Java的SPI引入Jar包简单例子

　　我们把之前在一个项目中实现SPI机制的简单例子（参见Java的SPI简单实例 ），换成实际使用中的jar包引入的简单例子。一拆为三，接口一个包，实现一个包，应用一个包，哦，还得加个骨架包。我们新增一个父项目：在IDEA中点击左上角File -> 点New Project -> 点Maven -> 点Next -> 输入Maven的groupId和artifactId后点Next -> 输入项目名后点Finish，只需在pom文件中新增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>

　　然后继续依葫芦画瓢，分别新增如上三个子项目：接口spi-api，实现spi-impl和应用spi-app。为了使用接口，spi-impl和spi-app都依赖了spi-api包，spi-app加载实现类，还得引入spi-impl包。看看子项目的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节点是为了把spi-impl包中的resources目录下的内容在打jar包时，复制到META-INF目录中，输出META-INF/services/com.wlf.api.ITest这个文件。这个文件的内容执行实现类：

　　其他的接口类、实现类和应用类代码不变，包路径改改就好：

 　　跑一把，结果跟之前在单一项目跑的结果是一样的：