我们知道，Maven 是通过仓库对依赖进行管理的，当 Maven 项目需要某个依赖时，只要其 POM 中声明了依赖的坐标信息，Maven 就会自动从仓库中去下载该构件使用。但在实际的开发过程中，经常会遇到一种情况：某一个项目需要依赖于存储在本地的某个 jar 包，该 jar 包无法从任何仓库中下载的，这种依赖被称为外部依赖或本地依赖。那么这种依赖是如何声明的呢？

下面我们通过一个实例实例来介绍如何导入本地 jar 包。

1. 打开命令行窗口，跳转到 D:\maven 目录下，执行以下 mvn 命令，创建一个名为 secondMaven 的项目。

​mvn archetype:generate -DgroupId=net.biancheng.www -DartifactId=secondMaven -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false

2. 更新 secondMaven 中 App 类的代码如下。

package net.biancheng.www;

import net.biancheng.www.Util;;

public class App {
    public static void main(String[] args) {
        Util.printMessage("secondMaven");
    }
}

由以上代码可以看出，secondMaven 中的 App 类需要使用 helloMaven 中独有的 Util 类，即 secondMaven 需要依赖于 helloMaven。

3. 跳转到 helloMaven 所在的目录，执行以下 mvn 命令， 将该项目打包成 jar 文件。

mvn clean package

命令执行执行结果如下。

[INFO] Scanning for projects...
[INFO]​
[INFO] --------------------< net.biancheng.www:helloMaven >--------------------
[INFO] Building helloMaven 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ helloMaven ---
[INFO] Deleting d:\maven\helloMaven\target
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ helloMaven ---
[WARNING] Using platform encoding (GBK actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory d:\maven\helloMaven\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ helloMaven ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding GBK, i.e. build is platform dependent!
[INFO] Compiling 2 source files to d:\maven\helloMaven\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ helloMaven ---
[WARNING] Using platform encoding (GBK actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory d:\maven\helloMaven\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:testCompile (default-testCompile) @ helloMaven ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding GBK, i.e. build is platform dependent!
[INFO] Compiling 1 source file to d:\maven\helloMaven\target\test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.12.4:test (default-test) @ helloMaven ---
[INFO] Surefire report directory: d:\maven\helloMaven\target\surefire-reports

-------------------------------------------------------
T E S T S
-------------------------------------------------------
Running net.biancheng.www.AppTest
Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.016 sec

Results :

Tests run: 1, Failures: 0, Errors: 0, Skipped: 0

[INFO]
[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ helloMaven ---
[INFO] Building jar: d:\maven\helloMaven\target\helloMaven-1.0-SNAPSHOT.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.940 s
[INFO] Finished at: 2021-03-03T13:27:34+08:00
[INFO] ------------------------------------------------------------------------

4. 命令执行完成后，进入 D:\maven\helloMaven\target 目录，可以看到 Maven 已经将 helloMaven 项目打包成 helloMaven-1.0-SNAPSHOT.jar。

图1： helloMaven 打包结果

5. 修改 secondMaven 中 pom.xml 的配置如下。

<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/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>net.biancheng.www</groupId>
    <artifactId>secondMaven</artifactId>
    <packaging>jar</packaging>
    <version>1.0-SNAPSHOT</version>
    <name>secondMaven</name>
    <url>http://maven.apache.org</url>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <!--外部依赖-->
        <dependency>
            <groupId>net.biancheng.www</groupId>
            <artifactId>helloMaven</artifactId>
             <!--依赖范围-->
            <scope>system</scope>
            <version>1.0-SNAPSHOT</version>
            <!--依赖所在位置-->
            <systemPath>D:\maven\helloMaven\target\helloMaven-1.0-SNAPSHOT.jar</systemPath>
        </dependency>
    </dependencies>
</project>

在以上配置中，除了依赖的坐标信息外，外部依赖还使用了 scope 和 systemPath 两个元素。

• scope 表示依赖范围，这里取值必须是 system，即系统。
• systemPath 表示依赖的本地构件的位置。

6. 打开命令行窗口，跳转到 secondMaven 所在目录，执行以下 mvn 命令，进行编译。

mvn clean compile

命令执行结果如下。

[WARNING]
[WARNING] Some problems were encountered while building the effective model for net.biancheng.www:secondMaven:jar:1.0-SNAPSHOT
[WARNING] 'dependencies.dependency.systemPath' for net.biancheng.www:helloMaven:jar should use a variable instead of a hard-coded path D:\maven\helloMaven\target\helloMaven-1.0-SNAPSHOT.jar @ line 23, column 16
[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] -------------------< net.biancheng.www:secondMaven >--------------------
[INFO] Building secondMaven 1.0-SNAPSHOT
[INFO] --------------------------------[ jar ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ secondMaven ---
[INFO] Deleting d:\maven\secondMaven\target
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ secondMaven ---
[WARNING] Using platform encoding (GBK actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory d:\maven\secondMaven\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.1:compile (default-compile) @ secondMaven ---
[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding GBK, i.e. build is platform dependent!
[INFO] Compiling 1 source file to d:\maven\secondMaven\target\classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.030 s
[INFO] Finished at: 2021-03-03T14:05:00+08:00
[INFO] ------------------------------------------------------------------------

7. 编译完成后，执行以下命令，设置临时环境变量。

set classpath=%classpath%;D:\maven\helloMaven\target\helloMaven-1.0-SNAPSHOT.jar

8. 执行以下 Java 命令。

java net.biancheng.www.App

9. 结果如下图。

图2：执行结果

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