Add CompositeAppender Logback Appender implementation to compose multiple Appenders as a single Appender.

Resolve gh-73.

spring-geode-starter-logging/src/main/java/org/springframework/geode/logging/slf4j/logback/CompositeAppender.java

@@ -0,0 +1,105 @@
+/*
+ * Copyright 2019 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
+ *
+ *     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 org.springframework.geode.logging.slf4j.logback;
+
+import ch.qos.logback.core.Appender;
+import ch.qos.logback.core.AppenderBase;
+import ch.qos.logback.core.Context;
+
+/**
+ * {@link CompositeAppender} is an {@link Appender} implementation implementing
+ * the <a href="https://en.wikipedia.org/wiki/Composite_pattern">Composite Software Design Pattern</a>
+ *
+ * The {@literal Composite Software Design Pattern} enables two or more {@link Appender} objects to be composed
+ * and treated as a single instance of {@link Appender}.
+ *
+ * @author John Blum
+ * @see ch.qos.logback.core.Appender
+ * @see ch.qos.logback.core.AppenderBase
+ * @since 1.3.0
+ */
+public class CompositeAppender<T> extends AppenderBase<T> {
+
+	protected static final String DEFAULT_NAME = "composite";
+
+	private final Appender<T> one;
+	private final Appender<T> two;
+
+	/**
+	 * Factory method used to compose two {@link Appender} objects into a {@literal Composite} {@link Appender}.
+	 *
+	 * @param <T> {@link Class type} of {@link Appender} to compose.
+	 * @param one first {@link Appender} to compose.
+	 * @param two second {@link Appender} to compose.
+	 * @return {@link Appender} one if {@link Appender} two is {@literal null};
+	 * Return {@link Appender} two if {@link Appender} one is {@literal null}.
+	 * Otherwise, return a {@literal Composite} {@link Appender} composed of {@link Appender} one
+	 * and {@link Appender} two.
+	 * @see ch.qos.logback.core.Appender
+	 */
+	public static <T> Appender<T> compose(Appender<T> one, Appender<T> two) {
+		return one == null ? two : two == null ? one : new CompositeAppender<>(one, two);
+	}
+
+	/**
+	 * Constructs a new instance of {@link CompositeAppender} composed of {@link Appender} one and {@link Appender} two.
+	 *
+	 * @param one first {@link Appender} in the composite.
+	 * @param two second {@link Appender} in the composite.
+	 * @see ch.qos.logback.core.Appender
+	 */
+	private CompositeAppender(Appender<T> one, Appender<T> two) {
+
+		this.one = one;
+		this.two = two;
+		this.name = DEFAULT_NAME;
+		this.started = true;
+	}
+
+	protected Appender<T> getAppenderOne() {
+		return this.one;
+	}
+
+	protected Appender<T> getAppenderTwo() {
+		return this.two;
+	}
+
+	@Override
+	public void setContext(Context context) {
+
+		super.setContext(context);
+
+		getAppenderOne().setContext(context);
+		getAppenderTwo().setContext(context);
+	}
+
+	@Override
+	public Context getContext() {
+
+		Context context = super.getContext();
+
+		context = context != null ? context : getAppenderOne().getContext();
+		context = context != null ? context : getAppenderTwo().getContext();
+
+		return context;
+	}
+
+	@Override
+	protected void append(T eventObject) {
+		getAppenderOne().doAppend(eventObject);
+		getAppenderTwo().doAppend(eventObject);
+	}
+}