chore(JavaDocs): detail thread saftey of SDK Client Builders

pom.xml

@@ -37,6 +37,14 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>
+<!--    Uncomment the next block for development with TestVectorsAwsCryptographicMaterialProviders in local mvn -->
+<!--    <repositories>-->
+<!--        <repository>-->
+<!--            <id>localRepository</id>-->
+<!--            <name>localRepository</name>-->
+<!--            <url>file://${user.home}/.m2/repository/</url>-->
+<!--        </repository>-->
+<!--    </repositories>-->
 
     <dependencyManagement>
         <dependencies>

src/examples/java/com/amazonaws/crypto/examples/v2/RestrictRegionExample.java

@@ -15,6 +15,7 @@
 import com.amazonaws.encryptionsdk.kmssdkv2.KmsMasterKeyProvider;
 import com.amazonaws.encryptionsdk.CommitmentPolicy;
 import com.amazonaws.encryptionsdk.kms.DiscoveryFilter;
+import com.amazonaws.encryptionsdk.kmssdkv2.RegionalClientSupplier;
 import software.amazon.awssdk.regions.Region;
 import software.amazon.awssdk.services.kms.KmsClient;
 
@@ -48,6 +49,8 @@ public static void main(final String[] args) {
         encryptAndDecrypt(keyName, partition, accountId, region);
     }
 
+
+
     static void encryptAndDecrypt(final String keyName, final String partition, final String accountId, final Region region) {
         // Instantiate the SDK.
         // This builds the AwsCrypto client with the RequireEncryptRequireDecrypt commitment policy,
@@ -101,15 +104,7 @@ static void encryptAndDecrypt(final String keyName, final String partition, fina
         // This example also configures the AWS KMS master key provider with a Discovery Filter to limit
         // the attempted AWS KMS CMKs to a particular partition and account.
         final KmsMasterKeyProvider decryptingKeyProvider = KmsMasterKeyProvider.builder()
-                .customRegionalClientSupplier(cmkRegion -> {
-                    if(cmkRegion.equals(region)) {
-                        // return the previously built AWS KMS client so that we do
-                        // not create a new client on every decrypt call.
-                        return kmsClient;
-                    }
-
-                    throw new AwsCryptoException("Only " + region.id() + " is supported");
-                })
+                .customRegionalClientSupplier(new ARegionalClientSupplier(region, kmsClient))
                 .buildDiscovery(discoveryFilter);
 
         // 8. Decrypt the data
@@ -127,4 +122,30 @@ static void encryptAndDecrypt(final String keyName, final String partition, fina
         // 10. Verify that the decrypted plaintext matches the original plaintext
         assert Arrays.equals(decryptResult.getResult(), EXAMPLE_DATA);
     }
+
+
+    /**
+     * This class is Thread Safe, as both of its members are thread safe.
+     * KMS Client Builders are NOT thread safe, and can lead to unexpected behavior if concurrently used.
+     */
+    private static class ARegionalClientSupplier implements RegionalClientSupplier {
+        private final Region region;
+        private final KmsClient kmsClient;
+
+        public ARegionalClientSupplier(Region region, KmsClient kmsClient) {
+            this.region = region;
+            this.kmsClient = kmsClient;
+        }
+
+        @Override
+        public KmsClient getClient(Region cmkRegion) {
+          if (cmkRegion.equals(region)) {
+            // return the previously built AWS KMS client so that we do
+            // not create a new client on every decrypt call.
+            return kmsClient;
+          }
+
+          throw new AwsCryptoException("Only " + region.id() + " is supported");
+        }
+    }
 }

src/main/java/com/amazonaws/encryptionsdk/kms/AwsKmsMrkAwareMasterKeyProvider.java

@@ -172,6 +172,10 @@ public AwsKmsMrkAwareMasterKeyProvider.Builder withCredentials(AWSCredentials cr
      * <p>This method will overwrite any credentials set using {@link
      * #withCredentials(AWSCredentialsProvider)}.
      *
+     * <p>WARNING: {@link AWSKMSClientBuilder}s are NOT thread safe. If {@link KmsMasterKeyProvider}
+     * is going to be used concurrently, a proper supplier MUST be provided, or use a {@link
+     * #withCustomClientFactory(KmsMasterKeyProvider.RegionalClientSupplier)}
+     *
      * @see KmsMasterKeyProvider.Builder#withClientBuilder(AWSKMSClientBuilder)
      */
     public AwsKmsMrkAwareMasterKeyProvider.Builder withClientBuilder(AWSKMSClientBuilder builder) {

src/main/java/com/amazonaws/encryptionsdk/kms/KmsMasterKeyProvider.java

@@ -173,6 +173,10 @@ public Builder withCredentials(AWSCredentials credentials) {
      * <p>This method will overwrite any credentials set using {@link
      * #withCredentials(AWSCredentialsProvider)}.
      *
+     * <p>WARNING: {@link AWSKMSClientBuilder}s are NOT thread safe. If {@link KmsMasterKeyProvider}
+     * is going to be used concurrently, a proper supplier MUST be provided, or use a {@link
+     * #withCustomClientFactory(KmsMasterKeyProvider.RegionalClientSupplier)}
+     *
      * @param builder
      * @return
      */

src/main/java/com/amazonaws/encryptionsdk/kmssdkv2/AwsKmsMrkAwareMasterKeyProvider.java

@@ -131,6 +131,10 @@ public Builder customRegionalClientSupplier(RegionalClientSupplier regionalClien
      * <p>Note: The AWS Encryption SDK for Java does not support the {@code KmsAsyncClient}
      * interface.
      *
+     * <p>WARNING: {@link KmsClientBuilder}s are NOT thread safe. If {@link KmsMasterKeyProvider} is
+     * going to be used concurrently, a proper supplier MUST be provided, or use a {@link
+     * #customRegionalClientSupplier(RegionalClientSupplier)}
+     *
      * @see KmsMasterKeyProvider.Builder#builderSupplier(Supplier)
      */
     public Builder builderSupplier(Supplier<KmsClientBuilder> supplier) {

src/main/java/com/amazonaws/encryptionsdk/kmssdkv2/KmsMasterKeyProvider.java

@@ -116,6 +116,10 @@ public Builder customRegionalClientSupplier(RegionalClientSupplier regionalClien
      * <p>Note: The AWS Encryption SDK for Java does not support the {@code KmsAsyncClient}
      * interface.
      *
+     * <p>WARNING: {@link KmsClientBuilder}s are NOT thread safe. If {@link KmsMasterKeyProvider} is
+     * going to be used concurrently, a proper supplier MUST be provided, or use a {@link
+     * #customRegionalClientSupplier(RegionalClientSupplier)}
+     *
      * @param supplier Should return a new {@link KmsClientBuilder} on each invocation.
      * @return
      */