Fix LoggingSystem package tangle

Introduce a new `LoggingSystemFactory` interface so that the
`LoggingSystem` class can find implementations without needing to
be directly tied to them.

Closes gh-23387

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/DelegatingLoggingSystemFactory.java

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2012-2020 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.boot.logging;
+
+import java.util.List;
+import java.util.function.Function;
+
+/**
+ * {@link LoggingSystemFactory} that delegates to other factories.
+ *
+ * @author Phillip Webb
+ */
+class DelegatingLoggingSystemFactory implements LoggingSystemFactory {
+
+	private final Function<ClassLoader, List<LoggingSystemFactory>> delegates;
+
+	/**
+	 * Create a new {@link DelegatingLoggingSystemFactory} instance.
+	 * @param delegates a function that provides the delegates
+	 */
+	DelegatingLoggingSystemFactory(Function<ClassLoader, List<LoggingSystemFactory>> delegates) {
+		this.delegates = delegates;
+	}
+
+	@Override
+	public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
+		List<LoggingSystemFactory> delegates = (this.delegates != null) ? this.delegates.apply(classLoader) : null;
+		if (delegates != null) {
+			for (LoggingSystemFactory delegate : delegates) {
+				LoggingSystem loggingSystem = delegate.getLoggingSystem(classLoader);
+				if (loggingSystem != null) {
+					return loggingSystem;
+				}
+			}
+		}
+		return null;
+	}
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/LoggingSystem.java

@@ -21,11 +21,8 @@ import java.util.Collections;
 import java.util.EnumSet;
 import java.util.List;
 import java.util.Set;
-import java.util.function.Function;
 
-import org.springframework.boot.logging.java.JavaLoggingSystem;
+import org.springframework.util.Assert;
-import org.springframework.boot.logging.log4j2.Log4J2LoggingSystem;
-import org.springframework.boot.logging.logback.LogbackLoggingSystem;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.StringUtils;
 
@@ -58,21 +55,7 @@ public abstract class LoggingSystem {
 	 */
 	public static final String ROOT_LOGGER_NAME = "ROOT";
 
-	private static final Function<ClassLoader, LoggingSystem> SYSTEM_FACTORY = getSystemFactory();
+	private static final LoggingSystemFactory SYSTEM_FACTORY = LoggingSystemFactory.fromSpringFactories();
-
-	private static Function<ClassLoader, LoggingSystem> getSystemFactory() {
-		ClassLoader classLoader = LoggingSystem.class.getClassLoader();
-		if (ClassUtils.isPresent("ch.qos.logback.core.Appender", classLoader)) {
-			return LogbackLoggingSystem::new;
-		}
-		if (ClassUtils.isPresent("org.apache.logging.log4j.core.impl.Log4jContextFactory", classLoader)) {
-			return Log4J2LoggingSystem::new;
-		}
-		if (ClassUtils.isPresent("java.util.logging.LogManager", classLoader)) {
-			return JavaLoggingSystem::new;
-		}
-		throw new IllegalStateException("No suitable logging system located");
-	}
 
 	/**
 	 * Reset the logging system to be limit output. This method may be called before
@@ -155,19 +138,21 @@ public abstract class LoggingSystem {
 	 * @return the logging system
 	 */
 	public static LoggingSystem get(ClassLoader classLoader) {
-		String loggingSystem = System.getProperty(SYSTEM_PROPERTY);
+		String loggingSystemClassName = System.getProperty(SYSTEM_PROPERTY);
-		if (StringUtils.hasLength(loggingSystem)) {
+		if (StringUtils.hasLength(loggingSystemClassName)) {
-			if (NONE.equals(loggingSystem)) {
+			if (NONE.equals(loggingSystemClassName)) {
 				return new NoOpLoggingSystem();
 			}
-			return get(classLoader, loggingSystem);
+			return get(classLoader, loggingSystemClassName);
 		}
-		return SYSTEM_FACTORY.apply(classLoader);
+		LoggingSystem loggingSystem = SYSTEM_FACTORY.getLoggingSystem(classLoader);
+		Assert.state(loggingSystem != null, "No suitable logging system located");
+		return loggingSystem;
 	}
 
