feat(project): add maven wrapper

* Provide a wrapper to allow developers to build the project without
  installing Maven.

Related to CAM-13361, PR camunda#1394

.mvn/wrapper/MavenWrapperDownloader.java

@@ -0,0 +1,117 @@
+/*
+ * Copyright 2007-present the original author or authors.
+ *
+ * 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.
+ */
+import java.net.*;
+import java.io.*;
+import java.nio.channels.*;
+import java.util.Properties;
+
+public class MavenWrapperDownloader {
+
+    private static final String WRAPPER_VERSION = "0.5.6";
+    /**
+     * Default URL to download the maven-wrapper.jar from, if no 'downloadUrl' is provided.
+     */
+    private static final String DEFAULT_DOWNLOAD_URL = "https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/"
+        + WRAPPER_VERSION + "/maven-wrapper-" + WRAPPER_VERSION + ".jar";
+
+    /**
+     * Path to the maven-wrapper.properties file, which might contain a downloadUrl property to
+     * use instead of the default one.
+     */
+    private static final String MAVEN_WRAPPER_PROPERTIES_PATH =
+            ".mvn/wrapper/maven-wrapper.properties";
+
+    /**
+     * Path where the maven-wrapper.jar will be saved to.
+     */
+    private static final String MAVEN_WRAPPER_JAR_PATH =
+            ".mvn/wrapper/maven-wrapper.jar";
+
+    /**
+     * Name of the property which should be used to override the default download url for the wrapper.
+     */
+    private static final String PROPERTY_NAME_WRAPPER_URL = "wrapperUrl";
+
+    public static void main(String args[]) {
+        System.out.println("- Downloader started");
+        File baseDirectory = new File(args[0]);
+        System.out.println("- Using base directory: " + baseDirectory.getAbsolutePath());
+
+        // If the maven-wrapper.properties exists, read it and check if it contains a custom
+        // wrapperUrl parameter.
+        File mavenWrapperPropertyFile = new File(baseDirectory, MAVEN_WRAPPER_PROPERTIES_PATH);
+        String url = DEFAULT_DOWNLOAD_URL;
+        if(mavenWrapperPropertyFile.exists()) {
+            FileInputStream mavenWrapperPropertyFileInputStream = null;
+            try {
+                mavenWrapperPropertyFileInputStream = new FileInputStream(mavenWrapperPropertyFile);
+                Properties mavenWrapperProperties = new Properties();
+                mavenWrapperProperties.load(mavenWrapperPropertyFileInputStream);
+                url = mavenWrapperProperties.getProperty(PROPERTY_NAME_WRAPPER_URL, url);
+            } catch (IOException e) {
+                System.out.println("- ERROR loading '" + MAVEN_WRAPPER_PROPERTIES_PATH + "'");
+            } finally {
+                try {
+                    if(mavenWrapperPropertyFileInputStream != null) {
+                        mavenWrapperPropertyFileInputStream.close();
+                    }
+                } catch (IOException e) {
+                    // Ignore ...
+                }
+            }
+        }
+        System.out.println("- Downloading from: " + url);
+
+        File outputFile = new File(baseDirectory.getAbsolutePath(), MAVEN_WRAPPER_JAR_PATH);
+        if(!outputFile.getParentFile().exists()) {
+            if(!outputFile.getParentFile().mkdirs()) {
+                System.out.println(
+                        "- ERROR creating output directory '" + outputFile.getParentFile().getAbsolutePath() + "'");
+            }
+        }
+        System.out.println("- Downloading to: " + outputFile.getAbsolutePath());
+        try {
+            downloadFileFromURL(url, outputFile);
+            System.out.println("Done");
+            System.exit(0);
+        } catch (Throwable e) {
+            System.out.println("- Error downloading");
+            e.printStackTrace();
+            System.exit(1);
+        }
+    }
+
+    private static void downloadFileFromURL(String urlString, File destination) throws Exception {
+        if (System.getenv("MVNW_USERNAME") != null && System.getenv("MVNW_PASSWORD") != null) {
+            String username = System.getenv("MVNW_USERNAME");
+            char[] password = System.getenv("MVNW_PASSWORD").toCharArray();
+            Authenticator.setDefault(new Authenticator() {
+                @Override
+                protected PasswordAuthentication getPasswordAuthentication() {
+                    return new PasswordAuthentication(username, password);
+                }
+            });
+        }
+        URL website = new URL(urlString);
+        ReadableByteChannel rbc;
+        rbc = Channels.newChannel(website.openStream());
+        FileOutputStream fos = new FileOutputStream(destination);
+        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
+        fos.close();
+        rbc.close();
+    }
+
+}

