Where to put persistence.xml in library jar using maven?

As @Dave mentions above src/main/resources is the place. When it comes to persistence.xml it should be placed in the META-INF folder. So to conclude: src/main/resources/META-INF/persistence.xml.

In the maven pom.xml file you have to add the resource directory, so you don't have to add it to the build-path in eclipse manually.

<project>
  ...
  <build>
    ...
    <resources>
      <resource>
        <directory>src/resources</directory>
      </resource>
    </resources>
    ...
  </build>
  ...
</project>

see: http://maven.apache.org/plugins/maven-resources-plugin/examples/resource-directory.html

XML configuration files almost always belong in src/main/resources, in a package hierarchy (just like Java source files). Those files will be copied into the artifact in the same hierarchy.

I have only gotten persistence.xml to work in my WAR when placed at WEB-INF/classes/META-INF, just as the documentation advises:

http://docs.oracle.com/cd/E19159-01/819-3669/bnbrj/index.html states the following:

If you package the persistence unit as a set of classes in a WAR file, persistence.xml should be located in the WAR file’s WEB-INF/classes/META-INF directory.

In my Eclipse project props, I added this to the build path:

src\main\resources

My Maven project has persistence.xml at the following location:

src\main\resources\META-INF\persistence.xml

After running Maven clean install, a WAR is built with:

WEB-INF\classes\META-INF\persistence.xml

When I drop this WAR into Tomcat 7's webapps folder, it is deployed properly and JPA works.