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在一些项目中,我们知道用exclusion排除一些依赖包,这属于依赖排除(Dependency Exclusions),还有一种就是今天所说的可选依赖(Optional Dependencies)。主要还是讲怎么用。
准备工作
准备两个工程,简单点,就是A和B
只看POM文件,这是A的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>
这是B的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>
在这种情况下,joda-time包在B工程会被正常引用。
加入optional
在A工程对joda-time添加optional选项,这时在B工程中,joda-time依赖包会消失.
<dependencies>
<dependency>
<groupId>joda-time</groupId>
<artifactId>joda-time</artifactId>
<version>2.9.9</version>
<optional>true</optional>
</dependency>
</dependencies>
如果想引这个包,需要在A项目中设optional为false或者去除optional,或者在B项目中显式调用。
parent 继承的情况
如果A的pom像下面这样配置
<?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>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>joda-time</groupId>
<artifactId>joda-time</artifactId>
<version>2.9.9</version>
<optional>true</optional>
</dependency>
</dependencies>
</dependencyManagement>
</project>
B再去引用的话,还是可以正常引用joda-time包,optional选项在统一控制版本的情况下会失效。
<?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>