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Spring源码解析之零配置

作者:互联网

前言

1.spring在配置时代,都是基于xml配置的,启动一个spring容器,通过ClassPathXmlApplicationContext类的refresh方法,然后解析bean标签,完成bean的注册,解析自定义标签<context:component-scan base-package="cn.com.dq.xml"></context:component-scan>对注解的支撑,完成bean的注册
2.spring在零配置时代,基于注解,如何启动容器?如何完成对注解的支撑?

AnnotationConfigApplicationContext

AnnotationConfigApplicationContext是基于注解启动spring容器的类,它有2个构造方法,1.参数是包路径,2.参数是一个类

构造方法参数是包路径

public AnnotationConfigApplicationContext(String... basePackages) {
		this();
		scan(basePackages);
		refresh();
	}

this方法

public AnnotationConfigApplicationContext() {
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

this方法是他的无参构造器,在无参构造器中,干了2件事情,1.new了一个AnnotatedBeanDefinitionReader,2.new 了一个ClassPathBeanDefinitionScanner

scan方法

public void scan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		this.scanner.scan(basePackages);
	}
	
public int scan(String... basePackages) {
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

		doScan(basePackages);

		// Register annotation config processors, if necessary.
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}

看到上面的代码又是似曾相识,我们又会联想到xml的方式
我们看xml方式下component-scan扫描的源码

public BeanDefinition parse(Element element, ParserContext parserContext) {
		String basePackage = element.getAttribute(BASE_PACKAGE_ATTRIBUTE);
		basePackage = parserContext.getReaderContext().getEnvironment().resolvePlaceholders(basePackage);
		String[] basePackages = StringUtils.tokenizeToStringArray(basePackage,
				ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);

		// Actually scan for bean definitions and register them.
		ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
		Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
		registerComponents(parserContext.getReaderContext(), beanDefinitions, element);

		return null;
	}

我们可以看到doScan是同一个方法,因为他俩的扫描器是同一个类
我们在看xml方式下组件注册registerComponents(parserContext.getReaderContext(), beanDefinitions, element);
最终调到的是AnnotationConfigUtils.registerAnnotationConfigProcessors(readerContext.getRegistry(), source);
而基于注解的方式小组件注册AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);跟下去调的是
registerAnnotationConfigProcessors(registry, null);发现又是同一个方法,只是基于注解的第二个参数null而已

refresh方法

发现与xml方式的又是同一个方法,因为AnnotationConfigApplicationContext与ClassPathXmlApplicationContext的父类是AbstractApplicationContext,refresh方法是AbstractApplicationContext里面的方法

构造方法参数是类

public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
		this();
		register(annotatedClasses);
		refresh();
	}

他与构造器参数是包路径唯一不同的是register方法,我们来看register方法

register

void doRegisterBean(Class<T> annotatedClass, @Nullable Supplier<T> instanceSupplier, @Nullable String name,
			@Nullable Class<? extends Annotation>[] qualifiers, BeanDefinitionCustomizer... definitionCustomizers) {

		AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
		if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
			return;
		}

		abd.setInstanceSupplier(instanceSupplier);
		ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
		abd.setScope(scopeMetadata.getScopeName());
		String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));

		AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
		if (qualifiers != null) {
			for (Class<? extends Annotation> qualifier : qualifiers) {
				if (Primary.class == qualifier) {
					abd.setPrimary(true);
				}
				else if (Lazy.class == qualifier) {
					abd.setLazyInit(true);
				}
				else {
					abd.addQualifier(new AutowireCandidateQualifier(qualifier));
				}
			}
		}
		for (BeanDefinitionCustomizer customizer : definitionCustomizers) {
			customizer.customize(abd);
		}

		BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
		definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
		BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
	}

1.传入的参数class封装成beanDefinition

AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);

2.判断是否存在@Condition注解,如果有,会判断该类上面是否有@Configuration,@Component,@ComponentScan,@Import,@ImportResource,如果都没有,然后判断方法上是否有@Bean注解
对于有其中任何一个注解,代码就不会往下面执行,为什么?因为@Condition是会进行条件判断。springboot中会大量运用该注解,我们这里是没有这个注解的,代码继续往下

public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
			return false;
		}

		if (phase == null) {
			if (metadata instanceof AnnotationMetadata &&
					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
			}
			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
		}
		
		public static boolean isConfigurationCandidate(AnnotationMetadata metadata) {
		return (isFullConfigurationCandidate(metadata) || isLiteConfigurationCandidate(metadata));
	}
	
	public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
		return metadata.isAnnotated(Configuration.class.getName());
	}
	
	public static boolean isLiteConfigurationCandidate(AnnotationMetadata metadata) {
		// Do not consider an interface or an annotation...
		if (metadata.isInterface()) {
			return false;
		}

		// Any of the typical annotations found?
		for (String indicator : candidateIndicators) {
			if (metadata.isAnnotated(indicator)) {
				return true;
			}
		}

		// Finally, let's look for @Bean methods...
		try {
			return metadata.hasAnnotatedMethods(Bean.class.getName());
		}
		catch (Throwable ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to introspect @Bean methods on class [" + metadata.getClassName() + "]: " + ex);
			}
			return false;
		}
	}

3.接着就是对beanDefinition的scope属性赋值,接着执行了这行代码
AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);,点进去看

static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd, AnnotatedTypeMetadata metadata) {
		//对@Lazy注解支持
		AnnotationAttributes lazy = attributesFor(metadata, Lazy.class);
		if (lazy != null) {
			abd.setLazyInit(lazy.getBoolean("value"));
		}
		else if (abd.getMetadata() != metadata) {
			lazy = attributesFor(abd.getMetadata(), Lazy.class);
			if (lazy != null) {
				abd.setLazyInit(lazy.getBoolean("value"));
			}
		}

		if (metadata.isAnnotated(Primary.class.getName())) {
			abd.setPrimary(true);
		}
		//对@DependsOn注解支持
		AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
		if (dependsOn != null) {
			abd.setDependsOn(dependsOn.getStringArray("value"));
		}

		AnnotationAttributes role = attributesFor(metadata, Role.class);
		if (role != null) {
			abd.setRole(role.getNumber("value").intValue());
		}
		AnnotationAttributes description = attributesFor(metadata, Description.class);
		if (description != null) {
			abd.setDescription(description.getString("value"));
		}
	}

发现和xml的又一样,就是对于一些注解的支撑,然后填充beanDefinition中的属性,包含了@Lazy,@DependsOn等注解的支撑。
4.接着就是这行代码BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);看方法名是注册bean

public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

		// Register bean definition under primary name.
		String beanName = definitionHolder.getBeanName();

		//完成BeanDefinition的注册
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		//建立别名和 id的映射,这样就可以根据别名获取到id
		// Register aliases for bean name, if any.
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				registry.registerAlias(beanName, alias);
			}
		}
	}

上述代码又回到了xml注册bean的方法,构造方法参数是类是,是将指定的类注册到spring容器中,并实例化。

总结

基于xml和基于注解bean的注册的相同与不同
不同点:
1.基于xml创建的解析器是XmlBeanDefinitionReader,而基于注解的是AnnotatedBeanDefinitionReader
2.基于xml他是解析component-scan标签创建扫描器,而基于注解的是直接new的扫描器
相同点:
1.他们创建的扫描器都是同一个,扫描注解,注册组件流程是一模一样。

标签:xml,null,abd,Spring,源码,注解,解析,class,metadata
来源: https://www.cnblogs.com/honeylemon/p/16251903.html