Add initial classes for Remote Config API

src/main/java/com/google/firebase/remoteconfig/FirebaseRemoteConfigClient.java

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+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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
+ *
+ *     http://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 com.google.firebase.remoteconfig;
+
+/**
+ * An interface for managing Firebase Remote Config templates.
+ */
+interface FirebaseRemoteConfigClient {
+
+  /**
+   * Gets the current active version of the Remote Config template.
+   *
+   * @return A {@link RemoteConfigTemplate}.
+   * @throws FirebaseRemoteConfigException If an error occurs while getting the template.
+   */
+  RemoteConfigTemplate getTemplate() throws FirebaseRemoteConfigException;
+}

src/main/java/com/google/firebase/remoteconfig/FirebaseRemoteConfigClientImpl.java

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+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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
+ *
+ *     http://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 com.google.firebase.remoteconfig;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.api.client.http.HttpRequestFactory;
+import com.google.api.client.http.HttpResponseInterceptor;
+import com.google.api.client.json.JsonFactory;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableMap;
+import com.google.firebase.FirebaseApp;
+import com.google.firebase.FirebaseException;
+import com.google.firebase.ImplFirebaseTrampolines;
+import com.google.firebase.IncomingHttpResponse;
+import com.google.firebase.internal.AbstractPlatformErrorHandler;
+import com.google.firebase.internal.ApiClientUtils;
+import com.google.firebase.internal.ErrorHandlingHttpClient;
+import com.google.firebase.internal.HttpRequestInfo;
+import com.google.firebase.internal.SdkUtils;
+import com.google.firebase.remoteconfig.internal.RemoteConfigServiceErrorResponse;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * A helper class for interacting with Firebase Remote Config service.
+ */
+final class FirebaseRemoteConfigClientImpl implements FirebaseRemoteConfigClient {
+
+  private static final String REMOTE_CONFIG_URL = "https://firebaseremoteconfig.googleapis.com/v1/projects/%s/remoteConfig";
+
+  private static final Map<String, String> COMMON_HEADERS =
+          ImmutableMap.of(
+                  "X-Firebase-Client", "fire-admin-java/" + SdkUtils.getVersion(),
+                  // There is a known issue in which the ETag is not properly returned in cases
+                  // where the request does not specify a compression type. Currently, it is
+                  // required to include the header `Accept-Encoding: gzip` or equivalent in all
+                  // requests. https://firebase.google.com/docs/remote-config/use-config-rest#etag_usage_and_forced_updates
+                  "Accept-Encoding", "gzip"
+          );
+
+  private final String remoteConfigUrl;
+  private final HttpRequestFactory requestFactory;
+  private final JsonFactory jsonFactory;
+  private final ErrorHandlingHttpClient<FirebaseRemoteConfigException> httpClient;
+
+  private FirebaseRemoteConfigClientImpl(Builder builder) {
+    checkArgument(!Strings.isNullOrEmpty(builder.projectId));
+    this.remoteConfigUrl = String.format(REMOTE_CONFIG_URL, builder.projectId);
+    this.requestFactory = checkNotNull(builder.requestFactory);
+    this.jsonFactory = checkNotNull(builder.jsonFactory);
+    HttpResponseInterceptor responseInterceptor = builder.responseInterceptor;
+    RemoteConfigErrorHandler errorHandler = new RemoteConfigErrorHandler(this.jsonFactory);
+    this.httpClient = new ErrorHandlingHttpClient<>(requestFactory, jsonFactory, errorHandler)
+            .setInterceptor(responseInterceptor);
+  }
+
+  @VisibleForTesting
+  String getRemoteConfigUrl() {
+    return remoteConfigUrl;
+  }
+
+  @VisibleForTesting
+  HttpRequestFactory getRequestFactory() {
+    return requestFactory;
+  }
+
+  @VisibleForTesting
+  JsonFactory getJsonFactory() {
+    return jsonFactory;
+  }
+
+  @Override
+  public RemoteConfigTemplate getTemplate() throws FirebaseRemoteConfigException {
+    HttpRequestInfo request = HttpRequestInfo.buildGetRequest(remoteConfigUrl)
+            .addAllHeaders(COMMON_HEADERS);
+    IncomingHttpResponse response = httpClient.send(request);
+    RemoteConfigTemplate parsed = httpClient.parse(response, RemoteConfigTemplate.class);
+    parsed.setETag(getETag(response));
+    return parsed;
+  }
+
+  private String getETag(IncomingHttpResponse response) {
+    List<String> etagList = (List<String>) response.getHeaders().get("etag");
+    checkState(etagList != null && !etagList.isEmpty(),
+            "ETag header is not available in the server response.");
+
+    String etag = etagList.get(0);
+    checkState(!Strings.isNullOrEmpty(etag),
+            "ETag header is not available in the server response.");
+
+    return etag;
+  }
+
+  static FirebaseRemoteConfigClientImpl fromApp(FirebaseApp app) {
+    String projectId = ImplFirebaseTrampolines.getProjectId(app);
+    checkArgument(!Strings.isNullOrEmpty(projectId),
+            "Project ID is required to access Remote Config service. Use a service "
+                    + "account credential or set the project ID explicitly via FirebaseOptions. "
+                    + "Alternatively you can also set the project ID via the GOOGLE_CLOUD_PROJECT "
+                    + "environment variable.");
+    return FirebaseRemoteConfigClientImpl.builder()
+            .setProjectId(projectId)
+            .setRequestFactory(ApiClientUtils.newAuthorizedRequestFactory(app))
+            .setJsonFactory(app.getOptions().getJsonFactory())
+            .build();
+  }
+
+  static Builder builder() {
+    return new Builder();
+  }
+
+  static final class Builder {
+
+    private String projectId;
+    private HttpRequestFactory requestFactory;
+    private JsonFactory jsonFactory;
+    private HttpResponseInterceptor responseInterceptor;
+
+    private Builder() { }
+
+    Builder setProjectId(String projectId) {
+      this.projectId = projectId;
+      return this;
+    }
+
+    Builder setRequestFactory(HttpRequestFactory requestFactory) {
+      this.requestFactory = requestFactory;
+      return this;
+    }
+
+    Builder setJsonFactory(JsonFactory jsonFactory) {
+      this.jsonFactory = jsonFactory;
+      return this;
+    }
+
+    Builder setResponseInterceptor(
+            HttpResponseInterceptor responseInterceptor) {
+      this.responseInterceptor = responseInterceptor;
+      return this;
+    }
+
+    FirebaseRemoteConfigClientImpl build() {
+      return new FirebaseRemoteConfigClientImpl(this);
+    }
+  }
+
+  private static class RemoteConfigErrorHandler
+          extends AbstractPlatformErrorHandler<FirebaseRemoteConfigException> {
+
+    private RemoteConfigErrorHandler(JsonFactory jsonFactory) {
+      super(jsonFactory);
+    }
+
+    @Override
+    protected FirebaseRemoteConfigException createException(FirebaseException base) {
+      String response = getResponse(base);
+      RemoteConfigServiceErrorResponse parsed = safeParse(response);
+      return FirebaseRemoteConfigException.withRemoteConfigErrorCode(
+              base, parsed.getRemoteConfigErrorCode());
+    }
+
+    private String getResponse(FirebaseException base) {
+      if (base.getHttpResponse() == null) {
+        return null;
+      }
+
+      return base.getHttpResponse().getContent();
+    }
+
+    private RemoteConfigServiceErrorResponse safeParse(String response) {
+      if (!Strings.isNullOrEmpty(response)) {
+        try {
+          return jsonFactory.createJsonParser(response)
+                  .parseAndClose(RemoteConfigServiceErrorResponse.class);
+        } catch (IOException ignore) {
+          // Ignore any error that may occur while parsing the error response. The server
+          // may have responded with a non-json payload.
+        }
+      }
+
+      return new RemoteConfigServiceErrorResponse();
+    }
+  }
+}

