dependencyManagement和dependencies的不同

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前言

随着Spring全家桶越做越大，和其他框架的结合的机会也越来越多，现在做Web基本是起手就是一个Spring。这就会出现版本号冲突的问题，比如新的Spring和旧的Quartz不兼容之类的情况。

Spring framework bom

为了解决这个问题，Spring推出了spring-framework-bom。Spring Boot有对应的spring-boot-dependencies。
只需要将以下pom.xml加入我们项目的pom.xml，就不用在dependencies标签中写版本号。dependencyManagement会自动帮我们引入版本号。

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<project>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-framework-bom</artifactId>
                <version>${spring.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
        </dependency>
    </dependencies>
</project>

我们可以看下spring-framework-bom的源码，可以看到它已经默认为我们填好了版本号。

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<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>org.springframework</groupId>
    <artifactId>spring-framework-bom</artifactId>
    <version>4.2.0.RELEASE</version>
    <packaging>pom</packaging>
    <name>Spring Framework (Bill of Materials)</name>
    <description>Spring Framework (Bill of Materials)</description>
    <url>
        https://github.com/spring-projects/spring-framework
    </url>
    <organization>
        <name>Spring IO</name>
        <url>http://projects.spring.io/spring-framework</url>
    </organization>
    <licenses>
        <license>
            <name>The Apache Software License, Version 2.0</name>
            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
            <distribution>repo</distribution>
        </license>
    </licenses>
    <developers>
        <developer>
            <id>jhoeller</id>
            <name>Juergen Hoeller</name>
            <email>jhoeller@pivotal.io</email>
        </developer>
    </developers>
    <scm>
        <connection>
            scm:git:git://github.com/spring-projects/spring-framework
        </connection>
        <developerConnection>
            scm:git:git://github.com/spring-projects/spring-framework
        </developerConnection>
        <url>
            https://github.com/spring-projects/spring-framework
        </url>
    </scm>
    <issueManagement>
        <system>Jira</system>
        <url>https://jira.springsource.org/browse/SPR</url>
    </issueManagement>
    <!--===================注意这里===================-->
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-aop</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-aspects</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-beans</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-context</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-context-support</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-core</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-expression</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-instrument</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-instrument-tomcat</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-jdbc</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-jms</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-messaging</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-orm</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-oxm</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-test</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-tx</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-web</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-webmvc</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-webmvc-portlet</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-websocket</artifactId>
                <version>4.2.0.RELEASE</version>
            </dependency>
        </dependencies>
    </dependencyManagement>
    <!--===================注意这里===================-->
</project>

总结

dependencyManagement是一个版本号管理器，官方说法是中央依赖版本管理。
dependencyManagement子标签中的dependency声明的版本号，可以被引用了该dependencyManagement的pom.xml继承，但是需要在pom.xml中声明dependency而不必声明版本号。
简单地说，就是自己的pom.xml不用再写dependencyManagement中拥有的dependency的版本号了，但是要写dependency的名称(这里的名称只是形象的说法)

!!注意!!
dependencyManagement并不会产生继承关系，引用了dependencyManagement的新项目的pom.xml，仍需要手动声明引用的dependency，只是不用再写版本号。

如果需要继承的话，就使用parent标签，这里不赘述。

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<parent>
    <artifactId>ahao-all</artifactId>
    <groupId>com.ahao.core</groupId>
    <version>1.0</version>
</parent>

参考资料