java 对象转JSON

public static String parseSimpleObject(Object obj) {
        StringBuffer json = new StringBuffer("{");
        Class aClass = obj.getClass();
        Field[] fields = aClass.getDeclaredFields();
        int i = 0;
        for (Field field : fields) {
            field.setAccessible(true);
            String key = field.getName();
            Object value = null;
            try {
                value = field.get(obj);
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
            if (i==0){
                json.append("""+key+"""+":"+"""+value+""");
            }else {
                json.append(",""+key+"""+":"+"""+value+""");
            }
            i++;
        }
        json.append("}");
        return json.toString();
    }

public class User {
    private Integer id;
    private String name;
    private String address;
    private Date birth;
    public User() {
    }
    public User(Integer id, String name, String address, Date birth) {
        this.id = id;
        this.name = name;
        this.address = address;
        this.birth = birth;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public Date getBirth() {
        return birth;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }
}