Hide 'No log4j2 configuration file found' error

Add a Log4J2 `ConfigurationFactory` that is always applied so that the
"No log4j2 configuration file found" error message does not appear.

Although the message was harmless it was quite annoying to Spring Boot
users who could safely omit the file.

Fixes gh-4809

spring-boot/src/main/java/org/springframework/boot/logging/log4j2/SpringBootConfigurationFactory.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2012-2016 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.
+ */
+
+package org.springframework.boot.logging.log4j2;
+
+import java.net.URI;
+
+import org.apache.logging.log4j.core.LoggerContext;
+import org.apache.logging.log4j.core.config.Configuration;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
+import org.apache.logging.log4j.core.config.ConfigurationSource;
+import org.apache.logging.log4j.core.config.DefaultConfiguration;
+import org.apache.logging.log4j.core.config.Order;
+import org.apache.logging.log4j.core.config.plugins.Plugin;
+
+/**
+ * Spring Boot {@link ConfigurationFactory} that prevents logger warnings from being
+ * printed when the application first starts.
+ *
+ * @author Phillip Webb
+ * @since 1.5.0
+ */
+@Plugin(name = "SpringBootConfigurationFactory", category = ConfigurationFactory.CATEGORY)
+@Order(4) // Order behind XmlConfigurationFactory
+public class SpringBootConfigurationFactory extends ConfigurationFactory {
+
+	private static final String[] ALL_TYPES = { "*" };
+
+	@Override
+	protected String[] getSupportedTypes() {
+		return ALL_TYPES;
+	}
+
+	@Override
+	public Configuration getConfiguration(LoggerContext loggerContext, String name,
+			URI configLocation) {
+		if (configLocation == null) {
+			return new DefaultConfiguration();
+		}
+		return null;
+	}
+
+	@Override
+	public Configuration getConfiguration(LoggerContext loggerContext, String name,
+			URI configLocation, ClassLoader loader) {
+		return null;
+	}
+
+	@Override
+	public Configuration getConfiguration(LoggerContext loggerContext,
+			ConfigurationSource source) {
+		return null;
+	}
+
+}