b/260769399 Extract email-specific logic from NotificationService (#203)

* Turn MailNotificationService into top-level class
* Move NotificationTemplate to MailNotificationService as it's 
   mostly relevant for formatting email

sources/src/main/java/com/google/solutions/jitaccess/core/services/MailNotificationService.java

@@ -0,0 +1,203 @@
+//
+// Copyright 2022 Google LLC
+//
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you 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.solutions.jitaccess.core.services;
+
+import com.google.common.base.Preconditions;
+import com.google.common.escape.Escaper;
+import com.google.common.html.HtmlEscapers;
+import com.google.solutions.jitaccess.core.AccessException;
+import com.google.solutions.jitaccess.core.adapters.SmtpAdapter;
+
+import java.io.IOException;
+import java.time.Instant;
+import java.time.OffsetDateTime;
+import java.time.ZoneId;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
+import java.time.temporal.ChronoUnit;
+import java.util.EnumSet;
+
+/**
+ * Concrete class that delivers notifications over SMTP.
+ */
+public class MailNotificationService extends NotificationService {
+  private final Options options;
+  private final SmtpAdapter smtpAdapter;
+
+  /**
+   * Load a resource from a JAR resource.
+   * @return null if not found.
+   */
+  public static String loadResource(String resourceName) throws NotificationException{
+    try (var stream = NotificationService.class
+      .getClassLoader()
+      .getResourceAsStream(resourceName)) {
+
+      if (stream == null) {
+        return null;
+      }
+
+      var content = stream.readAllBytes();
+      if (content.length > 3 &&
+        content[0] == (byte)0xEF &&
+        content[1] == (byte)0xBB &&
+        content[2] == (byte)0xBF) {
+
+        //
+        // Strip UTF-8 BOM.
+        //
+        return new String(content, 3, content.length - 3);
+      }
+      else {
+        return new String(content);
+      }
+    }
+    catch (IOException e) {
+      throw new NotificationException(
+        String.format("Reading the template %s from the JAR file failed", resourceName), e);
+    }
+  }
+
+  public MailNotificationService(
+    SmtpAdapter smtpAdapter,
+    Options options
+  ) {
+    Preconditions.checkNotNull(smtpAdapter);
+    Preconditions.checkNotNull(options);
+
+    this.smtpAdapter = smtpAdapter;
+    this.options = options;
+  }
+
+  // -------------------------------------------------------------------------
+  // NotificationService implementation.
+  // -------------------------------------------------------------------------
+
+  @Override
+  public boolean canSendNotifications() {
+    return true;
+  }
+
+  @Override
+  public void sendNotification(Notification notification) throws NotificationException {
+    Preconditions.checkNotNull(notification, "notification");
+
+    var htmlTemplate = loadResource(
+      String.format("notifications/%s.html", notification.getType()));
+    if (htmlTemplate == null) {
+      //
+      // Unknown kind of notification, ignore.
+      //
+      return;
+    }
+
+    var formattedMessage = new MessageTemplate(
+      htmlTemplate,
+      this.options.timeZone,
+      HtmlEscapers.htmlEscaper())
+      .format(notification);
+
+    try {
+      this.smtpAdapter.sendMail(
+        notification.getToRecipients(),
+        notification.getCcRecipients(),
+        notification.getSubject(),
+        formattedMessage,
+        notification.isReply()
+          ? EnumSet.of(SmtpAdapter.Flags.REPLY)
+          : EnumSet.of(SmtpAdapter.Flags.NONE));
+    }
+    catch (SmtpAdapter.MailException | AccessException | IOException e) {
+      throw new NotificationException("The notification could not be sent", e);
+    }
+  }
+
+  // -------------------------------------------------------------------------
+  // Inner classes.
+  // -------------------------------------------------------------------------
+
+  /**
+   * Template for turning a notification object into some textual representation.
+   */
+  public static class MessageTemplate {
+    private final String template;
+    private final Escaper escaper;
+    private final ZoneId timezoneId;
+
+    public MessageTemplate(
+      String template,
+      ZoneId timezoneId,
+      Escaper escaper
+    ) {
+      Preconditions.checkNotNull(template, "template");
+      Preconditions.checkNotNull(timezoneId, "timezoneId");
+      Preconditions.checkNotNull(escaper, "escaper");
+
+      this.template = template;
+      this.timezoneId = timezoneId;
+      this.escaper = escaper;
+    }
+
+    public String format(NotificationService.Notification notification) {
+      Preconditions.checkNotNull(notification, "notification");
+
+      //
+      // Replace all {{PROPERTY}} placeholders in the template.
+      //
+
+      var message = this.template;
+      for (var property : notification.properties.entrySet()) {
+        String propertyValue;
+        if (property.getValue() instanceof Instant) {
+          //
+          // Apply time zone and convert to string.
+          //
+          propertyValue = OffsetDateTime
+            .ofInstant((Instant) property.getValue(), this.timezoneId)
+            .truncatedTo(ChronoUnit.SECONDS)
+            .format(DateTimeFormatter.RFC_1123_DATE_TIME);
+        }
+        else {
+          //
+          // Convert to a safe string.
+          //
+          propertyValue = escaper.escape(property.getValue().toString());
+        }
+
+        message = message.replace("{{" + property.getKey() + "}}", propertyValue);
+      }
+
+      return message;
+    }
+  }
+
+  public static class Options {
+    public static final ZoneId DEFAULT_TIMEZONE = ZoneOffset.UTC;
+
+    private final ZoneId timeZone;
+
+    public Options(ZoneId timeZone) {
+      Preconditions.checkNotNull(timeZone);
+      this.timeZone = timeZone;
+    }
+  }
+}