Log4j2 - Unrecognized conversion specifier [xwEx] starting at position 160 in conversion pattern

There is an issue reported that says Spring Boot 1.5 is not compatible with Log4j 2.8. https://github.com/spring-projects/spring-boot/issues/9172

I was getting errors like yours with Spring Boot 1.5 and Log4j 2.8.2, and the errors away when I switched to Log4j 2.7

springboot1 default log4j.xml use pattern: %xwEx is not supported by 2.8+, but we can override the pattern by set environment variable LOG_EXCEPTION_CONVERSION_WORD like: %throwable

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        System.setProperty("LOG_EXCEPTION_CONVERSION_WORD", "%throwable");
        SpringApplication.run(Application.class, args);
    }
}

I think that existing answers are correct but incomplete.

You get that error if both the following conditions occur:

1. You are using an old version of spring-boot with a recent version of Log4J2 (not ideal but possible; you are upgrading Log4J2 due to "Log4shell" vulnerability, right?)
2. You are using the default logging config file provided by spring-boot (it is /org/springframework/boot/logging/log4j2/log4j2.xml in spring-boot-x.x.x.RELEASE.jar). This is the real problem.

The default config file exists to allow a quick setup of new applications. Unfortunately it is not compatible with Log4J 2.8+.

Probably you want to use your own configuration and, perhaps, you have already prepared your config file but the application hasn't found it.

Possible solutions:

1. Upgrade spring-boot or downgrade Log4J2 (downgrading is not recommended, of course).

2. If you really want to use the default cfg (really?), set this environment variable: LOG_EXCEPTION_CONVERSION_WORD=%throwable as suggested by tsingke

3. Ensure that your own log4j2.xml or, even better, log4j2-spring.xml is in the classpath or in a path pointed out by the logging.config environment variable (see Custom Log Configuration).

I have used solution #3.

Side note: if you upgrade Log4J but continue using spring-boot 1.x you suffer from other vulnerabilities, so it should be just the first step, even if your boss (or your customer) only cares about Log4shell!