Spring Boot 4.2 migrator 一收紧，旧配置键就把进程挂了

升到 Spring Boot 4.2.0-RC1 时，配置里还留着 logging.path、spring.application.index 这类旧键。想让 migrator 更严一点，把 spring.properties.migrator.fail 收到 on-error 或 on-warning，进程会直接挂：日志先打出旧键报告，再来 Application run failed 和 IllegalStateException: Found configuration keys that need to be migrated，退出码 1。默认 never 只警告，应用还能起来。

环境是 JDK 21.0.12.1、Boot 4.2.0-RC1。runtime 挂上 migrator：

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-properties-migrator</artifactId>
  <scope>runtime</scope>
</dependency>

RC1 发行说明「Properties Migrator Fail Support」给 migrator 加了失败开关；附录键名是 spring.properties.migrator.fail，取值 never（默认）、on-error、on-warning。migrator 模块本身很早就有，这次新的是能把报告升级成启动失败。升级窗口里可以先用它扫出还在用的旧键；CI 里再决定要不要长期开着。

先不碰 fail，只带着改名键启动：

java -jar app.jar --logging.path=logs-demo

2026-10-27T08:12:48.179+08:00  INFO 506771 --- [           main] c.e.m.MigratorFailApplication            : Started MigratorFailApplication in 0.581 seconds (process running for 0.829)
2026-10-27T08:12:48.181+08:00  WARN 506771 --- [           main] o.s.b.c.p.m.PropertiesMigrationListener  : 
The use of configuration keys that have been renamed was found in the environment:

Property source 'commandLineArgs':
	Key: logging.path
		Replacement: logging.file.path


Each configuration key has been temporarily mapped to its replacement for your convenience. To silence this warning, please update your configuration to use the new keys.

exit=0

进程起来了。WARN 写明 logging.path 已改名，替代键是 logging.file.path，当前只是临时映射。换成已删除的 spring.application.index，级别变成 ERROR，照样 exit=0：

java -jar app.jar --spring.application.index=1

2026-10-27T08:12:49.009+08:00  INFO 506821 --- [           main] c.e.m.MigratorFailApplication            : Started MigratorFailApplication in 0.538 seconds (process running for 0.792)
2026-10-27T08:12:49.012+08:00 ERROR 506821 --- [           main] o.s.b.c.p.m.PropertiesMigrationListener  : 
The use of configuration keys that are no longer supported was found in the environment:

Property source 'commandLineArgs':
	Key: spring.application.index
		Reason: Application context ids are now unique by default.


Please refer to the release notes or reference guide for potential alternatives.

exit=0

Reason 写得很直：Application context ids are now unique by default。日志里「renamed」和「no longer supported」是两类问题；默认 fail 都只报、不拦。

升级时若先只想挡住已删除键，打开 on-error，再带上 spring.application.index：

java -jar app.jar --spring.application.index=1 --spring.properties.migrator.fail=on-error

The use of configuration keys that are no longer supported was found in the environment:

Property source 'commandLineArgs':
	Key: spring.application.index
		Reason: Application context ids are now unique by default.

2026-10-27T08:12:49.598+08:00 ERROR 506855 --- [           main] o.s.boot.SpringApplication               : Application run failed

java.lang.IllegalStateException: Found configuration keys that need to be migrated
exit=1

同一条 unsupported 报告之后，Spring Boot 打出 Application run failed，异常文案是 Found configuration keys that need to be migrated，进程退出 1。只带着改名键 logging.path、同样开 on-error，则仍能启动：

2026-10-27T08:12:50.428+08:00  INFO 506923 --- [           main] c.e.m.MigratorFailApplication            : Started MigratorFailApplication in 0.547 seconds (process running for 0.791)
2026-10-27T08:12:50.431+08:00  WARN 506923 --- [           main] o.s.b.c.p.m.PropertiesMigrationListener  : 
The use of configuration keys that have been renamed was found in the environment:

Property source 'commandLineArgs':
	Key: logging.path
		Replacement: logging.file.path

exit=0

on-error 只认 unsupported 那一类。改名键还在用时，进程不会因为这一档停住。

若升级验收要求改名键也不能过，收到 on-warning：

java -jar app.jar --logging.path=logs-demo --spring.properties.migrator.fail=on-warning

2026-10-27T08:12:51.037+08:00  WARN 507011 --- [           main] o.s.b.c.p.m.PropertiesMigrationListener  : 
The use of configuration keys that have been renamed was found in the environment:

Property source 'commandLineArgs':
	Key: logging.path
		Replacement: logging.file.path


Each configuration key has been temporarily mapped to its replacement for your convenience. To silence this warning, please update your configuration to use the new keys.

2026-10-27T08:12:51.045+08:00 ERROR 507011 --- [           main] o.s.boot.SpringApplication               : Application run failed

java.lang.IllegalStateException: Found configuration keys that need to be migrated
exit=1

还是那条 renamed WARN，紧接着同样的 IllegalStateException 和退出码 1。读日志时先看 migrator 报的是 renamed 还是 no longer supported，再看后面有没有 Application run failed；有失败栈就说明当前 fail 档位已经把报告抬成了启动错误。

要解卡：把 logging.path 改成 logging.file.path；删掉或换掉 spring.application.index（metadata 给出的原因就是 Application context ids are now unique by default.）。旧键清干净后，再决定 CI 里是否继续留着 on-warning，避免以后又混进过期键。同一次 RC1 里默认 Ubuntu 26.04 builder 和 bindings 的启动问题，见 Spring Boot 4.2.0-RC1 默认 Ubuntu 26.04，bindings 会启动失败，那篇用的是另一组旧键。

依据：Spring Boot 4.2.0-RC1 Release Notes、附录 Application Properties、Upgrading Spring Boot。