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Microsoft JDBC Driver for SQL Server. The Azure Key Vault feature in Microsoft JDBC Driver for SQL Server depends on Azure SDK for JAVA and Azure Active Directory Library For Java.
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/*
 * Microsoft JDBC Driver for SQL Server
 * 
 * Copyright(c) Microsoft Corporation All rights reserved.
 * 
 * This program is made available under the terms of the MIT License. See the LICENSE file in the project root for more information.
 */

package com.microsoft.sqlserver.jdbc;

import static java.nio.charset.StandardCharsets.UTF_8;
import static java.util.concurrent.TimeUnit.SECONDS;

import java.text.MessageFormat;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;

import java.util.Base64;;

class CacheClear implements Runnable {

    private String keylookupValue;
    static private java.util.logging.Logger aeLogger = java.util.logging.Logger.getLogger("com.microsoft.sqlserver.jdbc.CacheClear");

    CacheClear(String keylookupValue) {
        this.keylookupValue = keylookupValue;
    }

    @Override
    public void run() {
        // remove() is a no-op if the key is not in the map.
        // It is a concurrentHashMap, update/remove operations are thread safe.
        synchronized (SQLServerSymmetricKeyCache.lock) {
            SQLServerSymmetricKeyCache instance = SQLServerSymmetricKeyCache.getInstance();
            if (instance.getCache().containsKey(keylookupValue)) {
                instance.getCache().get(keylookupValue).zeroOutKey();
                instance.getCache().remove(keylookupValue);
                if (aeLogger.isLoggable(java.util.logging.Level.FINE)) {
                    aeLogger.fine("Removed encryption key from cache...");
                }
            }
        }
    }
}

/**
 * 
 * Cache for the Symmetric keys
 *
 */
final class SQLServerSymmetricKeyCache {
    static Object lock = new Object();
    private final ConcurrentHashMap cache;
    private static final SQLServerSymmetricKeyCache instance = new SQLServerSymmetricKeyCache();
    private static ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1, new ThreadFactory() {
        @Override
        public Thread newThread(Runnable r) {
            Thread t = Executors.defaultThreadFactory().newThread(r);
            t.setDaemon(true);
            return t;
        }
    });

    static final private java.util.logging.Logger aeLogger = java.util.logging.Logger
            .getLogger("com.microsoft.sqlserver.jdbc.SQLServerSymmetricKeyCache");

    private SQLServerSymmetricKeyCache() {
        cache = new ConcurrentHashMap<>();
    }

    static SQLServerSymmetricKeyCache getInstance() {
        return instance;
    }

    ConcurrentHashMap getCache() {
        return cache;
    }

    /**
     * Retrieves key
     * 
     * @param keyInfo
     *            contains encryption meta data information
     * @param connection
     * @return plain text key
     */
    SQLServerSymmetricKey getKey(EncryptionKeyInfo keyInfo,
            SQLServerConnection connection) throws SQLServerException {
        SQLServerSymmetricKey encryptionKey = null;
        synchronized (lock) {
            String serverName = connection.getTrustedServerNameAE();
            assert null != serverName : "serverName should not be null in getKey.";

            StringBuilder keyLookupValuebuffer = new StringBuilder(serverName);
            String keyLookupValue;
            keyLookupValuebuffer.append(":");

            keyLookupValuebuffer.append(Base64.getEncoder().encodeToString((new String(keyInfo.encryptedKey, UTF_8)).getBytes()));

            keyLookupValuebuffer.append(":");
            keyLookupValuebuffer.append(keyInfo.keyStoreName);
            keyLookupValue = keyLookupValuebuffer.toString();
            keyLookupValuebuffer.setLength(0); // Get rid of the buffer, will be garbage collected.

            if (aeLogger.isLoggable(java.util.logging.Level.FINE)) {
                aeLogger.fine("Checking trusted master key path...");
            }
            Boolean[] hasEntry = new Boolean[1];
            List trustedKeyPaths = SQLServerConnection.getColumnEncryptionTrustedMasterKeyPaths(serverName, hasEntry);
            if (hasEntry[0]) {
                if ((null == trustedKeyPaths) || (0 == trustedKeyPaths.size()) || (!trustedKeyPaths.contains(keyInfo.keyPath))) {
                    MessageFormat form = new MessageFormat(SQLServerException.getErrString("R_UntrustedKeyPath"));
                    Object[] msgArgs = {keyInfo.keyPath, serverName};
                    throw new SQLServerException(this, form.format(msgArgs), null, 0, false);
                }
            }

            if (aeLogger.isLoggable(java.util.logging.Level.FINE)) {
                aeLogger.fine("Checking Symmetric key cache...");
            }

            // if ColumnEncryptionKeyCacheTtl is 0 no caching at all
            if (!cache.containsKey(keyLookupValue)) {

                // Check for the connection provider first.
                SQLServerColumnEncryptionKeyStoreProvider provider = connection.getSystemColumnEncryptionKeyStoreProvider(keyInfo.keyStoreName);

                // There is no connection provider of this name, check for the global system providers.
                if (null == provider) {
                    provider = SQLServerConnection.getGlobalSystemColumnEncryptionKeyStoreProvider(keyInfo.keyStoreName);
                }

                // There is no global system provider of this name, check for the global custom providers.
                if (null == provider) {
                    provider = SQLServerConnection.getGlobalCustomColumnEncryptionKeyStoreProvider(keyInfo.keyStoreName);
                }

                // No provider was found of this name.
                if (null == provider) {
                    String systemProviders = connection.getAllSystemColumnEncryptionKeyStoreProviders();
                    String customProviders = SQLServerConnection.getAllGlobalCustomSystemColumnEncryptionKeyStoreProviders();
                    MessageFormat form = new MessageFormat(SQLServerException.getErrString("R_UnrecognizedKeyStoreProviderName"));
                    Object[] msgArgs = {keyInfo.keyStoreName, systemProviders, customProviders};
                    throw new SQLServerException(this, form.format(msgArgs), null, 0, false);
                }

                byte[] plaintextKey;
                plaintextKey = provider.decryptColumnEncryptionKey(keyInfo.keyPath, keyInfo.algorithmName, keyInfo.encryptedKey);
                encryptionKey = new SQLServerSymmetricKey(plaintextKey);

                /*
                 * a ColumnEncryptionKeyCacheTtl value of '0' means no caching at all. The expected use case is to have the application set it once.
                 * The application could set it multiple times, in which case a key gets the TTL defined at the time of its entry into the cache.
                 */
                long columnEncryptionKeyCacheTtl = SQLServerConnection.getColumnEncryptionKeyCacheTtl();
                if (0 != columnEncryptionKeyCacheTtl) {
                    cache.putIfAbsent(keyLookupValue, encryptionKey);
                    if (aeLogger.isLoggable(java.util.logging.Level.FINE)) {
                        aeLogger.fine("Adding encryption key to cache...");
                    }
                    scheduler.schedule(new CacheClear(keyLookupValue), columnEncryptionKeyCacheTtl, SECONDS);
                }
            }
            else {
                encryptionKey = cache.get(keyLookupValue);
            }
        }
        return encryptionKey;
    }
}