Make it easier to override FILE_LOG_PATTERN

Closes gh-6782

spring-boot/src/main/resources/org/springframework/boot/logging/logback/defaults.xml

@@ -10,7 +10,7 @@ initialization performed by Boot
 	<conversionRule conversionWord="wex" converterClass="org.springframework.boot.logging.logback.WhitespaceThrowableProxyConverter" />
 	<conversionRule conversionWord="wEx" converterClass="org.springframework.boot.logging.logback.ExtendedWhitespaceThrowableProxyConverter" />
 	<property name="CONSOLE_LOG_PATTERN" value="${CONSOLE_LOG_PATTERN:-%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %clr(${LOG_LEVEL_PATTERN:-%5p}) %clr(${PID:- }){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} %m%n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}}"/>
-	<property name="FILE_LOG_PATTERN" value="%d{yyyy-MM-dd HH:mm:ss.SSS} ${LOG_LEVEL_PATTERN:-%5p} ${PID:- } --- [%t] %-40.40logger{39} : %m%n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}"/>
+	<property name="FILE_LOG_PATTERN" value="${FILE_LOG_PATTERN:-%d{yyyy-MM-dd HH:mm:ss.SSS} ${LOG_LEVEL_PATTERN:-%5p} ${PID:- } --- [%t] %-40.40logger{39} : %m%n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}}"/>
 
 	<appender name="DEBUG_LEVEL_REMAPPER" class="org.springframework.boot.logging.logback.LevelRemappingAppender">
 		<destinationLogger>org.springframework.boot</destinationLogger>

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

@@ -24,6 +24,7 @@ import ch.qos.logback.classic.joran.JoranConfigurator;
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.core.Appender;
 import ch.qos.logback.core.ConsoleAppender;
+import ch.qos.logback.core.FileAppender;
 import ch.qos.logback.core.encoder.Encoder;
 import ch.qos.logback.core.joran.spi.JoranException;
 import org.junit.Test;
@@ -52,4 +53,18 @@ public class LogbackConfigurationTests {
 		assertThat(((PatternLayoutEncoder) encoder).getPattern()).isEqualTo("foo");
 	}
 
+	@Test
+	public void filePatternCanBeOverridden() throws JoranException {
+		JoranConfigurator configurator = new JoranConfigurator();
+		LoggerContext context = new LoggerContext();
+		configurator.setContext(context);
+		configurator
+				.doConfigure(new File("src/test/resources/custom-file-log-pattern.xml"));
+		Appender<ILoggingEvent> appender = context.getLogger("ROOT").getAppender("FILE");
+		assertThat(appender).isInstanceOf(FileAppender.class);
+		Encoder<?> encoder = ((FileAppender<?>) appender).getEncoder();
+		assertThat(encoder).isInstanceOf(PatternLayoutEncoder.class);
+		assertThat(((PatternLayoutEncoder) encoder).getPattern()).isEqualTo("bar");
+	}
+
 }

spring-boot/src/test/resources/custom-file-log-pattern.xml

+<configuration>
+	<property name="FILE_LOG_PATTERN" value="bar" />
+	<include resource="org/springframework/boot/logging/logback/base.xml" />
+</configuration>