Eclipse: Add another Project as Dependency may Cause Unexpected Exception


The Problem
In local development, we run spring-boot application in eclipse tomcat - as we also deploy the project as a war.

But for some reason, one developer stills run it as a java application, and it fails with error - while it works well when run in (eclipse) tomcat.
Bean instantiation via factory method failed; nested exception is org.springframework.beans.BeanInstantiationException:
Failed to instantiate [com.amazonaws.services.s3.AmazonS3]: Factory method 's3Client' threw exception;
nested exception is java.lang.NoSuchMethodError: com.amazonaws.handlers.HandlerChainFactory.getGlobalHandlers()Ljava/util/List;

The Root Cause
Maven uses nearest wins strategy to determine which version to use, So we explicitly specify what version of aws-java-sdk-s3 to use in admin's pom.xml, but one library also implicitly depends on aws-java-sdk-s3 in common-module project. 

run mvn dependency:tree -Dverbose -Dincludes=com.amazonaws:aws-java-sdk-core, which shows that maven chooses the right version.
[INFO] |  \- com.amazonaws:aws-java-sdk-cloudfront:jar:1.11.32:compile
[INFO] |     \- (com.amazonaws:aws-java-sdk-core:jar:1.11.32:compile - omitted for conflict with 1.11.98)

- We can also get which version maven uses and why in eclipse: 
open pom,xml then go to dependency hierarchy tab , select the library in right panel.

I tried to run it as a java application, it works fine. But why it failed in his environment?

I compared the difference between his eclipse setup and mine, and found out that he manually added common-module in admin's Java Build Path -> Projects tab.

Now it's kind of clear why it failed: when we add a project as dependency, Eclipse also includes all libraries it depends on to the project. So now the project includes both versions, and Eclipse chooses the wrong version to use.

I created one bug Bug 514094 - Adding another Project as Dependency Causes Unexpected Exception to track it.

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