Why Maven dependency exclusion would not cause compile error?

Question

Newly exposed to Maven, I can understand the use case of the <exclusion> tag, but not sure why it wouldn't cause compile error:

<dependencies>
<dependency>
  <groupId>org.apache.maven</groupId>
  <artifactId>maven-embedder</artifactId>
  <version>2.0</version>
  <exclusions>
    <exclusion>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-core</artifactId>
    </exclusion>
  </exclusions>
</dependency>
...

Is this only possible only when you have another direct dependency on maven-core? otherwise, compile error should happen. (assuming maven-core is used somewhere in maven-embedder)

Answer 1

There are different possibilities:

1. As Carl said: Check your dependency:tree if the dependency is not pulled in from somewhere else.
2. It is possible that maven-core is not used at all, even if maven-embedder indeed uses it: Assume e.g. that maven-embedder has two classes A and B. You only use A, but maven-core is only used by B. Then (if A and B do not use each other), your project might be entirely independent of maven-core. (A side remark: some jars should logically be two separate jars, but where merged together by whatever reason - in our example, one should think about putting A and B in separate artifacts).
3. It is possible that transitive dependencies are not necessary at compile time, but are used at runtime.