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The Amazon Web Services SDK for Java provides Java APIs for building software on AWS' cost-effective, scalable, and reliable infrastructure products. The AWS Java SDK allows developers to code against APIs for all of Amazon's infrastructure web services (Amazon S3, Amazon EC2, Amazon SQS, Amazon Relational Database Service, Amazon AutoScaling, etc).

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/*
 * Copyright 2013-2014 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.
 * A copy of the License is located at
 *
 *  http://aws.amazon.com/apache2.0
 *
 * or in the "license" file accompanying this file. 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.s3.internal;

import java.io.IOException;
import java.io.InputStream;
import java.security.MessageDigest;
import java.util.Arrays;

import com.amazonaws.AmazonClientException;
import com.amazonaws.internal.SdkDigestInputStream;

/**
 * Input stream extends MD5DigestValidationInputStream, when you finish reading the stream, it
 * will validate whether the computed digest equals the one from the server
 * side.
 */
public class DigestValidationInputStream extends SdkDigestInputStream {

    private byte[] expectedHash;

    //Flag do we don't validate twice.  See validateMD5Digest()
    private boolean digestValidated = false;

    public DigestValidationInputStream(InputStream in, MessageDigest digest, byte[] serverSideHash) {
        super(in, digest);
        this.expectedHash = serverSideHash;
    }

    /**
     * @see java.io.InputStream#read()
     */
    @Override
    public int read() throws IOException {
        int ch = super.read();
        if (ch == -1) {
            validateMD5Digest();
        }
        return ch;
    }

    /**
     * @see java.io.InputStream#read(byte[], int, int)
     */
    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        int result = super.read(b, off, len);
        if (result == -1) {
            validateMD5Digest();
        }
        return result;
    }

    public byte[] getMD5Checksum() {
        return digest.digest();
    }

    private void validateMD5Digest() {
        /*
         * Some InputStream readers (e.g., java.util.Properties) read more than
         * once at the end of the stream. This class validates the digest once
         * -1 has been read so we must not validate twice.
         */
        if (expectedHash != null && !digestValidated ) {
            digestValidated = true;
            if (!Arrays.equals(digest.digest(), expectedHash)) {
                throw new AmazonClientException("Unable to verify integrity of data download.  "
                        + "Client calculated content hash didn't match hash calculated by Amazon S3.  "
                        + "The data may be corrupt.");
            }
        }
    }

}