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The AWS Java SDK for Amazon DynamoDB module holds the client classes that are used for communicating with Amazon DynamoDB Service

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/*
 * Copyright 2014-2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 *
 *    http://aws.amazon.com/apache2.0
 *
 * This file is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES
 * OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and
 * limitations under the License.
 */
package com.amazonaws.services.dynamodbv2.datamodeling.unmarshallers;

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMappingException;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMarshaller;
import com.amazonaws.services.dynamodbv2.model.AttributeValue;

/**
 * An unmarshaller that delegates to an instance of a
 * {@code DynamoDBMarshaller}-derived custom marshaler.
 */
public class CustomUnmarshaller extends SUnmarshaller {

    private final Class targetClass;
    private final Class> unmarshallerClass;

    public CustomUnmarshaller(
            Class targetClass,
            Class> unmarshallerClass) {

        this.targetClass = targetClass;
        this.unmarshallerClass = unmarshallerClass;
    }

    @Override
    @SuppressWarnings({ "rawtypes", "unchecked" })
    public Object unmarshall(AttributeValue value) {

        // TODO: Would be nice to cache this object, but not sure if we can
        // do that now without a breaking change; user's unmarshallers
        // might not all be thread-safe.

        DynamoDBMarshaller unmarshaller =
                createUnmarshaller(unmarshallerClass);

        return unmarshaller.unmarshall(targetClass, value.getS());
    }

    @SuppressWarnings({ "rawtypes" })
    private static DynamoDBMarshaller createUnmarshaller(Class clazz) {
        try {

            return (DynamoDBMarshaller) clazz.newInstance();

        } catch (InstantiationException e) {
            throw new DynamoDBMappingException(
                    "Failed to instantiate custom marshaler for class " + clazz,
                    e);

        } catch (IllegalAccessException e) {
            throw new DynamoDBMappingException(
                    "Failed to instantiate custom marshaler for class " + clazz,
                    e);
        }
    }
}