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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.jaxrs;

import static org.kiwiproject.base.KiwiPreconditions.checkArgumentNotNull;

import lombok.experimental.UtilityClass;
import lombok.extern.slf4j.Slf4j;

import javax.annotation.Nullable;
import javax.ws.rs.core.GenericType;
import javax.ws.rs.core.Response;
import java.util.Optional;
import java.util.function.Supplier;

/**
 * Static utilities related to reading entities from a {@link Response}.
 */
@UtilityClass
@Slf4j
public class KiwiEntities {

    /**
     * Read an entity as a String from the given response. If it cannot be read, return an empty {@link Optional}.
     *
     * @param response the response object
     * @return an Optional that contains the response as a String or an empty Optional
     */
    public static Optional safeReadEntity(Response response) {
        return safeReadEntity(response, String.class);
    }

    /**
     * Read an entity as a String from the given response. If it cannot be read, return a default message.
     *
     * @param response       the response object
     * @param defaultMessage a default message to return if the response entity cannot be read (can be null)
     * @return the response entity as a String or the given default message
     */
    public static String safeReadEntity(Response response, @Nullable String defaultMessage) {
        return safeReadEntity(response).orElse(defaultMessage);
    }

    /**
     * Read an entity as a String from the given response. If it cannot be read, call the given {@link Supplier} to
     * get the default message.
     *
     * @param response               the response object
     * @param defaultMessageSupplier supplies a default message if the response entity cannot be read
     *                               (the Supplier can return null, but the Supplier itself must not be null)
     * @return the response entity as a String or the message supplied by the given Supplier
     */
    public static String safeReadEntity(Response response, Supplier defaultMessageSupplier) {
        checkArgumentNotNull(defaultMessageSupplier, "defaultMessageSupplier must not be null");
        return safeReadEntity(response).orElseGet(defaultMessageSupplier);
    }

    /**
     * Read an entity as an instance of the given {@link Class} specified by {@code entityType}.
     * If it cannot be read, return an empty {@link Optional}.
     *
     * @param response   the response object
     * @param entityType the type of entity the response is expected to contain
     * @param         the entity type
     * @return an Optional that contains the response as a specific type or an empty Optional
     */
    public static  Optional safeReadEntity(Response response, Class entityType) {
        try {
            var entity = response.readEntity(entityType);
            return Optional.ofNullable(entity);
        } catch (Exception e) {
            return emptyOptional(e);
        }
    }

    /**
     * Read an entity as an instance of the given {@link GenericType} specified by {@code entityType}.
     * If it cannot be read, return an empty {@link Optional}.
     *
     * @param response   the response object
     * @param entityType the type of entity the response is expected to contain
     * @param         the entity type
     * @return an Optional that contains the response as a specific type or an empty Optional
     */
    public static  Optional safeReadEntity(Response response, GenericType entityType) {
        try {
            var entity = response.readEntity(entityType);
            return Optional.ofNullable(entity);
        } catch (Exception e) {
            return emptyOptional(e);
        }
    }

    private static  Optional emptyOptional(Exception e) {
        LOG.error("Error reading response entity", e);
        return Optional.empty();
    }
}