// ???
import com.amazonaws.annotation.ThreadSafe;

com.amazonaws.http.AmazonHttpClient

    // Logger providing detailed information on requests/responses. Users can enable this logger to
    // get access to AWS request IDs for responses, individual requests and parameters sent to AWS, etc.
    @SdkInternalApi
    public static final Log requestLog = LogFactory.getLog("com.amazonaws.request");

        // Returns the credentials from the execution if exists. Else returns null.
        private AWSCredentials getCredentialsFromContext() {
            final AWSCredentialsProvider credentialsProvider = executionContext.getCredentialsProvider();

            AWSCredentials credentials = null;
            if (credentialsProvider != null) {
                awsRequestMetrics.startEvent(Field.CredentialsRequestTime);
                try {
                    credentials = credentialsProvider.getCredentials();
                } finally {
                    awsRequestMetrics.endEvent(Field.CredentialsRequestTime);
                }
            }
            return credentials;
        }



package com.amazonaws.auth;

/**
 * {@link AWSCredentialsProvider} implementation that chains together multiple
 * credentials providers. When a caller first requests credentials from this provider,
 * it calls all the providers in the chain, in the original order specified,
 * until one can provide credentials, and then returns those credentials. If all
 * of the credential providers in the chain have been called, and none of them
 * can provide credentials, then this class will throw an exception indicated
 * that no credentials are available.
 * <p>
 * By default, this class will remember the first credentials provider in the chain
 * that was able to provide credentials, and will continue to use that provider when
 * credentials are requested in the future, instead of traversing the chain each time.
 * This behavior can be controlled through the {@link #setReuseLastProvider(boolean)} method.
 */
public class AWSCredentialsProviderChain implements AWSCredentialsProvider {
    ...
    private static final Log log = LogFactory.getLog(AWSCredentialsProviderChain.class);
    ..    
    @Override
    public AWSCredentials getCredentials() {
        if (reuseLastProvider && lastUsedProvider != null) {
            return lastUsedProvider.getCredentials();
        }

        List<String> exceptionMessages = null;
        for (AWSCredentialsProvider provider : credentialsProviders) {
            try {
                AWSCredentials credentials = provider.getCredentials();

                if (credentials.getAWSAccessKeyId() != null &&
                    credentials.getAWSSecretKey() != null) {
                    log.debug("Loading credentials from " + provider.toString());

                    lastUsedProvider = provider;
                    return credentials;
                }
            } catch (Exception e) {
                // Ignore any exceptions and move onto the next provider
                String message = provider + ": " + e.getMessage();
                log.debug("Unable to load credentials from " + message);
                if (exceptionMessages == null) {
                    exceptionMessages = new LinkedList<String>();
                }
                exceptionMessages.add(message);
            }
        }
        throw new SdkClientException("Unable to load AWS credentials from any provider in the chain: " + exceptionMessages);
    }
    ...
}