为什么abstract @service注解的类不被加载到beanfactory中

青旅半醒 2022-02-25 00:30 101阅读 0赞

使用过spring开发的开发者对@Service注解以及@Autowired注解不会陌生，系统在启动时会把@Service注解的类加载到BeanFactory中，然后就可以通过@Autowired注解的方式注入Service类实例，但并不是所以被@Service注解的类都会被加载到系统中，那么到底哪些类会被加载到系统中(也就是满足什么条件才会被加载呢)，这个看下ClassPathScanningCandidateComponentProvider类的findCandidateComponents方法：

public Set<BeanDefinition> findCandidateComponents(String basePackage) {
    		Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>();
    		try {
    			String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
    					resolveBasePackage(basePackage) + '/' + this.resourcePattern;
    			Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
    			boolean traceEnabled = logger.isTraceEnabled();
    			boolean debugEnabled = logger.isDebugEnabled();
    			for (Resource resource : resources) {
    				if (traceEnabled) {
    					logger.trace("Scanning " + resource);
    				}
    				if (resource.isReadable()) {
    					try {
    						MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
    						if (isCandidateComponent(metadataReader)) {
    							ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
    							sbd.setResource(resource);
    							sbd.setSource(resource);
    							if (isCandidateComponent(sbd)) {
    								if (debugEnabled) {
    									logger.debug("Identified candidate component class: " + resource);
    								}
    								candidates.add(sbd);
    							}
    							else {
    								if (debugEnabled) {
    									logger.debug("Ignored because not a concrete top-level class: " + resource);
    								}
    							}
    						}
    						else {
    							if (traceEnabled) {
    								logger.trace("Ignored because not matching any filter: " + resource);
    							}
    						}
    					}
    					catch (Throwable ex) {
    						throw new BeanDefinitionStoreException(
    								"Failed to read candidate component class: " + resource, ex);
    					}
    				}
    				else {
    					if (traceEnabled) {
    						logger.trace("Ignored because not readable: " + resource);
    					}
    				}
    			}
    		}
    		catch (IOException ex) {
    			throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
    		}
    		return candidates;
    	}

下面再看下下面这个方法：

protected boolean isCandidateComponent(AnnotatedBeanDefinition beanDefinition) {
    		AnnotationMetadata metadata = beanDefinition.getMetadata();
    		return (metadata.isIndependent() && (metadata.isConcrete() ||
    				(metadata.isAbstract() && metadata.hasAnnotatedMethods(Lookup.class.getName()))));
    	}

上面这段代码的意思就是：

1.类是独立的（独立类或者是嵌套内部类）

2.类是具体的（不是接口也不是抽象类）

3.类是抽象的且这个类都被Lookup注解的方法

也就是@Service被加载到系统需要满足条件1 && （条件2 || 条件3）