[Java] springboot 加载 META-INF/spring.factories方式

• springboot 加载 META-INF/spring.factories
  
  
  
  • 用户应用程序Application
    
  
  
• 建立META-INF/spring.factories文件的意义何在
  
  
  
  • 平常我们如何将Bean注入到容器当中
    
  
  

ConfigurableApplicationContext context = SpringApplication.run(NacosSpringBootYamlApplication.class, args);

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
  return run(new Class<?>[] { primarySource }, args);
 }

// 这里Class是数组
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
  return new SpringApplication(primarySources).run(args);
 }

public SpringApplication(Class<?>... primarySources) {
  this(null, primarySources);
 }

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
  this.resourceLoader = resourceLoader;
  Assert.notNull(primarySources, "PrimarySources must not be null");
  this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources)); // 这里就是SpringMvcApplication的实例
  this.webApplicationType = WebApplicationType.deduceFromClasspath();// deduce(推断)web类型(servlet、reactive、NoWeb)
  setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));// 这里会处理加载所有的spring.factories文件的内容到缓存 找到*META-INF/spring.factories*中声明的所有ApplicationContextInitializer的实现类并将其实例化
  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); //找到*META-INF/spring.factories*中声明的所有ApplicationListener的实现类并将其实例化
  this.mainApplicationClass = deduceMainApplicationClass(); //获得当前执行main方法的类对象，这里就是SpringMvcApplication的实例
 }

/** key:是spring.factories的key  value:是根据key分组，把同个key的不同value放到list里面 */
 private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
  MultiValueMap<String, String> result = cache.get(classLoader);
  if (result != null) { //已经处理过了  直接返回
   return result;
  }
//url: // file:/C:/Users/kongqi/.m2/repository/org/springframework/spring-beans/5.1.9.RELEASE/spring-beans-5.1.9.RELEASE.jar!/META-INF/spring.factories
  try { //得到classloader下的所有jar包中的spring.factories的文件
   Enumeration<URL> urls = (classLoader != null ?
   classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
   ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
   result = new LinkedMultiValueMap<>();
   while (urls.hasMoreElements()) {
  URL url = urls.nextElement();
  UrlResource resource = new UrlResource(url);
  Properties properties = PropertiesLoaderUtils.loadProperties(resource); // 得到spring.factories的内容
  for (Map.Entry<?, ?> entry : properties.entrySet()) { // key: spring.factories的key  value: spring.factories的value
   String factoryClassName = ((String) entry.getKey()).trim(); // spring.factories的key
   for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {//value根据逗号，分隔
    result.add(factoryClassName, factoryName.trim()); //factoryClassName其实就是spring.factories的key   由于value是List类型 MultiValueMap value有多个
   }
  }
   }
   cache.put(classLoader, result);
   return result;
  }
  catch (IOException ex) {
   throw new IllegalArgumentException("Unable to load factories from location [" +
   FACTORIES_RESOURCE_LOCATION + "]", ex);
  }
 }

@Configuration
@EnableConfigurationProperties(HelloProperties.class)
public class HelloServiceAutoConfiguration {
  @Autowired
  HelloProperties helloProperties;
  @Bean
  public HelloService helloService() {
    HelloService service = new HelloService();
    service.setHelloProperties( helloProperties  );
    return service;
  }
}

@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 {

/**
 * Return the auto-configuration class names that should be considered. By default
 * this method will load candidates using {@link SpringFactoriesLoader} with
 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return a list of candidate configurations
 */
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
  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;
}