Jackson ObjectMapper 核心用法大全：實體、集合、Map、數組、流式處理與日期配置

ObjectMapper 核心 API 速查

1. 單實體對象雙向轉換

public class Student {
    private int id;
    private String name;
    private boolean isStudent;
    // getter/setter/toString
}

Student s = new Student(1000, "小廖", true);
// 對象 -> JSON 文件
mapper.writeValue(new File("student.json"), s);
// 對象 -> JSON 字符串 (控制台輸出)
String json = mapper.writeValueAsString(s);
// {"id":1000,"name":"小廖","isStudent":true}

// JSON 字符串 -> 對象
Student s2 = mapper.readValue(json, Student.class);
// JSON 文件 -> 對象
Student s3 = mapper.readValue(new File("student.json"), Student.class);

2. 集合與數組處理

List 序列化

List list = Arrays.asList(
    new Student(1000, "小廖", true),
    new Student(200, "小米", false)
);
mapper.writeValue(new File("students.json"), list);
// [{"id":1000,"name":"小廖","isStudent":true},{"id":200,"name":"小米","isStudent":false}]

List 反序列化：TypeReference 必須用

String json = "[{\"id\":1000,\"name\":\"小廖\"},{\"id\":200,\"name\":\"小米\"}]";
// 錯誤：List

數組反序列化：直接用 Class[]

Student[] arr = mapper.readValue(json, Student[].class);

3. Map 處理

Map map = new LinkedHashMap<>();
map.put("stu1", new Student(1000, "小廖", true));
map.put("stu2", new Student(200, "小米", false));

// Map -> JSON (鍵為 JSON 對象屬性名)
mapper.writeValue(new File("map.json"), map);
// {"stu1":{"id":1000,...},"stu2":{"id":200,...}}

// JSON -> Map (值類型需 TypeReference)
Map parsed = mapper.readValue(new File("map.json"),
    new TypeReference>() {});

4. 混合結構處理

// List 包含：字符串、對象、Map、數組
List