maven中的可选依赖true

看了这三种情况的例子可能你就明白了

1、父模块的依赖中不添加true

父模块pom文件：

<?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>A</groupId>
    <artifactId>A</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
            <version>2.9.9</version>
        </dependency>
    </dependencies>
</project>

子模块pom文件：

<?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>B</groupId>
    <artifactId>B</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>A</groupId>
            <artifactId>A</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>
</project>

结论：
父模块A的依赖中不添加true，在这种情况下，在子模块B中会引用joda-time依赖jar包

2、父模块的依赖中添加true，不显式调用joda-time的jar包依赖

父模块pom文件：

<?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>A</groupId>
    <artifactId>A</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
            <version>2.9.9</version>
            <optional>true</optional>
        </dependency>
    </dependencies>
</project>

子模块pom文件：

<?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>B</groupId>
    <artifactId>B</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>A</groupId>
            <artifactId>A</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>
</project>

结论：
父模块A的依赖中不添加true，不显式调用joda-time的jar包依赖，在这种情况下，在子模块B中不会引用joda-time依赖jar包

3、父模块的依赖中添加true，显式调用joda-time的jar包依赖

父模块pom文件：

<?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>A</groupId>
    <artifactId>A</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
            <version>2.9.9</version>
            <optional>true</optional>
        </dependency>
    </dependencies>
</project>

子模块pom文件：

<?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>B</groupId>
    <artifactId>B</artifactId>
    <version>1.0-SNAPSHOT</version>

    <parent>
        <groupId>A</groupId>
        <artifactId>A</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <dependencies>
        <dependency>
            <groupId>joda-time</groupId>
            <artifactId>joda-time</artifactId>
        </dependency>
    </dependencies>
</project>

结论：
父模块A的依赖中添加true，显式调用joda-time的jar包依赖，在这种情况下，在子模块B中会正常引用joda-time依赖jar包，对于true来说，父模块中是否加上都一样，都是需要显示调用才能引用joda-time的jar包依赖

注意：如果你还是不明白，那就看https://segmentfault.com/a/1190000019266080?utm_source=tag-newest