMap);
    // {"1":[1,2,3],"2":[2,3,4]}
    System.out.println(result);

    Map<String, List<Integer>> finalMap = JsonUtil.string2Obj(result, Map.class);
    // {1=[1, 2, 3], 2=[2, 3, 4]}
    System.out.println(finalMap);
}

到现在为止，你可以用这3个函数进行序列化和反序列化操作

/** 将对象转为json */
public static <T> String obj2String(T obj) 

/** 将对象转为json,并格式化显示 */
public static <T> String obj2StringPretty(T obj)

/** 将json转为对象 */
public static <T> T string2Obj(String str, Class<T> clazz)

可能你还看过其他类型的解析方式，如下所示，但是没有必要，上面3个函数完全能满足你的需求，我这里演示一下

方式2

public static <T> T string2Obj(String str, TypeReference<T> typeReference){
    if(StringUtils.isEmpty(str) || typeReference == null){
        return null;
    }
    try {
        return (T)(typeReference.getType().equals(String.class)? str : objectMapper.readValue(str,typeReference));
    } catch (Exception e) {
        log.warn("Parse String to Object error",e);
        return null;
    }
}

@Test
public void string2Obj3() throws Exception {

    Student student1 = new Student();
    student1.setAge(10);
    student1.setName("name1");
    student1.setProfileImageUrl("link1");

    Student student2 = new Student();
    student2.setAge(20);
    student2.setName("name2");
    student2.setProfileImageUrl("link2");

    List<Student> studentList = new ArrayList<>();
    studentList.add(student1);
    studentList.add(student2);
    String result = JsonUtil.obj2String(studentList);
    // [{"name":"name1","age":10,"profileImageUrl":"link1"},{"name":"name2","age":20,"profileImageUrl":"link2"}]
    System.out.println(result);

    List<Student> finalList = JsonUtil.string2Obj(result, new TypeReference<List<Student>>() {});
    // [{name=name1, age=10, profileImageUrl=link1}, {name=name2, age=20, profileImageUrl=link2}]
    System.out.println(finalList);
}

方式3

public static <T> T string2Obj(String str, Class<?> collectionClass, Class<?>... elementClasses){
    JavaType javaType = objectMapper.getTypeFactory().constructParametricType(collectionClass,elementClasses);
    try {
        return objectMapper.readValue(str,javaType);
    } catch (Exception e) {
        log.warn("Parse String to Object error",e);
        return null;
    }
}

@Test
public void string2Obj4() throws Exception {

    Student student1 = new Student();
    student1.setAge(10);
    student1.setName("name1");
    student1.setProfileImageUrl("link1");

    Student student2 = new Student();
    student2.setAge(20);
    student2.setName("name2");
    student2.setProfileImageUrl("link2");

    List<Student> studentList = new ArrayList<>();
    studentList.add(student1);
    studentList.add(student2);
    String result = JsonUtil.obj2String(studentList);
    // [{"name":"name1","age":10,"profileImageUrl":"link1"},{"name":"name2","age":20,"profileImageUrl":"link2"}]
    System.out.println(result);

    List<Student> finalList = JsonUtil.string2Obj(result, List.class, Student.class);
    // [{name=name1, age=10, profileImageUrl=link1}, {name=name2, age=20, profileImageUrl=link2}]
    System.out.println(finalList);
}

常用注解及配置

@JsonIgnore忽略属性

@Data
public class Student {

    /** 名字 */
    private String name;
    /** 年龄 */
    private Integer age;
    /** 头像 */
    @JsonIgnore
    private String profileImageUrl;
}

@Test
public void jsonIgnore() throws Exception {
    Student student = new Student();
    student.setAge(10);
    student.setName("name");
    student.setProfileImageUrl("link");
    String result = JsonUtil.obj2String(student);
    // {"name":"name","age":10}
    System.out.println(result);

