Improve environment

base/src/main/java/io/spine/base/CustomEnvironmentType.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2020, TeamDev. All rights reserved.
+ *
+ * Redistribution and use in source and/or binary forms, with or without
+ * modification, must retain the above copyright notice and the following
+ * disclaimer.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package io.spine.base;
+
+/**
+ * Abstract base for custom environment types.
+ **
+ * <p>{@code Environment} allows to {@link Environment#register(Class) register custom types}.
+ * In this case the environment detection functionality iterates over all known types, starting
+ * with those registered by the framework user:
+ *
+ * <pre>
+ *
+ * public final class Application {
+ *
+ *     static {
+ *         Environment.instance()
+ *                    .register(Staging.class)
+ *                    .register(LoadTesting().class);
+ *     }
+ *
+ *     private final ConnectionPool pool;
+ *
+ *     private Application() {
+ *         Environment environment = Environment.instance();
+ *         if (environment.is(Tests.class) {
+ *              // Single connection is enough for tests.
+ *             this.pool = new ConnectionPoolImpl(PoolCapacity.of(1));
+ *         } else {
+ *             if(environment.is(LoadTesting.class) {
+ *                 this.pool =
+ *                         new ConnectionPoolImpl(PoolCapacity.fromConfig("load_tests.yml"));
+ *             } else {
+ *                 this.pool =
+ *                         new ConnectionPoolImpl(PoolCapacity.fromConfig("cloud_deployment.yml"));
+ *             }
+ *         }
+ *         //...
+ *     }
+ * }
+ * </pre>
+ *
+ * <p><b>When registering custom types, please ensure</b> their mutual exclusivity.
+ * If two or more environment types {@linkplain EnvironmentType#enabled() consider themselves
+ * enabled} at the same time, the behaviour of {@link Environment#is(Class)} is undefined.
+ */
+public abstract class CustomEnvironmentType extends EnvironmentType {
+
+    /**
+     * Custom environment types must have a public no-arg constructor.
+     *
+     * <p>This constructor is used by {@link Environment} upon
+     * the {@linkplain Environment#register(Class) registration}.
+     */
+    protected CustomEnvironmentType() {
+        super();
+    }
+}