SpringBoot学习之自动配置原理

￡神魔★判官ぃ 2022-03-25 03:16 244阅读 0赞

SpringBoot启动时会加载大量自动配置类，大量减少了手动编写配置注入功能组件的工作。  
SpringBoot会扫描所有类路径下的META-INF/spring.factories文件，从这获取EnableAutoConfiguration对应的值，这些值就对应了各自自动配置类的路径，将其导入到容器中，就可以进行自动配置了。  
接下来看分析源码来熟悉自动配置原理：

### 1、启动类 ###

@SpringBootApplication
    public class SpringbootApplication { 
    
        public static void main(String[] args) { 
            SpringApplication.run(SpringbootApplication.class, args);
        }
    
    }

@SpringBootApplication是一个组合注解

### 2、@SpringBootApplication ###

@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 { 
    ...

其中@EnableAutoConfiguration的作用就是启动自动配置

### 3、@EnableAutoConfiguration ###

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(AutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration { 
    ...

通过@Import(AutoConfigurationImportSelector.class)向容器中导入一些组件

### 4、AutoConfigurationImportSelector.class ###

public class AutoConfigurationImportSelector
    		implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,
    		BeanFactoryAware, EnvironmentAware, Ordered { 
    
    	private static final String[] NO_IMPORTS = { };
    
    	private static final Log logger = LogFactory
    			.getLog(AutoConfigurationImportSelector.class);
    
    	private static final String PROPERTY_NAME_AUTOCONFIGURE_EXCLUDE = "spring.autoconfigure.exclude";
    
    	private ConfigurableListableBeanFactory beanFactory;
    
    	private Environment environment;
    
    	private ClassLoader beanClassLoader;
    
    	private ResourceLoader resourceLoader;
    
    	@Override
    	public String[] selectImports(AnnotationMetadata annotationMetadata) { 
    		if (!isEnabled(annotationMetadata)) { 
    			return NO_IMPORTS;
    		}
    		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
    				.loadMetadata(this.beanClassLoader);
    		AnnotationAttributes attributes = getAttributes(annotationMetadata);
    		//获取配置名的字符串列表
    		List<String> configurations = getCandidateConfigurations(annotationMetadata,
    				attributes);
    		configurations = removeDuplicates(configurations);
    		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
    		checkExcludedClasses(configurations, exclusions);
    		configurations.removeAll(exclusions);
    		configurations = filter(configurations, autoConfigurationMetadata);
    		fireAutoConfigurationImportEvents(configurations, exclusions);
    		return StringUtils.toStringArray(configurations);
    	}
    
    ...

List configurations = getCandidateConfigurations(annotationMetadata,  
attributes);  
作用是扫描并获取类路径下的自动配置类的路径放到字符串列表中

### 5、getCandidateConfigurations(annotationMetadata,attributes); ###

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;
     }

*看注释也能知道该方法作用*,主要是List configurations = SpringFactoriesLoader.loadFactoryNames(  
getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());方法

### 6、SpringFactoriesLoader.loadFactoryNames(） ###

public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) { 
    		String factoryClassName = factoryClass.getName();
    		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    	}

主要是调用loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());方法

### 7、loadSpringFactories（） ###

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { 
    		MultiValueMap<String, String> result = cache.get(classLoader);
    		if (result != null) { 
    			return result;
    		}
    
    		try { 
    			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);
    				for (Map.Entry<?, ?> entry : properties.entrySet()) { 
    					List<String> factoryClassNames = Arrays.asList(
    							StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
    					result.addAll((String) entry.getKey(), factoryClassNames);
    				}
    			}
    			cache.put(classLoader, result);
    			return result;
    		}
    		catch (IOException ex) { 
    			throw new IllegalArgumentException("Unable to load factories from location [" +
    					FACTORIES_RESOURCE_LOCATION + "]", ex);
    		}
    	}

调用Enumeration urls = (classLoader != null ?  
classLoader.getResources(FACTORIES\_RESOURCE\_LOCATION) :  
ClassLoader.getSystemResources(FACTORIES\_RESOURCE\_LOCATION));方法，可以知道，FACTORIES\_RESOURCE\_LOCATION常量的值为

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";
    	...

loadSpringFactories方法会扫描所有jar包类路径下 META-INF/spring.factories文件，并把扫描到的这些文件的内容包装成properties对象，然后从properties中获取到EnableAutoConfiguration.class类（类名）对应的值，然后把他们添加在容器中  
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其他jar包下也类似

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