Make Logback fall back to JVM's default charset

The charset "default" is an alias for US-ASCII, not the JVM's default
charset. This commit updates the built-in Logback configuration to
use Charset.defaultCharset().name() in place of "default" in the
Java-based configuration. In the XML-based configuration where
Charset.defaultCharset().name() cannot be called, we emulate its
behaviour [1] by using the file.encoding system property, falling back
to UTF-8 when it's not set.

Fixes gh-27230

[1] https://github.com/openjdk/jdk8u/blob/19be6113dd0f658f950583b751284961d8ce0458/jdk/src/share/classes/java/nio/charset/Charset.java#L604-L617

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -72,11 +72,14 @@ private void defaults(LogbackConfigurator config) {
 				+ "%clr(%d{${LOG_DATEFORMAT_PATTERN:-yyyy-MM-dd HH:mm:ss.SSS}}){faint} %clr(${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}}"));
-		config.getContext().putProperty("CONSOLE_LOG_CHARSET", resolve(config, "${CONSOLE_LOG_CHARSET:-default}"));
+		String defaultCharset = Charset.defaultCharset().name();
+		config.getContext().putProperty("CONSOLE_LOG_CHARSET",
+				resolve(config, "${CONSOLE_LOG_CHARSET:-" + defaultCharset + "}"));
 		config.getContext().putProperty("FILE_LOG_PATTERN", resolve(config, "${FILE_LOG_PATTERN:-"
 				+ "%d{${LOG_DATEFORMAT_PATTERN:-yyyy-MM-dd HH:mm:ss.SSS}} ${LOG_LEVEL_PATTERN:-%5p} ${PID:- } --- [%t] "
 				+ "%-40.40logger{39} : %m%n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}}"));
-		config.getContext().putProperty("FILE_LOG_CHARSET", resolve(config, "${FILE_LOG_CHARSET:-default}"));
+		config.getContext().putProperty("FILE_LOG_CHARSET",
+				resolve(config, "${FILE_LOG_CHARSET:-" + defaultCharset + "}"));
 		config.logger("org.apache.catalina.startup.DigesterFactory", Level.ERROR);
 		config.logger("org.apache.catalina.util.LifecycleBase", Level.ERROR);
 		config.logger("org.apache.coyote.http11.Http11NioProtocol", Level.WARN);

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

@@ -10,9 +10,9 @@ Default logback configuration provided for import
 	<conversionRule conversionWord="wEx" converterClass="org.springframework.boot.logging.logback.ExtendedWhitespaceThrowableProxyConverter" />
 
 	<property name="CONSOLE_LOG_PATTERN" value="${CONSOLE_LOG_PATTERN:-%clr(%d{${LOG_DATEFORMAT_PATTERN:-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${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}}"/>
-	<property name="CONSOLE_LOG_CHARSET" value="${CONSOLE_LOG_CHARSET:-default}"/>
+	<property name="CONSOLE_LOG_CHARSET" value="${CONSOLE_LOG_CHARSET:-${file.encoding:-UTF-8}}"/>
 	<property name="FILE_LOG_PATTERN" value="${FILE_LOG_PATTERN:-%d{${LOG_DATEFORMAT_PATTERN:-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_CHARSET" value="${FILE_LOG_CHARSET:-default}"/>
+	<property name="FILE_LOG_CHARSET" value="${FILE_LOG_CHARSET:-${file.encoding:-UTF-8}}"/>
 
 	<logger name="org.apache.catalina.startup.DigesterFactory" level="ERROR"/>
 	<logger name="org.apache.catalina.util.LifecycleBase" level="ERROR"/>