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README.md
Apache-2.0

Slot

Spring Boot 可插件化拓展改造器,让 Spring-Boot 应用支持加载外部 jar 包,实现插件化拓展。

GitHub: https://github.com/core-lib/slot-maven-plugin

Slot: 在计算机行业指的就是周边元件扩展插槽。

问题描述

Spring-Boot 项目打包后是一个FatJar 即把所有依赖的第三方jar也打包进自身的jar中,运行时 classpath 包括 FatJar 中的 BOOT-INF/classes 目录和 BOOT-INF/lib 目录下的所有jar。

那么问题是要想加载外部化 jar 就只能打包期间把 jar 依赖进去,无法实现可插拔式插件化拓展。

Slot 就是一个可以将 Spring-Boot 项目升级为可支持加载外部 jar 的 Maven 插件。

原理说明

一个 Spring-Boot JAR 启动的流程可以分为以下几步:

  1. 通过 java -jar spring-boot-app.jar args... 命令启动
  2. JVM 读取该 jar 的 META-INF/MANIFEST.MF 文件中的 Main-Class,在 Spring-Boot JAR 中这个值通常为 org.springframework.boot.loader.JarLauncher
  3. JVM 调用该类的 main 方法,传入参数即上述命令中参数
  4. JarLauncher 构建 ClassLoader 并反射调用 META-INF/MANIFEST.MF 中的 Start-Class 类的 main 方法,通常为项目中的 Application 类
  5. Application 类的 main 方法调用 SpringApplication.run(Application.class, args); 以最终启动应用

Slot 的核心原理是:

  1. 拓展 org.springframework.boot.loader.JarLauncher 实现根据启动命令参数读取外部 jar 包并且加入至 classpath 中
  2. 修改 META-INF/MANIFEST.MF 中的 Main-Class 为拓展的 JarLauncher

环境依赖

  1. JDK 1.7 +
  2. Spring-Boot

使用说明

<project>
    <!-- 设置 jitpack.io 插件仓库 -->
    <pluginRepositories>
        <pluginRepository>
            <id>jitpack.io</id>
            <url>https://jitpack.io</url>
        </pluginRepository>
    </pluginRepositories>
    <!-- 添加 Slot Maven 插件 -->
    <build>
        <plugins>
            <plugin>
                <groupId>com.github.core-lib</groupId>
                <artifactId>slot-maven-plugin</artifactId>
                <version>1.0.2</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>transform</goal>
                        </goals>
                        <phase>package</phase>
                        <configuration>
                            <!-- optional
                            <sourceDir/>
                            <sourceJar/>
                            <targetDir/>
                            <targetJar/>
                            -->
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

参数说明

参数名称 命令参数名称 参数说明 参数类型 缺省值 示例值
sourceDir -Dslot.sourceDir 源jar所在目录 File ${project.build.directory} 文件目录
sourceJar -Dslot.sourceJar 源jar名称 String ${project.build.finalName}.jar 文件名称
targetDir -Dslot.targetDir 目标jar存放目录 File ${project.build.directory} 文件目录
targetJar -Dslot.targetJar 目标jar名称 String ${project.build.finalName}.slot 文件名称

插件的默认执行阶段是 package , 当然也可以通过使用以下命令来单独执行。

mvn slot:transform

mvn slot:transform -Dslot.targetJar=your-spring-boot-app-slot.jar

默认情况下,通过 slot 升级后的 jar 名称为 ${project.build.finalName}-slot.jar ,可以通过插件配置或命令参数修改。

注意事项

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <!-- 需要将executable和embeddedLaunchScript参数删除,目前还不能支持对该模式Jar的升级!
    <configuration>
        <executable>true</executable>
        <embeddedLaunchScript>...</embeddedLaunchScript>
    </configuration>
    -->
</plugin>

启动应用

Slot 支持使用两个参数来指定要加载的外部 jar 包:

  1. --slot.root 即外部 jar 的根路径,缺省情况下为 Spring-Boot JAR 包的目录。
  2. --slot.path 即外部 jar 的路径,支持设置多个,支持 ANT 表达式风格。
java -jar spring-boot-app-slot.jar --slot.root=/absolute/root/ --slot.path=foo.jar  --slot.path=bar.jar

java -jar spring-boot-app-slot.jar --slot.path=/relative/path/to/plugin.jar

java -jar spring-boot-app-slot.jar --slot.path=/relative/path/to/**.jar

ANT 表达式通配符说明

通配符 含义 示例
** 任意个字符及目录 /plugins/**.jar 即 /plugins 目录及子目录的所有 .jar 后缀的文件
* 任意个字符 /plugins/*.jar 即 /plugins 目录的所有 .jar 后缀的文件
? 单个字符 ???.jar 即当前目录所有名称为三个任意字符及以 .jar 为后缀的文件

通配符可以随意组合使用! 例如 /plugins/**/plugin-*-v???.jar

使用技巧

由于通过 Slot 加载后的外部 jar 实际上和 Spring-Boot JAR 中的 jar 处于同一个 ClassLoader 所以外部插件和母体应用之间是一个平级的关系, 外部插件可以引用母体应用中的 class 同样母体应用也可以引用外部插件的 class。

由于外部插件项目或模块通常也会依赖另外的第三方jar,所以外部插件与母体应用集成运行时也需要把另外的第三方jar通过--slot.path参数加载进来。 推荐使用 maven-dependency-plugin 在打包时将需要用到的第三方jar拷贝到指定目录,最后通过ANT表达式方式一起加载运行。

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-dependency-plugin</artifactId>
    <executions>
        <execution>
            <phase>package</phase>
            <goals>
                <goal>copy-dependencies</goal>
            </goals>
            <configuration>
                <includeScope>runtime</includeScope>
                <outputDirectory>${project.build.directory}/dependencies</outputDirectory>
            </configuration>
        </execution>
    </executions>
</plugin>

或者使用 maven-shade-plugin 插件把相关的第三方jar资源通通打包进一个。

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-shade-plugin</artifactId>
    <version>3.1.1</version>
    <executions>
        <execution>
            <phase>package</phase>
            <goals>
                <goal>shade</goal>
            </goals>
            <configuration>
                ...
            </configuration>
        </execution>
    </executions>
</plugin>

另外需要注意的是,当母体应用和外部插件有相同的第三方依赖时,推荐让外部插件模块以 <scope>provided</scope> 的方式依赖之。

下面是作者想到的一些插件化拓展的方案:

  1. IoC方式:母体应用声明接口,外部插件实现接口并且通过 @Component @Service 或其他注解让Spring 容器管理, 母体应用通过 @Resource @Autowired 来注入。

  2. SPI方式:母体应用声明接口,外部插件实现接口并且配置于 META-INF/services/ 下,母体应用通过 ServiceLoader 加载接口的实现类。

  3. AOP方式:外部插件通过 Spring Aspect 技术实现对母体应用的切面拦截。

版本记录

  • 1.0.2 升级LoadKit依赖版本解决ANT表达式无法正确匹配**/*通配符的问题

  • 1.0.1 bug 修复

  • 1.0.0 第一个正式版发布

协议声明

Apache-2.0

联系作者

QQ 646742615 不会钓鱼的兔子

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Spring Boot 可插件化拓展改造器,让 Spring-Boot 应用支持加载外部 jar 包,实现插件化拓展。 展开 收起
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