-	private static LoggingSystem get(ClassLoader classLoader, String loggingSystemClass) {
+	private static LoggingSystem get(ClassLoader classLoader, String loggingSystemClassName) {
 		try {
-			Class<?> systemClass = ClassUtils.forName(loggingSystemClass, classLoader);
+			Class<?> systemClass = ClassUtils.forName(loggingSystemClassName, classLoader);
 			Constructor<?> constructor = systemClass.getDeclaredConstructor(ClassLoader.class);
 			constructor.setAccessible(true);
 			return (LoggingSystem) constructor.newInstance(classLoader);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/LoggingSystemFactory.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2012-2020 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.boot.logging;
+
+import org.springframework.boot.env.EnvironmentPostProcessorsFactory;
+import org.springframework.core.io.support.SpringFactoriesLoader;
+
+/**
+ * Factory class used by {@link LoggingSystem#get(ClassLoader)} to find an actual
+ * implementation.
+ *
+ * @author Phillip Webb
+ * @since 2.4.0
+ */
+public interface LoggingSystemFactory {
+
+	/**
+	 * Return a logging system implementation or {@code null} if not logging system is
+	 * available.
+	 * @param classLoader the class loader to use
+	 * @return a logging system
+	 */
+	LoggingSystem getLoggingSystem(ClassLoader classLoader);
+
+	/**
+	 * Return a {@link EnvironmentPostProcessorsFactory} backed by
+	 * {@code spring.factories}.
+	 * @return an {@link LoggingSystemFactory} instance
+	 */
+	static LoggingSystemFactory fromSpringFactories() {
+		return new DelegatingLoggingSystemFactory(
+				(classLoader) -> SpringFactoriesLoader.loadFactories(LoggingSystemFactory.class, classLoader));
+	}
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/java/JavaLoggingSystem.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -34,7 +34,11 @@ import org.springframework.boot.logging.LogLevel;
 import org.springframework.boot.logging.LoggerConfiguration;
 import org.springframework.boot.logging.LoggingInitializationContext;
 import org.springframework.boot.logging.LoggingSystem;
+import org.springframework.boot.logging.LoggingSystemFactory;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.FileCopyUtils;
 import org.springframework.util.ResourceUtils;
 import org.springframework.util.StringUtils;
@@ -177,4 +181,20 @@ public class JavaLoggingSystem extends AbstractLoggingSystem {
 
 	}
 
+	/**
+	 * {@link LoggingSystemFactory} that returns {@link JavaLoggingSystem} if possible.
+	 */
+	@Order(Ordered.LOWEST_PRECEDENCE)
+	public static class Factory implements LoggingSystemFactory {
+
+		@Override
+		public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
+			if (ClassUtils.isPresent("java.util.logging.LogManager", classLoader)) {
+				return new JavaLoggingSystem(classLoader);
+			}
+			return null;
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystem.java

@@ -47,7 +47,10 @@ import org.springframework.boot.logging.LogLevel;
 import org.springframework.boot.logging.LoggerConfiguration;
 import org.springframework.boot.logging.LoggingInitializationContext;
 import org.springframework.boot.logging.LoggingSystem;
+import org.springframework.boot.logging.LoggingSystemFactory;
 import org.springframework.boot.logging.Slf4JLoggingSystem;
+import org.springframework.core.Ordered;
+import org.springframework.core.annotation.Order;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ResourceUtils;
@@ -326,4 +329,20 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 
 	}
 
