Print RuntimeException raised by StructuredLogFormatter::format to system err

[quaff]

Before this commit, runtime exception is swallowed silently.

For example, if two StructuredLoggingJsonMembersCustomizer add same key, exception is printed now:

Exception in thread "main" java.lang.IllegalStateException: The name 'test' has already been written
	at org.springframework.util.Assert.state(Assert.java:101)
	at org.springframework.boot.json.JsonValueWriter.writePair(JsonValueWriter.java:228)
	at org.springframework.boot.json.JsonValueWriter.write(JsonValueWriter.java:83)
	at org.springframework.boot.json.JsonWriter$Member.write(JsonWriter.java:650)
	at org.springframework.boot.json.JsonWriter$Members.write(JsonWriter.java:339)

[philwebb]

This is an interesting problem. I'm not sure if JsonWriterStructuredLogFormatter is the best place to fix it. Perhaps org.springframework.boot.logging.logback.StructuredLogEncoder and org.springframework.boot.logging.log4j2.StructuredLogLayout might be better. It's interesting that logback doesn't handle encoder problems directly.

[quaff]

This is an interesting problem. I'm not sure if JsonWriterStructuredLogFormatter is the best place to fix it. Perhaps org.springframework.boot.logging.logback.StructuredLogEncoder and org.springframework.boot.logging.log4j2.StructuredLogLayout might be better. It's interesting that logback doesn't handle encoder problems directly.

Amended as your suggestion.

[mhalbritter]

I assumed that Logback would handle any exception raised in an encoder. If that's not the case, StructuredLogEncoder and StructuredLogLayout should print them.

[philwebb]

Thanks for the PR @quaff. Looking into this in more detail, I'm not sure that we should be printing the exception ourselves. It looks like we perhaps should be hooking into more of the logging library features to deal with errors. I've opened #43384 to track that.