src/main/java/com/google/firebase/remoteconfig/FirebaseRemoteConfigException.java

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+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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
+ *
+ *     http://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 com.google.firebase.remoteconfig;
+
+import com.google.firebase.ErrorCode;
+import com.google.firebase.FirebaseException;
+import com.google.firebase.IncomingHttpResponse;
+import com.google.firebase.internal.NonNull;
+import com.google.firebase.internal.Nullable;
+
+/**
+ * Generic exception related to Firebase Remote Config. Check the error code and message for more
+ * details.
+ */
+public final class FirebaseRemoteConfigException extends FirebaseException {
+
+  private final RemoteConfigErrorCode errorCode;
+
+  public FirebaseRemoteConfigException(
+          @NonNull ErrorCode errorCode,
+          @NonNull String message,
+          @Nullable Throwable cause,
+          @Nullable IncomingHttpResponse response,
+          @Nullable RemoteConfigErrorCode remoteConfigErrorCode) {
+    super(errorCode, message, cause, response);
+    this.errorCode = remoteConfigErrorCode;
+  }
+
+  static FirebaseRemoteConfigException withRemoteConfigErrorCode(
+          FirebaseException base, @Nullable RemoteConfigErrorCode errorCode) {
+    return new FirebaseRemoteConfigException(
+            base.getErrorCode(),
+            base.getMessage(),
+            base.getCause(),
+            base.getHttpResponse(),
+            errorCode);
+  }
+
+  @Nullable
+  public RemoteConfigErrorCode getRemoteConfigErrorCode() {
+    return errorCode;
+  }
+}

src/main/java/com/google/firebase/remoteconfig/RemoteConfigErrorCode.java

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+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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
+ *
+ *     http://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 com.google.firebase.remoteconfig;
+
+/**
+ * Error codes that can be raised by the Remote Config APIs.
+ */
+public enum RemoteConfigErrorCode {
+
+  /**
+   * One or more arguments specified in the request were invalid.
+   */
+  INVALID_ARGUMENT,
+
+  /**
+   * Internal server error.
+   */
+  INTERNAL,
+}

src/main/java/com/google/firebase/remoteconfig/RemoteConfigTemplate.java

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+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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
+ *
+ *     http://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 com.google.firebase.remoteconfig;
+
+import com.google.api.client.util.Key;
+
+public final class RemoteConfigTemplate {
+
+  @Key("etag")
+  private String etag;
+
+  public String getETag() {
+    return this.etag;
+  }
+
+  void setETag(String etag) {
+    this.etag = etag;
+  }
+}