Java SPI實作
可參考上一篇文章：
https://blog.csdn.net/shang_xs/article/details/86560469
Spring Cloud SPI 實作
1. Spring Cloud SPI
SPI英文為Service Provider Interface單從字面可以了解為Service提供者接口，正如從SPI的名字去了解SPI就是Service提供者接口；我對SPI的定義：提供給服務提供廠商與擴充架構功能的開發者使用的接口。
SPI是供給架構開發者以友善的對架構進行額外拓展的；API是提供給架構使用者的，使用者可以實作API接口來定義自己的功能元件。
2. 其他架構SPI思想
很多架構都使用了java的SPI機制，如java.sql.Driver的SPI實作（MySQL驅動、oracle驅動等）、common-logging的日志接口實作、dubbo的擴充實作等等架構；
說到Spring，那麼就離不開Bean的概念，SpringCloud的SPI實質上也是将一些Bean引入Spring容器中，也就是将netflix的配置類以Bean的形式整合進SpringCloud中。
我們以Eureka為例
3. Spring Cloud Eureka
3.1 pom檔案
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-eureka-server</artifactId>
<version>1.4.4.RELEASE</version>
</dependency>
3.2 spring.factories檔案
以1.4.4.RELEASE版本為例說明SpringCloud如何通過SPI機制整合Eureka。
在spring-cloud-starter-eureka-server包下”/META-INF“有一個”spring.factories“檔案：
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
其内部有兩個鍵值對，
org.springframework.boot.autoconfigure.EnableAutoConfiguration就是注解@EnableAutoConfiguration的類全名。
org.springframework.cloud.bootstrap.BootstrapConfiguration就是注解@BootstrapConfiguration的類全名，這兩個注解都是Spring的啟動注解，一個引導父容器，一個引導子容器(mvc容器)。
到這裡基本就有眉目了，兩個注解對應不同的Class，也就是說可以通過這兩個注解來将相應的Class引入Spring容器，定義成Bean。
3.3 原理實作
一般在Springboot的啟動類上會有這個注解：@SpringBootApplication
@EnableEurekaServer @SpringBootApplication public class SpringCloudEurekaServerApplication {
public static void main(String[] args) {
SpringApplication.run(SpringCloudEurekaServerApplication.class, args);
}
} Spring在初始化時，會逐層的解析定義的注解，看@SpringBootApplication的内部結構：
@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 {
......
其上辨別有@EnableAutoConfiguration注解，我們接着看這個注解的内部結構：
@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 内部有兩個可定義屬性，從名稱上看是排除類，他們的用法就是排除這個注解對應在spring.factories裡的鍵值對。 也就是我通過@EnableAutoConfiguration引入的配置類，可以通過其内部屬性來排除不需要的，這樣能夠提供一定的靈活性。代碼實作在後面會講。
@EnableAutoConfiguration上有一個@Import注解，引入了EnableAutoConfigurationImportSelector，看其代碼：
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);
// 根據@EnableAutoConfiguration 注解資料來擷取候選的配置類
List<String> configurations = getCandidateConfigurations(annotationMetadata,
attributes);
configurations = removeDuplicates(configurations);
configurations = sort(configurations, autoConfigurationMetadata);
// 擷取要排除的類名稱
Set<String> exclusions = getExclusions(annotationMetadata, attributes);
checkExcludedClasses(configurations, exclusions);
// 從候選類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) {
// 通過SpringFactoriesLoader來擷取指定Class對應的配置類名稱
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間接實作了ImportSelector，這個類的作用就是可選擇的引入一個Class，将這個Class注冊到Spring容器中。
在其selectImports(…)方法中，最終通過SpringFactoriesLoader來擷取EnableAutoConfiguration.class對應的配置類名稱，看SpringFactoriesLoader的擷取邏輯：
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，擷取根路徑下 “META-INF/spring.factories“檔案，然後将檔案内容解析成Properties對象，擷取指定類名對應的字元串，然後用”,“拆分字元串，将獲得的字元串數組加入集合中。
也就是說在引入SpringCloud-Eureka的Jar包後，通過@EnableAutoConfiguration 注解引入其對應的字元串：
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
然後拆分字元串，得到對應的配置類名稱，然後移除@EnableAutoConfiguration 裡指定排除的類名，最後将剩下的類注冊入Spring容器中。
通過這種形式，當改變版本時，可以很容易的修改引入的配置類，也就是更換拓展的元件。
更多擴充實作參見GitHub：https://github.com/dwyanewede/project-learn
Spring Boot擴充點加載：com.learn.demo.spi.ISpi
Dubbo擴充點加載：com.learn.demo.dubbo.extension.MyExtension
更多技術文檔
https://blog.csdn.net/shang_xs/article/details/86560469
https://blog.csdn.net/shang_xs/article/details/86560691
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