.mvn/wrapper/maven-wrapper.properties

@@ -0,0 +1,2 @@
+distributionUrl=https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/3.2.5/apache-maven-3.2.5-bin.zip
+wrapperUrl=https://repo.maven.apache.org/maven2/io/takari/maven-wrapper/0.5.6/maven-wrapper-0.5.6.jar

TESTING.md

@@ -77,6 +77,35 @@ There is a special profile for the WildFly Application Servers:
 
 Camunda supports all database technologies listed on [Supported Database Products](https://docs.camunda.org/manual/latest/introduction/supported-environments/#supported-database-products), and in all environments, they are operating in as specified. Support means we guarantee the Camunda Platform integrates well with the database technology’s JDBC behavior (there are some [documented](https://docs.camunda.org/manual/latest/user-guide/process-engine/database/) limitations, e.g., isolation level `READ_COMMITTED` is required for all databases except CockroachDB, which in turns requires `SERIALIZABLE`). We test a database technology with a specific database, i.e., we test it in one environment, not all possible environments that you can imagine (e.g., we test Postgres on local Docker containers, but not as hosted databases on AWS or Azure).
 
+# No Maven? No problem!
+
+This project provides a [Maven Wrapper](https://github.com/takari/maven-wrapper). This feature is useful for developers
+to build and test the project with the same version that Camunda uses. It's also useful for developers that don't want
+to install Maven at all. By executing the `mvnw` script (Unix), or `mvnw.cmd` script (Windows), a Maven distro will be 
+downloaded and installed in the `$USER_HOME/.m2/wrapper/dists` folder of the system. You can check the download URL in
+the [.mvn/wrapper/maven-wrapper.properties](.mvn/wrapper/maven-wrapper.properties) file.
+
+The Maven Wrapper requires Maven commands to be executed from the root of the project. As the Camunda Platform project
+is a multi-module (Maven Reactor) project, this is also a good best practice to apply.
+
+To build the whole project, or just a module, one of the following commands may be executed:
+
+```shell
+# build the whole project
+./mvnw clean install
+
+# build the engine module
+./mvnw clean install -f engine/pom.xml
+
+# run the rolling-update IT tests with the H2 database
+./mvnw verify -f qa/test-db-rolling-update/pom.xml -Prolling-update,h2
+```
+
+> Note: Above the `mvn -f` command line option is recommended over the `mvn -pl` option. The reason is that `-pl` will
+build only the specified module, and will ignore any sub-modules that it might contain (unless the `-amd` option is also
+added). As the Camunda Platform project has a multi-tiered module hierarchy (e.g. the [qa](qa/) module has modules of 
+it's own), the `mvn -f` command option is simpler. 
+
 ## What about database technology X in environment Y?
 
 To make a statement regarding Camunda Platform support, we need to understand if technology X is one of the technologies we already support or different technology. Several databases may share the same or a similar name, but they can still be different technologies: For example, IBM DB2 z/OS behaves quite differently from IBM DB2 on Linux, Unix, Windows. Amazon Aurora Postgres is different from a standard Postgres.

model-api/bpmn-model/README.asciidoc

@@ -112,7 +112,7 @@ Read the https://camunda.com/bpmn/[BPMN 2.0 Tutorial on camunda.com].
 
 === Which Java (JRE) Version is required?
 
-Java JRE 1.6+ is required. We test camunda BPMN model API on Oracle and IBM JVMs.
+Java JRE 1.8+ is required. We test camunda BPMN model API on Oracle and IBM JVMs.
 
 === Does this API support all BPMN 2.0 Elements?