ObjectMapper是Spring Boot中的一个关键组件，用于将Java对象序列化为JSON，并将JSON反序列化为Java对象。以下是一些常见的ObjectMapper用例及代码示例：

1. 基本对象到JSON的转换

此用例涉及将Java对象转换为JSON。

import com.fasterxml.jackson.databind.ObjectMapper;

public class ObjectToJsonExample {
    public static void main(String[] args) throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();
        Employee employee = new Employee(1, "John", "Developer");

        // Convert Object to JSON
        String jsonString = objectMapper.writeValueAsString(employee);
        System.out.println(jsonString);
    }
}

2. 基本JSON到对象的转换

此用例演示了将JSON字符串反序列化为Java对象。

import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonToObjectExample {
    public static void main(String[] args) throws Exception {
        String jsonString = "{\"id\":1,\"name\":\"John\",\"designation\":\"Developer\"}";
        ObjectMapper objectMapper = new ObjectMapper();

        // Convert JSON string to Java object
        Employee employee = objectMapper.readValue(jsonString, Employee.class);
        System.out.println(employee.getName());  // Output: John
    }
}

3. 使用注解自定义JSON字段名称

在某些情况下，Java对象中的字段名称可能与JSON中的字段名称不匹配。使用@JsonProperty来处理此问题。

import com.fasterxml.jackson.annotation.JsonProperty;

public class Employee {
    private int id;

    @JsonProperty("full_name")
    private String name;

    private String designation;

    // constructor, getters, setters
}

JSON:

{
  "id": 1,
  "full_name": "John",
  "designation": "Developer"
}

4. 序列化时忽略字段

在将对象转换为JSON时，可能需要忽略某些字段。使用@JsonIgnore来跳过这些字段。

import com.fasterxml.jackson.annotation.JsonIgnore;

public class Employee {
    private int id;

    private String name;

    @JsonIgnore
    private String password;  // This will be ignored during serialization

    // constructor, getters, setters
}

5. 处理未知的JSON字段

在反序列化时，JSON中可能存在Java对象中不存在的字段。使用@JsonIgnoreProperties(ignoreUnknown = true)来避免因未知字段而导致的错误。

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public class Employee {
    private int id;
    private String name;
    private String designation;

    // constructor, getters, setters
}

6. 将对象列表转换为JSON

您可以使用ObjectMapper轻松地序列化和反序列化对象列表。

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.Arrays;
import java.util.List;

public class ListToJsonExample {
    public static void main(String[] args) throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();
        List<Employee> employees = Arrays.asList(
                new Employee(1, "John", "Developer"),
                new Employee(2, "Jane", "Tester")
        );

        // Convert List to JSON
        String jsonString = objectMapper.writeValueAsString(employees);
        System.out.println(jsonString);

        // Convert JSON back to List
        List<Employee> employeeList = objectMapper.readValue(jsonString, new TypeReference<List<Employee>>() {});
        System.out.println(employeeList.get(0).getName());  // Output: John
    }
}

7. 在Spring Boot控制器中使用ObjectMapper

在Spring Boot中，ObjectMapper可以用于REST API中处理JSON输入和输出。

@RestController
@RequestMapping("/employees")
public class EmployeeController {

    @Autowired
    private ObjectMapper objectMapper;

    @PostMapping("/create")
    public String createEmployee(@RequestBody String employeeJson) throws Exception {
        Employee employee = objectMapper.readValue(employeeJson, Employee.class);
        return "Employee created: " + employee.getName();
    }

    @GetMapping("/get")
    public String getEmployee() throws Exception {
        Employee employee = new Employee(1, "John", "Developer");
        return objectMapper.writeValueAsString(employee);
    }
}

8. 将JSON文件转换为对象

您可以直接将JSON文件反序列化为Java对象。

import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.File;

public class JsonFileToObjectExample {
    public static void main(String[] args) throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();

        // Read JSON from file and convert to Employee object
        Employee employee = objectMapper.readValue(new File("employee.json"), Employee.class);
        System.out.println(employee.getName());
    }
}

9. 美化打印JSON输出

使用writerWithDefaultPrettyPrinter()来美化打印JSON输出，以提高可读性。

public class PrettyPrintJsonExample {
    public static void main(String[] args) throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();
        Employee employee = new Employee(1, "John", "Developer");

        // Convert Object to Pretty JSON
        String prettyJson = objectMapper.writerWithDefaultPrettyPrinter().writeValueAsString(employee);
        System.out.println(prettyJson);
    }
}

10. 自定义ObjectMapper配置

您可以根据特定需求配置ObjectMapper，例如处理空值、日期格式等。

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;

public class CustomObjectMapperExample {
    public static void main(String[] args) throws Exception {
        ObjectMapper objectMapper = new ObjectMapper();

        // Register Java 8 Time module to handle date/time
        objectMapper.registerModule(new JavaTimeModule());
        
        // Disable timestamp formatting for dates
        objectMapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);

        // Convert Employee with LocalDate field
        Employee employee = new Employee(1, "John", LocalDate.now());
        String jsonString = objectMapper.writeValueAsString(employee);
        System.out.println(jsonString);
    }
}