+	/**
+	 * {@link LoggingSystemFactory} that returns {@link Log4J2LoggingSystem} if possible.
+	 */
+	@Order(Ordered.LOWEST_PRECEDENCE)
+	public static class Factory implements LoggingSystemFactory {
+
+		@Override
+		public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
+			if (ClassUtils.isPresent("org.apache.logging.log4j.core.impl.Log4jContextFactory", classLoader)) {
+				return new Log4J2LoggingSystem(classLoader);
+			}
+			return null;
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -47,11 +47,15 @@ import org.springframework.boot.logging.LogLevel;
 import org.springframework.boot.logging.LoggerConfiguration;
 import org.springframework.boot.logging.LoggingInitializationContext;
 import org.springframework.boot.logging.LoggingSystem;
+import org.springframework.boot.logging.LoggingSystemFactory;
 import org.springframework.boot.logging.LoggingSystemProperties;
 import org.springframework.boot.logging.Slf4JLoggingSystem;
+import org.springframework.core.Ordered;
 import org.springframework.core.SpringProperties;
+import org.springframework.core.annotation.Order;
 import org.springframework.core.env.Environment;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ResourceUtils;
 import org.springframework.util.StringUtils;
 
@@ -331,4 +335,20 @@ public class LogbackLoggingSystem extends Slf4JLoggingSystem {
 
 	}
 
+	/**
+	 * {@link LoggingSystemFactory} that returns {@link LogbackLoggingSystem} if possible.
+	 */
+	@Order(Ordered.LOWEST_PRECEDENCE)
+	public static class Factory implements LoggingSystemFactory {
+
+		@Override
+		public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
+			if (ClassUtils.isPresent("ch.qos.logback.core.Appender", classLoader)) {
+				return new LogbackLoggingSystem(classLoader);
+			}
+			return null;
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/main/resources/META-INF/spring.factories

@@ -1,3 +1,9 @@
+# Logging Systems
+org.springframework.boot.logging.LoggingSystemFactory=\
+org.springframework.boot.logging.logback.LogbackLoggingSystem.Factory,\
+org.springframework.boot.logging.log4j2.Log4J2LoggingSystem.Factory,\
+org.springframework.boot.logging.java.JavaLoggingSystem.Factory
+
 # PropertySource Loaders
 org.springframework.boot.env.PropertySourceLoader=\
 org.springframework.boot.env.PropertiesPropertySourceLoader,\

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/DelegatingLoggingSystemFactoryTests.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2012-2020 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.boot.logging;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
+
+/**
+ * Tests for {@link DelegatingLoggingSystemFactory}.
+ *
+ * @author Phillip Webb
+ */
+class DelegatingLoggingSystemFactoryTests {
+
+	private ClassLoader classLoader = getClass().getClassLoader();
+
+	@Test
+	void getLoggingSystemWhenDelegatesFunctionIsNullReturnsNull() {
+		DelegatingLoggingSystemFactory factory = new DelegatingLoggingSystemFactory(null);
+		assertThat(factory.getLoggingSystem(this.classLoader)).isNull();
+	}
+
+	@Test
+	void getLoggingSystemWhenDelegatesFunctionReturnsNullReturnsNull() {
+		DelegatingLoggingSystemFactory factory = new DelegatingLoggingSystemFactory((cl) -> null);
+		assertThat(factory.getLoggingSystem(this.classLoader)).isNull();
+	}
+
+	@Test
+	void getLoggingSystemReturnsFirstNonNullLoggingSystem() {
+		List<LoggingSystemFactory> delegates = new ArrayList<>();
+		delegates.add(mock(LoggingSystemFactory.class));
+		delegates.add(mock(LoggingSystemFactory.class));
+		delegates.add(mock(LoggingSystemFactory.class));
+		LoggingSystem result = mock(LoggingSystem.class);
+		given(delegates.get(1).getLoggingSystem(this.classLoader)).willReturn(result);
+		DelegatingLoggingSystemFactory factory = new DelegatingLoggingSystemFactory((cl) -> delegates);
+		assertThat(factory.getLoggingSystem(this.classLoader)).isSameAs(result);
+		verify(delegates.get(0)).getLoggingSystem(this.classLoader);
+		verify(delegates.get(1)).getLoggingSystem(this.classLoader);
+		verifyNoInteractions(delegates.get(2));
+	}
+
+}