Java создает аннотацию

Что такое аннотации?

Удержание и цель

Параметры аннотации

Как создавать аннотации?

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface JsonField {
    public String value() default "";
}

Как использовать аннотации?

public class Car {
    @JsonField("manufacturer")
    private final String make;
    @JsonField
    private final String model;
    private final String year;

    public Car(String make, String model, String year) {
        this.make = make;
        this.model = model;
        this.year = year;
    }

    public String getMake() {
        return make;
    }

    public String getModel() {
        return model;
    }

    public String getYear() {
        return year;
    }

    @Override
    public String toString() {
        return year + " " + make + " " + model;
    }
} 

Как работать с аннотациями?

public class JsonSerializer {
    public String serialize(Object object) throws JsonSerializeException {
        try {
            Class<?> objectClass = requireNonNull(object).getClass();
            Map<String, String> jsonElements = new HashMap<>();
            for (Field field : objectClass.getDeclaredFields()) {
                field.setAccessible(true);
                if (field.isAnnotationPresent(JsonField.class)) {
                    jsonElements.put(getSerializedKey(field), (String) field.get(object));
                }
            }
            System.out.println(toJsonString(jsonElements));
            return toJsonString(jsonElements);
        } catch (IllegalAccessException e) {
            throw new JsonSerializeException(e.getMessage());
        }
    }

    private String toJsonString(Map<String, String> jsonMap) {
        String elementsString = jsonMap.entrySet().stream().map(entry -> "\"" + entry.getKey() + "\":\"" + entry.getValue() + "\"").collect(Collectors.joining(","));
        return "{" + elementsString + "}";
    }

    private static String getSerializedKey(Field field) {
        String annotationValue = field.getAnnotation(JsonField.class).value();
        if (annotationValue.isEmpty()) {
            return field.getName();
        } else {
            return annotationValue;
        }
    }
} 

public class JsonSerializeException extends Exception {
    private static final long serialVersionUID = -8845242379503538623L;

    public JsonSerializeException(String message) {
        super(message);
    }
} 

Car car=new Car("Ford","F150","2018");
JsonSerializer serializer=new JsonSerializer();
serializer.serialize(car); 

{"model":"F150","manufacturer":"Ford"}

В заключение