ValueExtractor Type — spring-boot Architecture

Source Code

core/spring-boot/src/main/java/org/springframework/boot/json/JsonWriter.java lines 692–766

		@FunctionalInterface
		interface ValueExtractor<T extends @Nullable Object> {

			/**
			 * Represents a skipped value.
			 */
			Object SKIP = new Object();

			/**
			 * Extract the value from the given instance.
			 * @param instance the source instance
			 * @return the extracted value or {@link #SKIP}
			 */
			@Nullable T extract(@Nullable Object instance);

			/**
			 * Only extract when the given predicate matches.
			 * @param predicate the predicate to test
			 * @return a new {@link ValueExtractor}
			 */
			default ValueExtractor<T> when(Predicate<? super @Nullable T> predicate) {
				return (instance) -> test(extract(instance), predicate);
			}

			@SuppressWarnings("unchecked")
			private @Nullable T test(@Nullable T extracted, Predicate<? super @Nullable T> predicate) {
				return (!skip(extracted) && predicate.test(extracted)) ? extracted : (T) SKIP;
			}

			/**
			 * Adapt the extracted value.
			 * @param <R> the result type
			 * @param extractor the extractor to use
			 * @return a new {@link ValueExtractor}
			 */
			default <R> ValueExtractor<R> as(Extractor<T, R> extractor) {
				return (instance) -> apply(extract(instance), extractor);
			}

			@SuppressWarnings("unchecked")
			private <R> @Nullable R apply(@Nullable T value, Extractor<T, R> extractor) {
				if (skip(value)) {
					return (R) SKIP;
				}
				return (value != null) ? extractor.extract(value) : null;
			}

			/**
			 * Create a new {@link ValueExtractor} based on the given {@link Function}.
			 * @param <S> the source type
			 * @param <T> the extracted type
			 * @param extractor the extractor to use
			 * @return a new {@link ValueExtractor} instance
			 */
			@SuppressWarnings("unchecked")
			static <S, T> ValueExtractor<T> of(Extractor<S, T> extractor) {
				return (instance) -> {
					if (instance == null) {
						return null;
					}
					return (skip(instance)) ? (T) SKIP : extractor.extract((S) instance);
				};
			}

			/**
			 * Return if the extracted value should be skipped.
			 * @param <T> the value type
			 * @param extracted the value to test
			 * @return if the value is to be skipped
			 */
			static <T> boolean skip(@Nullable T extracted) {
				return extracted == SKIP;
			}

		}