[LOG4J2-2670] Add an option to customize the ObjectMapper for JSONLayout

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 2.11.2, 2.12.0
• Component/s: Layouts

Environment

Java 11, Spring Boot 2.1.5 with Log4J 2.11.2.

• Epic Link: Plugin Ergonomics
• Assignee: Unassigned
• Reporter: Dominik Sandjaja
• Created: 05/Aug/19 08:26
• Updated: 07/Oct/19 15:27

Description

We are using the JSON layout in our Spring Boot application to easily ingest the logs in Elasticsearch.
 
One message that we log is about failed conversions, when an incoming request parameter cannot be converted correctly. As we use Spring's Converter functionality for this, these IllegalArgumentExceptions are wrapped in a ConversionFailedException. This exception has fields of type TypeDescriptor which hold information about the source and the target type of the conversion.
 
The problem is, that these TypeDescriptor objects cannot be serialized to JSON. The simple call
 

com.fasterxml.jackson.databind.ObjectMapper().writeValueAsString(org.springframework.core.convert.TypeDescriptor.valueOf(String.javaClass))

 
throws an exception like
 

ERROR StatusLogger com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Direct self-reference leading to cycle (through reference chain: org.springframework.core.ResolvableType["componentType"]->org.springframework.core.ResolvableType["componentType"])
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Direct self-reference leading to cycle (through reference chain: org.springframework.core.ResolvableType["componentType"]->org.springframework.core.ResolvableType["componentType"])
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)
    at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:944)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:721)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)

The error can be prevented if Mixins are added to the ObjectMapper which ignore the corresponding class.
 
The Log4jJsonObjectMapper is initialized in the JacksonFactory.JSON class and I currently do not see any way to somehow configure this ObjectMapper with additional MixIns.

As discussed on the Mailinglist, this might be solved in several different ways, e.g. with the Plugin mechanism or with an explicit modifyObjectMapper hook.