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A1）groupId：定义当前 maven 项目隶属的实际项目；maven 项目和 实际项目不是一一对应的，如spring framework是一个实际项目，而 maven 项目有很多如 spring-core， spring-context等；A2）artifactId：定义实际项目中的一个 maven 项目（模块），使用实际项目名称作为 artifactId的前缀，以便于寻找实际构件；（其groupId 为spring，而artifaceId 为 spring-aop 或 spring-core 等等 ）
A3）packaging：该元素定义 maven 项目的打包方式；打包方式有 jar 和 war ，前者是默认的；A4）classifier：用来帮助定义构建输出的一些附属构件。附属构件 与 主构件 相对应，如主构件 spring-aop-4.2.6.RELEASE.jar ，其附属构件还可以是 spring-aop-4.2.6.RELEASE-javadoc.jar，spring-aop-4.2.6.RELEASE-sources.jar；（干货——引入了主构件和附属构件）
A1）以上坐标的5个元素：前3个是必须的，而 packaging 是可选的，而 classifier 是不能直接定义的；A2）项目构件的文件名和坐标是对应的：规则为 artifactId-version [-classfier].packaging ；
<project>
......
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.7</version>
<type>....</type>
<scope>test</scope>
<optional>...</optional>
<exclusions>
<exclusion>...</exclusion>
</exclusions>
</dependency>
</dependencies>
...
</project>A1）type：是依赖类型 ，默认为jar ，不必声明；A2）scope：依赖的范围（test，compile(默认)）；A3）optional：标记是否可选；A4）exclusions：用来排除传递性依赖；
classpath1）首先：编译项目主代码的时候需要用到 spring-core，该文件以依赖的方式被引入到 classpath中；classpath2）其次：maven 在编译和执行测试的时候会使用另外一套 classpath；classpath3）最后：实际运行maven 项目的时候，又会使用一套 classpath；
scope1）compile：编译依赖范围scope2）test：测试依赖范围：scope3）provided：已提供依赖范围；scope4）runtime：运行时 依赖范围；scope5）system：系统依赖范围；（最好不要使用）scope6）import：导入依赖范围；
1.1）problem：考虑一个基于 spring framework 的项目，如果不使用 maven ，那么在项目中就需要 手动下载相关依赖；1.2）solution：maven 的 传递性依赖就可以很好解决这个问题；
荔枝1）如项目A 有这样的依赖关系： A->B->C->X(1.0) AND A->D->X(2.0)，则 依赖调节的第一原则是：路径最近者优先（长度最短者），所以会选择 X(2.0)；荔枝2）如项目A 有这样的依赖关系： A->B->X(1.0) AND A->D->X(2.0)；则依赖调节的第二原则是：第一声明者优先，在长度相等的case下，在pom 中声明的顺序决定了谁会被使用；（干货——调节依赖的第一和第二原则）
C1）在理想case下，是不应该使用可选依赖的。因为在面向对象设计中，有一个 单一职责性原则，指一个类应该只有一个职责，而不是柔和太多的职责；C2）那所以，在上面的case上，就应该 分别为 mysql 和 oracle 创建一个 maven项目，基于同样的 groupId 不同的 artifactId ，在各自的 pom 中声明对应的 jdbc 驱动依赖，而不是使用可选依赖；
D:\classical_books\java_set\maven_in_action\mycode\chapter3>mvn dependency:list [INFO] Scanning for projects... [WARNING] [WARNING] Some problems were encountered while building the effective model for com.maven.chapter3:service:jar:1.0-SNAPSHOT [WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 22, column 15 [WARNING] [WARNING] It is highly recommended to fix these problems because they threaten the stability of your build. [WARNING] [WARNING] For this reason, future Maven versions might no longer support building such malformed projects. [WARNING] [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building service says hello maven. 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] [INFO] --- maven-dependency-plugin:2.8:list (default-cli) @ service --- [INFO] [INFO] The following files have been resolved: [INFO] junit:junit:jar:4.7:test [INFO] [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------ [INFO] Total time: 7.462 s [INFO] Finished at: 2016-06-18T20:52:40+08:00 [INFO] Final Memory: 13M/153M [INFO] ------------------------------------------------------------------------
D:\classical_books\java_set\maven_in_action\mycode\chapter3>mvn dependency:tree [INFO] Scanning for projects... [WARNING] [WARNING] Some problems were encountered while building the effective model for com.maven.chapter3:service:jar:1.0-SNAPSHOT [WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 22, column 15 [WARNING] [WARNING] It is highly recommended to fix these problems because they threaten the stability of your build. [WARNING] [WARNING] For this reason, future Maven versions might no longer support building such malformed projects. [WARNING] [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building service says hello maven. 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] [INFO] --- maven-dependency-plugin:2.8:tree (default-cli) @ service --- [INFO] com.maven.chapter3:service:jar:1.0-SNAPSHOT [INFO] \- junit:junit:jar:4.7:test // 传递性依赖，该项目会传递依赖于 junit; [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------ [INFO] Total time: 1.172 s [INFO] Finished at: 2016-06-18T20:54:26+08:00 [INFO] Final Memory: 13M/217M [INFO] ------------------------------------------------------------------------
D:\classical_books\java_set\maven_in_action\mycode\chapter3>mvn dependency:analyze [INFO] Scanning for projects... [WARNING] [WARNING] Some problems were encountered while building the effective model for com.maven.chapter3:service:jar:1.0-SNAPSHOT [WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-compiler-plugin is missing. @ line 22, column 15 [WARNING] [WARNING] It is highly recommended to fix these problems because they threaten the stability of your build. [WARNING] [WARNING] For this reason, future Maven versions might no longer support building such malformed projects. [WARNING] [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building service says hello maven. 1.0-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] [INFO] >>> maven-dependency-plugin:2.8:analyze (default-cli) > test-compile @ service >>> [INFO] [INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ service --- [WARNING] Using platform encoding (GBK actually) to copy filtered resources, i.e. build is platform dependent! [INFO] skip non existing resourceDirectory D:\classical_books\java_set\maven_in_action\mycode\chapter3\src\main\resources [INFO] [INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ service --- [INFO] Nothing to compile - all classes are up to date [INFO] [INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ service --- [WARNING] Using platform encoding (GBK actually) to copy filtered resources, i.e. build is platform dependent! [INFO] skip non existing resourceDirectory D:\classical_books\java_set\maven_in_action\mycode\chapter3\src\test\resources [INFO] [INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ service --- [INFO] Nothing to compile - all classes are up to date [INFO] [INFO] <<< maven-dependency-plugin:2.8:analyze (default-cli) < test-compile @ service <<< [INFO] [INFO] --- maven-dependency-plugin:2.8:analyze (default-cli) @ service --- [INFO] No dependency problems found [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------ [INFO] Total time: 3.028 s [INFO] Finished at: 2016-06-18T20:56:17+08:00 [INFO] Final Memory: 15M/222M [INFO] ------------------------------------------------------------------------
A1）Used undeclared dependencies：意指项目中使用到 的，但没有显式声明的依赖；A2）Unused declared dependencies：意指项目中未使用到的，但显式声明的依赖；
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原文地址：http://blog.csdn.net/pacosonswjtu/article/details/51707190
