Maven says I have a cyclic reference in multi-module project but can't figure out why

Question

I have a multi-module project that looks like this:

• module1
  • pom.xml
• module2
  • pom.xml
• pom.xml

The pom.xml in module2 has a dependency on module1.

When I run mvn clean compile I get the following error:

The projects in the reactor contain a cyclic reference.

Here are my dependencies in module1:

<dependencies>
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.14</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
        <version>1.6.1</version>
    </dependency>
    <dependency>
        <groupId>com.jcraft</groupId>
        <artifactId>jsch</artifactId>
        <version>0.1.48</version>
    </dependency>
</dependencies>

I can't figure out why it says there is a cyclic reference. Even when I do mvn dependency:tree on module1 I get the following:

[INFO] +- log4j:log4j:jar:1.2.14:compile
[INFO] +- org.slf4j:slf4j-log4j12:jar:1.6.1:compile
[INFO] |  \- org.slf4j:slf4j-api:jar:1.6.1:compile
[INFO] +- com.jcraft:jsch:jar:0.1.48:compile
[INFO] \- junit:junit:jar:4.8.2:test

It looks to me like there aren't any references to module2 in module1. So where is the cyclic reference coming from?

Edit: Here is the log with debug on:

+ Error stacktraces are turned on.
Apache Maven 2.2.1 (r801777; 2009-08-06 15:16:01-0400)
Java version: 1.6.0_31
Default locale: en_US, platform encoding: Cp1252
OS name: "windows 7" version: "6.1" arch: "amd64" Family: "windows"
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] The projects in the reactor contain a cyclic reference: Edge between 'Vertex{label='com.myorg:module2'}' and 'Vertex{label='com.myorg:module1'}' introduces to cycle in the graph com.myorg:module1 --> com.myorg:module2 --> com.myorg:module1
[INFO] ------------------------------------------------------------------------
[DEBUG] Trace
org.apache.maven.BuildFailureException: The projects in the reactor contain a cyclic reference: Edge between 'Vertex{label='com.myorg:module2'}' and 'Vertex{label='com.myorg:module1'}' introduces to cycle in the graph com.myorg:module1 --> com.myorg:module2 --> com.myorg:module1
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:295)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:138)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:362)
at org.apache.maven.cli.compat.CompatibleMain.main(CompatibleMain.java:60)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at org.codehaus.classworlds.Launcher.mainWithExitCode(Launcher.java:430)
at org.codehaus.classworlds.Launcher.main(Launcher.java:375)
Caused by: hidden.org.codehaus.plexus.util.dag.CycleDetectedException: Edge between 'Vertex{label='com.myorg:module2'}' and 'Vertex{label='com.myorg:module1'}' introduces to cycle in the graph com.myorg:module1 --> com.myorg:module2 --> com.myorg:module1
at hidden.org.codehaus.plexus.util.dag.DAG.addEdge(DAG.java:143)
at hidden.org.codehaus.plexus.util.dag.DAG.addEdge(DAG.java:123)
at org.apache.maven.project.ProjectSorter.<init>(ProjectSorter.java:118)
at org.apache.maven.execution.ReactorManager.<init>(ReactorManager.java:99)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:288)
... 11 more
[INFO] ------------------------------------------------------------------------
[INFO] Total time: < 1 second
[INFO] Finished at: Thu Jul 05 17:21:21 EDT 2012    
[INFO] Final Memory: 3M/244M
[INFO] ------------------------------------------------------------------------

Answer 1

It happened to me in this circumstances.

The module_child_X was specified 2 times at module_root pom.xml:

- As a module

(pom.xml of the root module)
<dependency>
    <groupId>module_root</groupId>
    <artifactId>module_child_X</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <type>jar</type>
</dependency>

- As a dependency

(pom.xml of the root module)
<modules>
    <module>module_child_X</module>
    ...
</modules>

Solution?

Removed the module_child_X as a dependency. It is already specified as a module.

Answer 2

Ah! It was a misleading error.

The problem wasn't that there both module1 and module2 depended on each other. The problem was that module2 is a Maven plugin and in my root pom.xml I had the plugin in the section. I removed that plugin from the build and it started working.

Answer 3

I do nearly the same, and I use IDEA.

I have a project A, which depends on a module B. In the pom file of A, B was declared as a dependency. This is OK. In the pom file of B, A was declared as its parent. I removed this information, and as it was requested in the error message I added the version number of B in its pom file.

And now it is OK.

Answer 4

I had exactly the same issue in a multimodule ear project. The ejb pom had a dependency on the web module (compile scope) and the web pom a dependency on the ejb module. As soon as i removed the ejb's pom dependency on the web module, the project build fine. It makes sense that the intramodule dependencies must be unidirectional after all, in order to avoid cyclic references.