Properties config with partial “appenders” list fails in >= 2.25.0 with ConfigurationException: No type attribute provided for component appender

[justinas-dabravolskas]

Description

Upgrading to Log4j 2.25.0 introduces a regression in properties-based configuration when the top-level appenders property lists only a subset of the defined appenders. At startup, Log4j throws:
ConfigurationException: No type attribute provided for component appender

This appears to be caused by the new processing of “remaining” root-level properties in PropertiesConfigurationBuilder.build(), which now folds leftover dotted prefixes (like appender.) into synthetic components and then requires a type on the container key appender. In prior releases, extra appender. groups not listed in appenders did not cause a failure.
It was introduced by commit

Expected behavior

• Properties configs that specify appenders = … should not fail if additional appender..* groups exist but are not listed.

1. Only the listed (or referenced) appenders should be instantiated; unlisted groups should not trigger a root-level component parse or an exception.

Configuration

Version: >= 2.25.0

# Root logger configuration
status = warn
name = PropertiesConfig

appenders = console

# Console appender
appender.console.type = Console
appender.console.name = Console
appender.console.target = SYSTEM_OUT
appender.console.layout.type = PatternLayout
appender.console.layout.pattern = %d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n

appender.console2.type = Console
appender.console2.name = Console
appender.console2.target = SYSTEM_OUT
appender.console2.layout.type = PatternLayout
appender.console2.layout.pattern = %d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n

Logs

Caused by: org.apache.logging.log4j.core.config.ConfigurationException: No type attribute provided for component appender
	at org.apache.logging.log4j.core.config.properties.PropertiesConfigurationBuilder.createComponent(PropertiesConfigurationBuilder.java:362)
	at org.apache.logging.log4j.core.config.properties.PropertiesConfigurationBuilder.processRemainingProperties(PropertiesConfigurationBuilder.java:206)
	at org.apache.logging.log4j.core.config.properties.PropertiesConfigurationBuilder.build(PropertiesConfigurationBuilder.java:190)
Caused by: org.apache.logging.log4j.core.config.ConfigurationException: No type attribute provided for component appender

	at org.apache.logging.log4j.core.config.properties.PropertiesConfigurationFactory.getConfiguration(PropertiesConfigurationFactory.java:56)
	at org.apache.logging.log4j.core.config.properties.PropertiesConfigurationFactory.getConfiguration(PropertiesConfigurationFactory.java:34)
	at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:544)
	at org.apache.logging.log4j.core.config.ConfigurationFactory$Factory.getConfiguration(ConfigurationFactory.java:463)
	at org.apache.logging.log4j.core.config.ConfigurationFactory.getConfiguration(ConfigurationFactory.java:321)
	at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:778)
	at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:808)
	at org.apache.logging.log4j.core.LoggerContext.start(LoggerContext.java:311)
	at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:160)
	at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:48)
	at org.apache.logging.log4j.LogManager.getContext(LogManager.java:139)
	at org.apache.logging.log4j.LogManager.getLogger(LogManager.java:557)
	at com.l4jmre.App.<clinit>(App.java:11)

Reproduction

An example configuration throws the above exception.

Workarounds

• Remove the appenders line entirely (let names be inferred), or
• List all defined appenders: appenders = console, console2

Both avoid the synthetic root-level “component appender” and the resulting exception.