    String str = "{\"name\":\"name\",\"age\":10,\"profileImageUrl\":\"link\"}";
    Student student1 = JsonUtil.string2Obj(str, Student.class);
    // Student(name=name, age=10, profileImageUrl=null)
    System.out.println(student1);
}

@JsonProperty

当Json的属性值和Java的属性值不一样时，会映射失败，用这个注解指定映射关系，在属性上用这个注解，则序列化和反序列化都会用这个值。如果序列化和反序列化的属性不一致，可以在get方法或者set方法上用这个注解，set方法影响反序列化，get方法影响序列化。

去掉@Data注解

public class Student {

    /** 名字 */
    private String name;
    /** 年龄 */
    private Integer age;
    /** 头像 */
    private String profileImageUrl;

    @JsonProperty("getImage")
    public String getProfileImageUrl(){
        return profileImageUrl;
    }

    @JsonProperty("setImage")
    public void setProfileImageUrl(String profileImageUrl){
        this.profileImageUrl = profileImageUrl;
    }

}

@Test
public void jsonProperty() throws Exception {

    String str = "{\"name\":\"name\",\"age\":10,\"setImage\":\"link\"}";
    Student student1 = JsonUtil.string2Obj(str, Student.class);
    // name
    System.out.println(student1.getName());
    // 10
    System.out.println(student1.getAge());
    // link
    System.out.println(student1.getProfileImageUrl());
    // {"name":"name","age":10,"getImage":"link"}
    System.out.println(JsonUtil.obj2String(student1));
}

@JsonFormat

日期格式化注解，不再演示

@JsonIgnoreProperties(ignoreUnknown = true)

如果json字符串中的属性个数小于java对象中的属性个数，可以顺利转换，java中多的那个属性为null

如果json字符串中出现java对象中没有的属性，则在将json转换为java对象时会报错：Unrecognized field, not marked as ignorable

解决方法1：
在目标对象的类级别上添加注解：@JsonIgnoreProperties(ignoreUnknown = true)

解决方法2：

@Slf4j
public class JsonUtil {

    private static ObjectMapper objectMapper = new ObjectMapper();

    static {
        //忽略 在json字符串中存在，但是在java对象中不存在对应属性的情况。防止错误
        objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    }

}

这样就不用在目标类上加@JsonIgnoreProperties属性了，体现了全局配置的优势，配置项很多，包括格式化显示日期等，不再详细介绍，百度配置即可

用Tree Mode方式解析JSON

除了将json转为对象外，还可以用Tree Mode方式解析JSON，全程无脑操作，除了一些特别复杂的JSON，或者只取JSON中的一部分，千万不要用这种二B方式解析JSON。正确的方式是将JSON直接转为对象。我这里演示一下解析的方式

{
    "name": "zhansan", 
    "age": 100, 
    "schools": [
        {
            "name": "tsinghua", 
            "location": "beijing"
        }, 
        {
            "name": "pku", 
            "location": "beijing"
        }
    ]
}

@Test
public void parseJson() throws Exception {
    String jsonStr = "{\"name\":\"zhansan\",\"age\":100,\"schools\":[{\"name\":\"tsinghua\",\"location\":\"beijing\"},{\"name\":\"pku\",\"location\":\"beijing\"}]}";

    ObjectMapper objectMapper = new ObjectMapper();
    JsonNode jsonNode = objectMapper.readTree(jsonStr);

    String name = jsonNode.get("name").asText();
    int age = jsonNode.get("age").asInt();
    // name is zhansan age is 100
    System.out.println("name is " + name + " age is " + age);

    JsonNode schoolsNode = jsonNode.get("schools");
    for (int i = 0; i < schoolsNode.size(); i++) {
        String schooleName = schoolsNode.get(i).get("name").asText();
        String schooleLocation = schoolsNode.get(i).get("location").asText();
        // schooleName is tsinghua schooleLocation is beijing
        // schooleName is pku schooleLocation is beijing
        System.out.println("schooleName is " + schooleName + " schooleLocation is " + schooleLocation);
    }
}

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