我們把之前在一個專案中實現SPI機制的簡單例子（參見Java的SPI簡單例項），換成實際使用中的jar包引入的簡單例子。一拆為三，介面一個包，實現一個包，應用一個包，哦，還得加個骨架包。我們新增一個父專案：在IDEA中點選左上角File -> 點New Project -> 點Maven -> 點Next -> 輸入Maven的groupId和artifactId後點Next -> 輸入專案名後點Finish，只需在pom檔案中新增module：

<?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>com.wlf.spi</groupId>
    <artifactId
 
>spi-parent</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <modules>
        <module>spi-api</module>
        <module
 
>spi-impl</module>
        <module>spi-app</module>
    </modules>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

　　然後繼續依葫蘆畫瓢，分別新增如上三個子專案：介面spi-api，實現spi-impl和應用spi-app。為了使用介面，spi-impl和spi-app都依賴了spi-api包，spi-app載入實現類，還得引入spi-impl包。看看子專案的pom.xml：

<?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>
    <parent>
        <groupId>com.wlf.spi</groupId>
        <artifactId>spi-parent</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>spi-api</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

</project>

<?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>
    <parent>
        <groupId>com.wlf.spi</groupId>
        <artifactId>spi-parent</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>spi-app</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>com.wlf.spi</groupId>
            <artifactId>spi-api</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>com.wlf.spi</groupId>
            <artifactId>spi-impl</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

</project>

<?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>
    <parent>
        <groupId>com.wlf.spi</groupId>
        <artifactId>spi-parent</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>spi-impl</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>com.wlf.spi</groupId>
            <artifactId>spi-api</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

    <build>
        <resources>
            <resource>
                <directory>src/main/resources</directory>
                <targetPath>META-INF/</targetPath>
            </resource>
        </resources>
    </build>
</project>

　　上面resource節點是為了把spi-impl包中的resources目錄下的內容在打jar包時，複製到META-INF目錄中，輸出META-INF/services/com.wlf.api.ITest這個檔案。這個檔案的內容執行實現類：

　　其他程式碼不變：

　　跑一把，結果跟單一專案中一樣：