坐标
在maven中,任何构件都有明确定义的坐标。这些坐标是通过一些元素定义的,下面是nexus-indexer的坐标定义:
<groupId>org.sonatype.nexus</groupId>
<artifactId>nexus-indexer</artifactId>
<version>2.0.0</version>
<packing>jar</packing>
groupId:定义当前maven项目隶属的实际项目,一般由公司前缀+隶属项目名称组成
artifactId:定义当前maven项目模块,一般由隶属项目名称+当前maven模块名称组成
version:版本号
packing:打包方式。有jar,war等,默认为jar classifier:定义构件输出的一些附属构件。本例主构件输出nexus-indexer-2.0.0.jar,附属构件比如有:nexus-indexer-2.0.0-javadoc.jar,nexus-indexer-2.0.0-source.jar等
依赖配置
除了上面的基本坐标定义,依赖还可以包含:
<project>
...
<dependency>
<dependency>
<groupId>... </groupId>
<artifactId>... </artifactId>
<version>... </version>
<type>... </type>
<scope>... </scope>
<optional>... </optional>
<exclusions>
<exclusion>
...
</exclusion>
...
</exclusions>
</dependency>
...
</dependency>
...
</project>
type:依赖的类型,对应坐标中的packing,默认为jar,一般不用声明
scope:依赖的范围
optional:标记依赖是否可选,若为true则为可选依赖。可选依赖不能传递
exclusions:用来排除传递性依赖
依赖范围
在一个maven项目中主要有三个classpath:编译classpath,测试classpath,运行时classpath。下表列出了依赖范围和classpath的关系:
依赖范围 (Scope) | 对于编译 classpath有效 | 对于测试 classpath有效 | 对于运行时 classpath有效 | 例子 |
---|---|---|---|---|
compile | √ | √ | √ | spring-core |
test | × | √ | × | JUnit |
provided | √ | √ | × | servlet-api |
runtime | × | √ | √ | JDBC驱动实现 |
system | √ | √ | × | 本地的,Maven仓库外的 类库文件 |
compile:编译依赖范围
test:测试依赖范围
provided:已提供依赖范围
runtime:运行时依赖范围
system:系统依赖范围
import:导入依赖范围
传递性依赖
现有pom如下:
<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>com.juven.mvnbook.account</groupId>
<artifactId>account-email</artifactId>
<name>Account Email</name>
<version>1.0.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>4.3.3.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-beans</artifactId>
<version>4.3.3.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>4.3.3.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context-support</artifactId>
<version>4.3.3.RELEASE</version>
</dependency>
<dependency>
<groupId>javax.mail</groupId>
<artifactId>mail</artifactId>
<version>1.4.7</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.7</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.icegreen</groupId>
<artifactId>greenmail</artifactId>
<version>1.5.2</version>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-resources-plugin</artifactId>
<configuration>
<encoding>UTF-8</encoding>
</configuration>
</plugin>
</plugins>
</build>
</project>
上面的xml中有一个org.springframework:spring-core.4.3.3.RELEASE.jar依赖。在中央仓库查看其xml,会发现其还包含了:
<dependency>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
<version>1.2</version>
<scope>compile</scope>
</dependency>
该依赖的范围为compile,而spring-core依赖没有直接声明依赖范围,默认为compile。现在account-email有一个compile范围的spring-core依赖,spring-core有一个compile范围的comms-logging依赖,所以comms-logging会成为account-email的compile范围的传递性依赖。
现假设A依赖于B,B依赖于C,那么A相对于B是第一直接依赖,B相对于C是第二直接依赖,A相对于C是传递性依赖,其依赖范围按下表定义:
compile | test | provided | runtime | |
---|---|---|---|---|
compile | compile | — | — | compile |
test | test | — | — | test |
provided | provided | — | provided | provided |
runtime | runtime | — | — | runtime |
表中,第一列是第一直接依赖范围,第一行是第二直接依赖范围,交叉部分是最终传递性依赖范围。
依赖调解
依赖调解有两个原则:
1.路径最近的优先
现在有如下两个依赖:
A → B → C → X(1.0.0)
A → B → X(1.2.1)
A有两个版本的传递性依赖X,因为X(1.2.1)路径较短,所以最终X(1.2.1)会被解析使用。
2.第一声明者优先
现在有如下两个依赖:
A → B → X(1.0.0)
A → B → X(1.2.1)
两个版本X的依赖路径一样长,这时候谁被解析取决于谁先定义!
