从Java SPI机制实现到Spring Cloud SPI扩展

た入场券 2022-03-25 10:20 186阅读 0赞

## Java SPI实现 ##

可参考上一篇文章：  
**[https://blog.csdn.net/shang\_xs/article/details/86560469][https_blog.csdn.net_shang_xs_article_details_86560469]**

## Spring Cloud SPI 实现 ##

### 1. Spring Cloud SPI ###

SPI英文为Service Provider Interface单从字面可以理解为Service提供者接口，正如从SPI的名字去理解SPI就是Service提供者接口；我对SPI的定义：提供给服务提供厂商与扩展框架功能的开发者使用的接口。

SPI是供给框架开发者以方便的对框架进行额外拓展的；API是提供给框架使用者的，使用者可以实现API接口来定义自己的功能组件。

### 2. 其他框架SPI思想 ###

很多框架都使用了java的SPI机制，如java.sql.Driver的SPI实现（MySQL驱动、oracle驱动等）、common-logging的日志接口实现、dubbo的扩展实现等等框架；

说到Spring，那么就离不开Bean的概念，SpringCloud的SPI实质上也是将一些Bean引入Spring容器中，也就是将netflix的配置类以Bean的形式整合进SpringCloud中。

我们以Eureka为例

### 3. Spring Cloud Eureka ###

#### 3.1 pom文件 ####

<dependency>
    
        <groupId>org.springframework.cloud</groupId>
    
        <artifactId>spring-cloud-starter-eureka-server</artifactId>
    
        <version>1.4.4.RELEASE</version>
    
    </dependency>

#### 3.2 spring.factories文件 ####

以1.4.4.RELEASE版本为例说明SpringCloud如何通过SPI机制整合Eureka。

在spring-cloud-starter-eureka-server包下”/META-INF“有一个”spring.factories“文件：

其内容如下：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
    
    org.springframework.cloud.netflix.eureka.config.EurekaClientConfigServerAutoConfiguration,\
    
    org.springframework.cloud.netflix.eureka.config.EurekaDiscoveryClientConfigServiceAutoConfiguration,\
    
    org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration,\
    
    org.springframework.cloud.netflix.ribbon.eureka.RibbonEurekaAutoConfiguration,\
    
    org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration
    
    org.springframework.cloud.bootstrap.BootstrapConfiguration=\
    
    org.springframework.cloud.netflix.eureka.config.EurekaDiscoveryClientConfigServiceBootstrapConfiguration

其内部有两个键值对，

org.springframework.boot.autoconfigure.EnableAutoConfiguration就是注解@EnableAutoConfiguration的类全名。
    org.springframework.cloud.bootstrap.BootstrapConfiguration就是注解@BootstrapConfiguration的类全名，这两个注解都是Spring的启动注解，一个引导父容器，一个引导子容器(mvc容器)。

到这里基本就有眉目了，两个注解对应不同的Class，也就是说可以通过这两个注解来将相应的Class引入Spring容器，定义成Bean。

#### 3.3 原理实现 ####

一般在Springboot的启动类上会有这个注解：@SpringBootApplication

@EnableEurekaServer @SpringBootApplication public class SpringCloudEurekaServerApplication { 
    
    public static void main(String[] args) { 
        SpringApplication.run(SpringCloudEurekaServerApplication.class, args);
    }
    } Spring在初始化时，会逐层的解析定义的注解，看@SpringBootApplication的内部结构：
    
    @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @SpringBootConfiguration @EnableAutoConfiguration @ComponentScan(excludeFilters = {  @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) }) public @interface SpringBootApplication { 
    
    ......

其上标识有@EnableAutoConfiguration注解，我们接着看这个注解的内部结构：

@SuppressWarnings("deprecation") @Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @AutoConfigurationPackage @Import(EnableAutoConfigurationImportSelector.class) public @interface EnableAutoConfiguration { 
    
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
    /** * Exclude specific auto-configuration classes such that they will never be applied. * @return the classes to exclude */
    Class<?>[] exclude() default { };
    
    /** * Exclude specific auto-configuration class names such that they will never be * applied. * @return the class names to exclude * @since 1.3.0 */
    String[] excludeName() default { };

@EnableAutoConfiguration 内部有两个可定义属性，从名称上看是排除类，他们的用法就是排除这个注解对应在spring.factories里的键值对。 也就是我通过@EnableAutoConfiguration引入的配置类，可以通过其内部属性来排除不需要的，这样能够提供一定的灵活性。代码实现在后面会讲。  
@EnableAutoConfiguration上有一个@Import注解，引入了EnableAutoConfigurationImportSelector，看其代码：

public class EnableAutoConfigurationImportSelector extends AutoConfigurationImportSelector { 
    
