add chained credential provider (#1043)

api/src/main/java/io/minio/credentials/ChainedProvider.java

@@ -0,0 +1,60 @@
+/*
+ * MinIO Java SDK for Amazon S3 Compatible Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * 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
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License 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 io.minio.credentials;
+
+import java.util.Arrays;
+import java.util.List;
+import javax.annotation.Nonnull;
+
+/** Chained credential provider work with list of credential providers. */
+public class ChainedProvider implements Provider {
+  private final List<Provider> providers;
+  private Provider currentProvider;
+  private Credentials credentials;
+
+  public ChainedProvider(@Nonnull Provider... providers) {
+    this.providers = Arrays.asList(providers);
+  }
+
+  @Override
+  public synchronized Credentials fetch() {
+    if (credentials != null && !credentials.isExpired()) {
+      return credentials;
+    }
+
+    if (currentProvider != null) {
+      try {
+        credentials = currentProvider.fetch();
+        return credentials;
+      } catch (IllegalStateException e) {
+        // Ignore and fallback to iteration.
+      }
+    }
+
+    for (Provider provider : providers) {
+      try {
+        credentials = provider.fetch();
+        currentProvider = provider;
+        return credentials;
+      } catch (IllegalStateException e) {
+        // Ignore and continue to next iteration.
+      }
+    }
+
+    throw new IllegalStateException("All providers fail to fetch credentials");
+  }
+}

examples/MinioClientWithChainedProvider.java

@@ -0,0 +1,42 @@
+/*
+ * MinIO Java SDK for Amazon S3 Compatible Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * 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
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License 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.
+ */
+
+import io.minio.MinioClient;
+import io.minio.StatObjectArgs;
+import io.minio.StatObjectResponse;
+import io.minio.credentials.AwsEnvironmentProvider;
+import io.minio.credentials.ChainedProvider;
+import io.minio.credentials.MinioEnvironmentProvider;
+import io.minio.credentials.Provider;
+
+public class MinioClientWithChainedProvider {
+  public static void main(String[] args) throws Exception {
+    Provider provider =
+        new ChainedProvider(new AwsEnvironmentProvider(), new MinioEnvironmentProvider());
+
+    MinioClient minioClient =
+        MinioClient.builder()
+            .endpoint("https://MINIO-HOST:MINIO-PORT")
+            .credentialsProvider(provider)
+            .build();
+
+    // Get information of an object.
+    StatObjectResponse stat =
+        minioClient.statObject(
+            StatObjectArgs.builder().bucket("my-bucketname").object("my-objectname").build());
+    System.out.println(stat);
+  }
+}