排除依赖
使用compile:tree查看依赖树:
[INFO] --- maven-dependency-plugin:2.8:tree (default-cli) @ account-email ---
[INFO] com.juven.mvnbook.account:account-email:jar:1.0.0-SNAPSHOT
[INFO] +- org.springframework:spring-core:jar:4.3.3.RELEASE:compile
[INFO] | - commons-logging:commons-logging:jar:1.2:compile
[INFO] +- org.springframework:spring-beans:jar:4.3.3.RELEASE:compile
[INFO] +- org.springframework:spring-context:jar:4.3.3.RELEASE:compile
[INFO] | +- org.springframework:spring-aop:jar:4.3.3.RELEASE:compile
[INFO] | - org.springframework:spring-expression:jar:4.3.3.RELEASE:compile
[INFO] +- org.springframework:spring-context-support:jar:4.3.3.RELEASE:compile
[INFO] +- javax.mail:mail:jar:1.4.7:compile
[INFO] | - javax.activation:activation:jar:1.1:compile
[INFO] +- junit:junit:jar:4.7:test
[INFO] - com.icegreen:greenmail:jar:1.5.2:test
[INFO] +- com.sun.mail:javax.mail:jar:1.5.6:test
[INFO] - org.slf4j:slf4j-api:jar:1.7.21:test
如上所述的,spring-core隐式依赖于commons-logging,如果不想传递此依赖,可以为spring-core添加exclusions元素:
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>4.3.3.RELEASE</version>
<exclusions>
<exclusion>
<groupId>commons-logging</groupId>
<artifactId>commons-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
再次执行dependency:tree命令:
[INFO] --- maven-dependency-plugin:2.8:tree (default-cli) @ account-email ---
[INFO] com.juven.mvnbook.account:account-email:jar:1.0.0-SNAPSHOT
[INFO] +- org.springframework:spring-core:jar:4.3.3.RELEASE:compile
[INFO] +- org.springframework:spring-beans:jar:4.3.3.RELEASE:compile
[INFO] +- org.springframework:spring-context:jar:4.3.3.RELEASE:compile
[INFO] | +- org.springframework:spring-aop:jar:4.3.3.RELEASE:compile
[INFO] | - org.springframework:spring-expression:jar:4.3.3.RELEASE:compile
[INFO] +- org.springframework:spring-context-support:jar:4.3.3.RELEASE:compile
[INFO] +- javax.mail:mail:jar:1.4.7:compile
[INFO] | - javax.activation:activation:jar:1.1:compile
[INFO] +- junit:junit:jar:4.7:test
[INFO] - com.icegreen:greenmail:jar:1.5.2:test
[INFO] +- com.sun.mail:javax.mail:jar:1.5.6:test
[INFO] - org.slf4j:slf4j-api:jar:1.7.21:test
可发现spring-core已经不依赖于commons-logging了,当然这里只是单纯演示排除依赖,commons-logging对于Spring框架是必须的。
归类依赖
在上述的pom中,spring的版本都为4.3.3.RELEASE版本,而且对于一个框架来说,其各个模块的版本一般都是一样的,这里我们可以统一声明其版本,然后引用即可:
<properties>
<springframework.version>4.3.3.RELEASE</springframework.version>
</properties>
...
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>${springframework.version}</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-beans</artifactId>
<version>${springframework.version}</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>${springframework.version}</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context-support</artifactId>
<version>${springframework.version}</version>
</dependency>
这样在以后更新spring版本的时候,只需要修改properties即可。
优化依赖
除了使用dependency:tree命令外,还可使是dependency:list查看所有依赖:
dependency:list
[INFO] --- maven-dependency-plugin:2.8:list (default-cli) @ account-email ---
[INFO]
[INFO] The following files have been resolved:
[INFO] org.springframework:spring-beans:jar:4.3.3.RELEASE:compile
[INFO] org.springframework:spring-context-support:jar:4.3.3.RELEASE:compile
[INFO] org.slf4j:slf4j-api:jar:1.7.21:test
[INFO] junit:junit:jar:4.7:test
[INFO] com.sun.mail:javax.mail:jar:1.5.6:test
[INFO] com.icegreen:greenmail:jar:1.5.2:test
[INFO] commons-logging:commons-logging:jar:1.2:compile
[INFO] org.springframework:spring-aop:jar:4.3.3.RELEASE:compile
[INFO] javax.activation:activation:jar:1.1:compile
[INFO] javax.mail:mail:jar:1.4.7:compile
[INFO] org.springframework:spring-core:jar:4.3.3.RELEASE:compile
[INFO] org.springframework:spring-context:jar:4.3.3.RELEASE:compile
[INFO] org.springframework:spring-expression:jar:4.3.3.RELEASE:compile
使用dependency:analyze分析依赖:
[INFO] <<< maven-dependency-plugin:2.8:analyze (default-cli) @ account-email <<<
[INFO]
[INFO] --- maven-dependency-plugin:2.8:analyze (default-cli) @ account-email ---
[WARNING] Unused declared dependencies found:
[WARNING] org.springframework:spring-core:jar:4.3.3.RELEASE:compile
[WARNING] org.springframework:spring-beans:jar:4.3.3.RELEASE:compile
[INFO] ------------------------------------------------------------------------
《Maven实战》读书笔记