    @Override
    protected boolean isEnabled(AnnotationMetadata metadata) { 
        if (getClass().equals(EnableAutoConfigurationImportSelector.class)) { 
            return getEnvironment().getProperty(
                    EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
                    true);
        }
        return true;
    }
    }

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered { 
    
    public String[] selectImports(AnnotationMetadata annotationMetadata) { 
        if (!isEnabled(annotationMetadata)) { 
            return NO_IMPORTS;
        }
        try { 
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = getAttributes(annotationMetadata);
            // 根据@EnableAutoConfiguration 注解数据来获取候选的配置类
            List<String> configurations = getCandidateConfigurations(annotationMetadata,
                    attributes);
            configurations = removeDuplicates(configurations);
            configurations = sort(configurations, autoConfigurationMetadata);
            // 获取要排除的类名称
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);
            checkExcludedClasses(configurations, exclusions);
            // 从候选类中移除那些待排除的
            configurations.removeAll(exclusions);
            configurations = filter(configurations, autoConfigurationMetadata);
            fireAutoConfigurationImportEvents(configurations, exclusions);
            return configurations.toArray(new String[configurations.size()]);
        }
        catch (IOException ex) { 
            throw new IllegalStateException(ex);
        }
    }
    
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
            AnnotationAttributes attributes) { 
        // 通过SpringFactoriesLoader来获取指定Class对应的配置类名称
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
                getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
        Assert.notEmpty(configurations,
                "No auto configuration classes found in META-INF/spring.factories. If you "
                        + "are using a custom packaging, make sure that file is correct.");
        return configurations;
    }
    
    protected Class<?> getSpringFactoriesLoaderFactoryClass() { 
        return EnableAutoConfiguration.class;
    }

AutoConfigurationImportSelector间接实现了ImportSelector，这个类的作用就是可选择的引入一个Class，将这个Class注册到Spring容器中。

在其selectImports(…)方法中，最终通过SpringFactoriesLoader来获取EnableAutoConfiguration.class对应的配置类名称，看SpringFactoriesLoader的获取逻辑：

public abstract class SpringFactoriesLoader { 
    
    /** * The location to look for factories. * <p>Can be present in multiple JAR files. */
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
    
    /** * Load the fully qualified class names of factory implementations of the * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given * class loader. * @param factoryClass the interface or abstract class representing the factory * @param classLoader the ClassLoader to use for loading resources; can be * {@code null} to use the default * @see #loadFactories * @throws IllegalArgumentException if an error occurs while loading factory names */
    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) { 
        String factoryClassName = factoryClass.getName();
        try { 
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) { 
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) { 
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

在SpringFactoriesLoader 的loadFactoryNames(…)方法中，获取根路径下 “META-INF/spring.factories“文件，然后将文件内容解析成Properties对象，获取指定类名对应的字符串，然后用”,“拆分字符串，将获得的字符串数组加入集合中。

也就是说在引入SpringCloud-Eureka的Jar包后，通过@EnableAutoConfiguration 注解引入其对应的字符串：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=
    org.springframework.cloud.netflix.eureka.config.EurekaClientConfigServerAutoConfiguration,
    org.springframework.cloud.netflix.eureka.config.EurekaDiscoveryClientConfigServiceAutoConfiguration,
    org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration,
    org.springframework.cloud.netflix.ribbon.eureka.RibbonEurekaAutoConfiguration,
    org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration

然后拆分字符串，得到对应的配置类名称，然后移除@EnableAutoConfiguration 里指定排除的类名，最后将剩下的类注册入Spring容器中。

通过这种形式，当改变版本时，可以很容易的修改引入的配置类，也就是更换拓展的组件。

**更多扩展实现参见GitHub：[https://github.com/dwyanewede/project-learn][https_github.com_dwyanewede_project-learn]**  
**Spring Boot扩展点加载：com.learn.demo.spi.ISpi**  
**Dubbo扩展点加载：com.learn.demo.dubbo.extension.MyExtension**

**更多技术文档**  
[https://blog.csdn.net/shang\_xs/article/details/86560469][https_blog.csdn.net_shang_xs_article_details_86560469]  
[https://blog.csdn.net/shang\_xs/article/details/86560691][https_blog.csdn.net_shang_xs_article_details_86560691]

[https_blog.csdn.net_shang_xs_article_details_86560469]: https://blog.csdn.net/shang_xs/article/details/86560469
[https_github.com_dwyanewede_project-learn]: https://github.com/dwyanewede/project-learn
[https_blog.csdn.net_shang_xs_article_details_86560691]: https://blog.csdn.net/shang_xs/article/details/86560691