Java中Map转对象

在工作中遇到一个场景：从某数据库中拿到一批数据，拿到的形式是 List<Map<String, Object>>，现有对应的自定义对象，假设为 User.class，但 Map 中的参数与 User 的属性并不是一一对应，且对某些特殊的字段要进行特殊处理。在此情况下，如何实现从 Map 到 User 对象的转换。

正好趁这个机会，总结一下之前用到过的几种方法。

一. 定义 User 对象与 Map

java 复制代码

@Data
public class User {

    private String _key;

    private String _id;

    private String _rev;

    private String name;

    private String surname;

    private Boolean alive;

    private Integer age;

    private List<String> traits;

    @JsonIgnore
    private String test;

}

@JsonIgnore 注解的作用是在 json 序列化时将被注解的属性忽略掉

java 复制代码

private static Map<String, Object> getMap() {
        Map<String, Object> map = new HashMap<>();
        map.put("_key", "qwertyui");
        map.put("_id", "ttt");
        map.put("_rev", "this is a test");
        map.put("name", "lizhou");
        map.put("surname", null);
        map.put("alive", true);
        map.put("age", 12);
        List<String> list = new ArrayList<>();
        list.add("aaa");
        list.add("bbb");
        list.add("ccc");
        map.put("traits", list);
        map.put("test", "test");
        return map;
    }

二. 五种转化方式

java 复制代码

    private static void useBeanMap(Map<String, Object> map) {
        User user = new User();
        BeanMap beanMap = BeanMap.create(user);
        beanMap.putAll(map);
        System.out.println(user);
    }

    private static void useObjectMapper(Map<String, Object> map) {
        ObjectMapper objectMapper = new ObjectMapper();
        User user = objectMapper.convertValue(map, User.class);
        System.out.println(user);
    }

    //将map转化为json字符串，然后转为对象
    private static void useFastJson(Map<String, Object> map) {
        String s = JSON.toJSONString(map);
        User user = JSON.parseObject(s, User.class);
        System.out.println(user);
    }

    private static void useGson(Map<String, Object> map) {
        Gson gson = new Gson();
        String s = gson.toJson(map);
        User user = gson.fromJson(s, User.class);
        System.out.println(user);
    }

    private static void useField(Map<String, Object> map) {
        try {
            User user = new User();
            Field[] fields = User.class.getDeclaredFields();
            for (Field field : fields) {
                field.setAccessible(true);
                field.set(user, map.get(field.getName()));
            }
            System.out.println(user);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }

1. 调用以上五种方法，查看结果

可以发现，都可以正常将 Map 转化为 User 对象，但是只有 ObjectMapper 方法会受到 @JsonIgnore 注解影响

2. 删除 User 类部分属性

将 User 类中的 test 属性注释掉

java 复制代码

@Data
public class User {

    private String _key;

    private String _id;

    private String _rev;

    private String name;

    private String surname;

    private Boolean alive;

    private Integer age;

    private List<String> traits;

//    @JsonIgnore
//    private String test;

}

再次调用方法，查看结果

可以发现，除了 ObjectMapper 方法以外，其他方法都可以正常运行，ObjectMapper 方法会报错：

java 复制代码

Exception in thread "main" java.lang.IllegalArgumentException: Unrecognized field "test" (class com.example.demo.bean.User), not marked as ignorable (8 known properties: "_key", "_rev", "traits", "_id", "name", "alive", "age", "surname"])
 at [Source: UNKNOWN; line: -1, column: -1] (through reference chain: com.example.demo.bean.User["test"])
	at com.fasterxml.jackson.databind.ObjectMapper._convert(ObjectMapper.java:3922)
	at com.fasterxml.jackson.databind.ObjectMapper.convertValue(ObjectMapper.java:3853)
	at com.example.demo.test.Test16.useObjectMapper(Test16.java:35)
	at com.example.demo.test.Test16.main(Test16.java:23)
Caused by: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "test" (class com.example.demo.bean.User), not marked as ignorable (8 known properties: "_key", "_rev", "traits", "_id", "name", "alive", "age", "surname"])
 at [Source: UNKNOWN; line: -1, column: -1] (through reference chain: com.example.demo.bean.User["test"])
	at com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException.from(UnrecognizedPropertyException.java:61)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnknownProperty(DeserializationContext.java:840)
	at com.fasterxml.jackson.databind.deser.std.StdDeserializer.handleUnknownProperty(StdDeserializer.java:1206)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownProperty(BeanDeserializerBase.java:1592)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.handleUnknownVanilla(BeanDeserializerBase.java:1570)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:294)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:151)
	at com.fasterxml.jackson.databind.ObjectMapper._convert(ObjectMapper.java:3917)
	... 3 more

所以 ObjectMapper 方法需要 Map 和转化对象的字段完全对应。