Support <springProperty> in logback configurations

Include support for a new <springProperty> element which can be used in
`logback-spring.xml` files to add properties from the Spring
Environment. For example:

	<configuration>
		...
		<springProperty name="destination" source="my.loggger.extradest"/>
		<appender name="FILE" class="ch.qos.logback.core.FileAppender">
			<file>${destination}</file>
				...
			</file>
		</appender>
		...
	</configuration>

Fixes gh-1788

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

@@ -17,6 +17,7 @@
 package org.springframework.boot.logging.logback;
 
 import org.springframework.boot.logging.LoggingInitializationContext;
+import org.springframework.core.env.Environment;
 
 import ch.qos.logback.classic.joran.JoranConfigurator;
 import ch.qos.logback.core.joran.action.NOPAction;
@@ -40,6 +41,9 @@ class SpringBootJoranConfigurator extends JoranConfigurator {
 	@Override
 	public void addInstanceRules(RuleStore rs) {
 		super.addInstanceRules(rs);
+		Environment environment = this.initializationContext.getEnvironment();
+		rs.addRule(new ElementSelector("configuration/springProperty"),
+				new SpringPropertyAction(environment));
 		rs.addRule(new ElementSelector("*/springProfile"), new SpringProfileAction(
 				this.initializationContext.getEnvironment()));
 		rs.addRule(new ElementSelector("*/springProfile/*"), new NOPAction());

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

@@ -0,0 +1,81 @@
+/*
+ * Copyright 2012-2015 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.logback;
+
+import org.springframework.boot.bind.RelaxedPropertyResolver;
+import org.springframework.core.env.Environment;
+import org.xml.sax.Attributes;
+
+import ch.qos.logback.core.joran.action.Action;
+import ch.qos.logback.core.joran.action.ActionUtil;
+import ch.qos.logback.core.joran.action.ActionUtil.Scope;
+import ch.qos.logback.core.joran.spi.ActionException;
+import ch.qos.logback.core.joran.spi.InterpretationContext;
+import ch.qos.logback.core.util.OptionHelper;
+
+/**
+ * Lockback {@link Action} to support {@code <springProperty>} tags. Allows logback
+ * properties to be sourced from the Spring environment.
+ *
+ * @author Phillip Webb
+ */
+class SpringPropertyAction extends Action {
+
+	private static final String SOURCE_ATTRIBUTE = "source";
+
+	private final Environment environment;
+
+	public SpringPropertyAction(Environment environment) {
+		this.environment = environment;
+	}
+
+	@Override
+	public void begin(InterpretationContext ic, String elementName, Attributes attributes)
+			throws ActionException {
+		String name = attributes.getValue(NAME_ATTRIBUTE);
+		String source = attributes.getValue(SOURCE_ATTRIBUTE);
+		Scope scope = ActionUtil.stringToScope(attributes.getValue(SCOPE_ATTRIBUTE));
+		if (OptionHelper.isEmpty(name) || OptionHelper.isEmpty(source)) {
+			addError("The \"name\" and \"source\" attributes of <springProperty> must be set");
+		}
+		ActionUtil.setProperty(ic, name, getValue(source), scope);
+	}
+
+	private String getValue(String source) {
+		if (this.environment == null) {
+			addWarn("No Spring Environment available to resolve " + source);
+			return null;
+		}
+		String value = this.environment.getProperty(source);
+		if (value != null) {
+			return value;
+		}
+		int lastDot = source.lastIndexOf(".");
+		if (lastDot > 0) {
+			String prefix = source.substring(0, lastDot + 1);
+			RelaxedPropertyResolver resolver = new RelaxedPropertyResolver(
+					this.environment, prefix);
+			return resolver.getProperty(source.substring(lastDot + 1));
+		}
+		return null;
+	}
+
+	@Override
+	public void end(InterpretationContext ic, String name) throws ActionException {
+	}
+
+}

spring-boot/src/test/java/org/springframework/boot/logging/logback/SpringBootJoranConfiguratorTests.java

@@ -23,6 +23,7 @@ import org.junit.Test;
 import org.slf4j.Logger;
 import org.slf4j.impl.StaticLoggerBinder;
 import org.springframework.boot.logging.LoggingInitializationContext;
+import org.springframework.boot.test.EnvironmentTestUtils;
 import org.springframework.boot.test.OutputCapture;
 import org.springframework.mock.env.MockEnvironment;
 
@@ -31,7 +32,9 @@ import ch.qos.logback.classic.joran.JoranConfigurator;
 import ch.qos.logback.core.joran.spi.JoranException;
 
 import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.not;
+import static org.junit.Assert.assertThat;
 
 /**
  * Tests for {@link SpringBootJoranConfigurator}.
@@ -103,6 +106,20 @@ public class SpringBootJoranConfiguratorTests {
 		doTestNestedProfile(false);
 	}
 
+	@Test
+	public void springProperty() throws Exception {
+		EnvironmentTestUtils.addEnvironment(this.environment, "my.example-property:test");
+		initialize("property.xml");
+		assertThat(this.context.getProperty("MINE"), equalTo("test"));
+	}
+
+	@Test
+	public void relaxedSpringProperty() throws Exception {
+		EnvironmentTestUtils.addEnvironment(this.environment, "my.EXAMPLE_PROPERTY:test");
+		initialize("property.xml");
+		assertThat(this.context.getProperty("MINE"), equalTo("test"));
+	}
+
 	private void doTestNestedProfile(boolean expected, String... profiles)
 			throws JoranException {
 		this.environment.setActiveProfiles(profiles);

spring-boot/src/test/resources/org/springframework/boot/logging/logback/property.xml

@@ -0,0 +1,5 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<configuration>
+	<include resource="org/springframework/boot/logging/logback/base.xml" />
+	<springProperty scope="context" name="MINE" source="my.example-property"/>
+</configuration>