java.lang.LinkageError - loader constraints violated

Symptom: "BeanCreationException: Error creating bean with name", "java.lang.LinkageError: loader constraints violated"

Cause: the class has already been loaded from different location than the plugin

Solution: see Troubleshooting a LinkageError

Example:

1
2
2010-02-23 13:29:51,421 http-8080-Processor25 ERROR admin 48590x7x1 b0gp1r 
http://localhost:8080/rest/gadget/1.0/stats/generate [atlassian.plugin.servlet.DefaultServletModuleManager] Unable to create filter
com.atlassian.plugin.servlet.util.LazyLoadedReference$InitializationException: 
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'com.atlassian.jira.gadgets.stats.IssueTableResource': 
Instantiation of bean failed; nested exception is java.lang.LinkageError: loader constraints violated when linking org/apache/log4j/Logger class
    at com.atlassian.plugin.servlet.util.LazyLoadedReference.get(LazyLoadedReference.java:94)
...

In this case the log4j class appears to be the cause of the error. Note that this error will also occur if your plugin has imported but not otherwise used the class that is using the missing class (so beware of import foo.* statements).