Maven多模块项目：解决子模块自引用错误并排除根POM依赖

问题描述 #

我有一个Maven多模块项目，根pom.xml中声明了包括resources子模块在内的依赖。resources子模块仅用于存放各子模块的公共资源，不需要任何依赖。现在出现了子模块自引用错误，错误信息如下：

The project com.github.alopatin.kafka_learning.java:resources:1.0 (D:\Wokrspace\kafka-learing\java\resources\pom.xml) has 1 error
[ERROR]
'dependencies.dependency.[com.github.alopatin.kafka_learning.java:resources:1.0]' for com.github.alopatin.kafka_learning.java:resources:1.0 is referencing itself. @ com.github.alopatin.kafka_learning.java:java:1.0, D:\Wokrspace\kafka-learing\java\pom.xml, line 36, column 21

根POM代码如下：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.github.alopatin.kafka_learning.java</groupId>
    <artifactId>java</artifactId>
    <version>1.0</version>
    <modules>
        <module>resources</module>
        <module>producer-simple</module>
        <module>producer-json</module>
    </modules>

    <properties>
        <maven.compiler.source>23</maven.compiler.source>
        <maven.compiler.target>23</maven.compiler.target>
    </properties>

    <packaging>pom</packaging>

    <dependencies>
        <dependency>
            <groupId>com.github.alopatin.kafka_learning.java</groupId>
            <artifactId>resources</artifactId>
            <version>1.0</version>
        </dependency>
        <dependency>
            <groupId>com.typesafe</groupId>
            <artifactId>config</artifactId>
            <version>1.4.3</version>
        </dependency>
        <dependency>
            <groupId>org.apache.kafka</groupId>
            <artifactId>kafka-clients</artifactId>
            <version>3.8.1</version>
        </dependency>
    </dependencies>

    <build>
        ...
    </build>
</project>

问题原因 #

Maven中，根POM的<dependencies>块内的依赖会被所有子模块自动继承。你的根POM里声明了resources子模块的依赖，而resources子模块本身继承根POM，这就导致它自己引入了自己的依赖，触发自引用错误。

解决方案 #

方法1：使用<dependencyManagement>统一管理依赖（推荐） #

把根POM中的依赖移到<dependencyManagement>块，这个块仅负责声明依赖版本，不会让子模块自动继承，需要依赖的子模块再单独引入。

修改后的根POM：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.github.alopatin.kafka_learning.java</groupId>
    <artifactId>java</artifactId>
    <version>1.0</version>
    <modules>
        <module>resources</module>
        <module>producer-simple</module>
        <module>producer-json</module>
    </modules>

    <properties>
        <maven.compiler.source>23</maven.compiler.source>
        <maven.compiler.target>23</maven.compiler.target>
    </properties>

    <packaging>pom</packaging>

    <!-- 用dependencyManagement统一管理依赖版本，子模块不会自动继承 -->
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>com.github.alopatin.kafka_learning.java</groupId>
                <artifactId>resources</artifactId>
                <version>1.0</version>
            </dependency>
            <dependency>
                <groupId>com.typesafe</groupId>
                <artifactId>config</artifactId>
                <version>1.4.3</version>
            </dependency>
            <dependency>
                <groupId>org.apache.kafka</groupId>
                <artifactId>kafka-clients</artifactId>
                <version>3.8.1</version>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <build>
        ...
    </build>
</project>

然后在需要依赖的子模块（比如producer-simple、producer-json）的pom.xml中引入所需依赖：

<dependencies>
    <!-- 引入resources模块，无需写版本，会继承根POM的声明 -->
    <dependency>
        <groupId>com.github.alopatin.kafka_learning.java</groupId>
        <artifactId>resources</artifactId>
    </dependency>
    <!-- 引入其他需要的依赖 -->
    <dependency>
        <groupId>com.typesafe</groupId>
        <artifactId>config</artifactId>
    </dependency>
    <dependency>
        <groupId>org.apache.kafka</groupId>
        <artifactId>kafka-clients</artifactId>
    </dependency>
</dependencies>

resources子模块的pom.xml保持极简，不需要任何依赖：

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <groupId>com.github.alopatin.kafka_learning.java</groupId>
        <artifactId>java</artifactId>
        <version>1.0</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>resources</artifactId>
    <!-- 纯资源模块设置为jar打包即可 -->
    <packaging>jar</packaging>

</project>

方法2：在resources子模块中排除自引用依赖 #

如果不想改动根POM的结构，可以在resources子模块的pom.xml中排除自身的依赖：

<dependencies>
    <dependency>
        <groupId>com.github.alopatin.kafka_learning.java</groupId>
        <artifactId>java</artifactId>
        <version>1.0</version>
        <type>pom</type>
        <exclusions>
            <exclusion>
                <groupId>com.github.alopatin.kafka_learning.java</groupId>
                <artifactId>resources</artifactId>
            </exclusion>
        </exclusions>
    </dependency>
</dependencies>

不过这种方法不够优雅，更推荐使用第一种方案。

内容的提问来源于stack exchange，提问作者Alexander Lopatin