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<optional>true</optional>
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<optional>true</optional>，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<optional>true</optional>，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<optional>true</optional>，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<optional>true</optional>，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<optional>true</optional>，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<optional>true</optional>来说，父模块中是否加上<dependencyManagement>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
