I am experiencing the same behavior. I have a custom spring mediator that calls out to a database for role based authNZ. I am running esb v 4.7.0.
I downloaded spring-tx-3.1.0.RELEASE.jar from the maven repository and copied it to /usr/local/wso2/wso2esb-4.7.0/repository/components/lib in my environment, then restarted the ESB. When I issue requests to my proxy service, the same class not found exception occurs.
I was examining jar contents today and when I checked the spring-tx jar in /usr/local/wso2/wso2esb-4.7.0/repository/components/lib, the class in present:
jar tf spring-tx-3.1.0.RELEASE.jar | grep TransactionSynchronization
org/springframework/transaction/support/TransactionSynchronization.class
When I do the same in /usr/local/wso2/wso2esb-4.7.0/repository/components/dropins after restart of the esb, the class is not present:
jar tf ../dropins/spring_tx_3.1.0.RELEASE_1.0.0.jar
spring-tx-3.1.0.RELEASE.jar
META-INF/
META-INF/p2.inf
META-INF/MANIFEST.MF
Notice that the spring-tx jar is not included in the OSGi bundle after restart of Synapse.
I will investigate further tomorrow, including building an OSGi bundle that contains the Spring dependencies I need via Eclipse