Add main method to print build info

Author: Michael-A-McMahon

pom.xml

@@ -56,6 +56,35 @@
           <target>${java.version}</target>
         </configuration>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-jar-plugin</artifactId>
+        <version>2.1</version>
+        <configuration>
+          <archive>
+            <manifestEntries>
+              <Implementation-Title>Oracle R2DBC</Implementation-Title>
+              <Implementation-Version>
+                ${project.version}
+              </Implementation-Version>
+              <Implementation-Vendor>Oracle Corporation</Implementation-Vendor>
+              <Specification-Title>
+                R2DBC - Reactive Relational Database Connectivity
+              </Specification-Title>
+              <Specification-Version>${r2dbc.version}</Specification-Version>
+              <Specification-Vendor>
+                Pivotal Software, Inc
+              </Specification-Vendor>
+              <Build-Info>
+                Oracle R2DBC ${project.version} compiled with JDK ${java.vm.version} from ${java.vm.vendor} on ${maven.build.timestamp}
+              </Build-Info>
+              <Main-Class>
+                oracle.r2dbc.impl.Main
+              </Main-Class>
+            </manifestEntries>
+          </archive>
+        </configuration>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-deploy-plugin</artifactId>

src/main/java/oracle/r2dbc/impl/Main.java

@@ -0,0 +1,84 @@
+/*
+  Copyright (c) 2020, 2021, Oracle and/or its affiliates.
+
+  This software is dual-licensed to you under the Universal Permissive License
+  (UPL) 1.0 as shown at https://oss.oracle.com/licenses/upl or Apache License
+  2.0 as shown at http://www.apache.org/licenses/LICENSE-2.0. You may choose
+  either license.
+
+  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 oracle.r2dbc.impl;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Objects;
+import java.util.jar.Manifest;
+
+/**
+ * <p>
+ * A public class implementing a main method that may be executed from a
+ * command line. This class is specified as the Main-Class attribute in
+ * the META-INF/MANIFEST.MF of the Oracle R2DBC jar.
+ * </p><p><i>
+ * The behavior implemented by this class may change between minor and patch
+ * version release updates.
+ * </i></p><p>
+ * The following command, in
+ * which "x.y.z" is the semantic version number of the jar,
+ * executes the main method of this class:
+ * </p><pre>
+ *   java -jar oracle-r2dbc-x.y.z.jar
+ * </pre><p>
+ * Since version 0.1.1, the main method is implemented to exit after printing
+ * a message to the standard output stream. The message includes the version
+ * numbers of the Oracle R2DBC Driver along with the JDK that compiled it. A
+ * timestamp captured at the moment when the jar was compiled is also included.
+ * </p>
+ *
+ * @since 0.1.1
+ * @author Michael-A-McMahon
+ */
+public final class Main {
+
+  private Main() {/*This class has no instance fields*/}
+
+  /**
+   * Prints information about this build of Oracle R2DBC. This method attempts
+   * to read a "Build-Info" attribute from META-INF/MANIFEST.MF. If the
+   * manifest is not available to the class loader, or if the manifest does
+   * not contain a Build-Info attribute, then this method prints a message
+   * indicating that build information can not be located.
+   * @param args ignored
+   * @throws IOException If the META-INF/MANIFEST.MF resource can not be read.
+   */
+  public static void main(String[] args) throws IOException {
+
+    InputStream manifestStream =
+      Main.class.getModule().getResourceAsStream("META-INF/MANIFEST.MF");
+
+    if (manifestStream == null) {
+      System.out.println("META-INF/MANIFEST.MF not found");
+      return;
+    }
+
+    try (manifestStream) {
+      System.out.println(Objects.requireNonNullElse(
+        new Manifest(manifestStream)
+          .getMainAttributes()
+          .getValue("Build-Info"),
+        "Build-Info is missing from the manifest"));
+    }
+  }
+}