spring boot 为什么以静态方法返回为空？

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.0.6</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>locale.demo</groupId>
    <artifactId>demo02</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo02</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>20</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

// controller.java
package locale.demo.demo02.controller;

import locale.demo.demo02.CommonResult;
import locale.demo.demo02.UserVO;
import org.springframework.web.bind.annotation.*;

import java.util.UUID;

@RestController
@RequestMapping("/users")
public class UserController {
    @GetMapping("/get2")
    public CommonResult<UserVO> get2(@RequestParam("id") Integer id) {
        UserVO user = new UserVO().setId(id).setUsername(UUID.randomUUID().toString());
        // 返回结果
        return CommonResult.success(user);
    }
}

// CommonResult.java
package locale.demo.demo02;

import com.fasterxml.jackson.annotation.JsonIgnore;
import org.springframework.util.Assert;

import java.io.Serializable;

public class CommonResult<T> implements Serializable {
    public static Integer CODE_SUCCESS = 0;

    private Integer code;
    private String message;
    private T data;

    public Integer getCode() {
        return code;
    }

    public String getMessage() {
        return message;
    }

    public static <K> CommonResult<K> success(K input_data) {
        CommonResult<K> result = new CommonResult<>();
        result.code = CODE_SUCCESS;
        result.data = input_data;
        result.message = "";
        return result;
    }
}

按以上方法写，则只能得到{"code":0,"message":""}这样的结果；只有将commonresult里的data从private改为public才能返回{"code":0,"message":"","data":{"id":3,"username":"1e29a45d-6363-43b3-a08d-3989ea70d1ae"}}这样的结果请问这是为什么？