开篇
这篇文章的目的主要目的是总结下BeanDefinition的保存数据结构以及Bean创建相关的流程（着重介绍流程而非流程细节），在这篇文章当中我们可以宏观看到BeanDefinition的保存数据结构、不同Scope的BeanDefinition的创建流程等。
整篇文章的脉络是：1、通过BeanFactory的类图观察核心的类变量；2、分析类变量当中的存储结果了解数据存储，3、最后介绍不同的scope的bean对象的创建流程。
BeanFactory的类图
DefaultListableBeanFactory
说明：
- 1、DefaultListableBeanFactory作为Spring的所有Bean的管控中心。
- 2、DefaultSingletonBeanRegistry作为Spring单例对象Bean的管控中心。
DefaultListableBeanFactory核心变量
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
@Nullable
private static Class<?> javaxInjectProviderClass;
/** Map from serialized id to factory instance. */
private static final Map<String, Reference<DefaultListableBeanFactory>> serializableFactories =
new ConcurrentHashMap<>(8);
/** Optional id for this factory, for serialization purposes. */
@Nullable
private String serializationId;
/** Whether to allow re-registration of a different definition with the same name. */
private boolean allowBeanDefinitionOverriding = true;
/** Whether to allow eager class loading even for lazy-init beans. */
private boolean allowEagerClassLoading = true;
/** Optional OrderComparator for dependency Lists and arrays. */
@Nullable
private Comparator<Object> dependencyComparator;
/** Resolver to use for checking if a bean definition is an autowire candidate. */
private AutowireCandidateResolver autowireCandidateResolver = new SimpleAutowireCandidateResolver();
/** Map from dependency type to corresponding autowired value. */
private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<>(16);
/** Map of bean definition objects, keyed by bean name. */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
/** Map of singleton and non-singleton bean names, keyed by dependency type. */
private final Map<Class<?>, String[]> allBeanNamesByType = new ConcurrentHashMap<>(64);
/** Map of singleton-only bean names, keyed by dependency type. */
private final Map<Class<?>, String[]> singletonBeanNamesByType = new ConcurrentHashMap<>(64);
/** List of bean definition names, in registration order. */
private volatile List<String> beanDefinitionNames = new ArrayList<>(256);
/** List of names of manually registered singletons, in registration order. */
private volatile Set<String> manualSingletonNames = new LinkedHashSet<>(16);
/** Cached array of bean definition names in case of frozen configuration. */
@Nullable
private volatile String[] frozenBeanDefinitionNames;
/** Whether bean definition metadata may be cached for all beans. */
private volatile boolean configurationFrozen = false;
}
说明：
- Map<String, BeanDefinition> beanDefinitionMap负责存储所有的BeanDefinition对象。
DefaultSingletonBeanRegistry核心变量
public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
/** Set of registered singletons, containing the bean names in registration order. */
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
/** Names of beans that are currently in creation. */
private final Set<String> singletonsCurrentlyInCreation =
Collections.newSetFromMap(new ConcurrentHashMap<>(16));
/** Names of beans currently excluded from in creation checks. */
private final Set<String> inCreationCheckExclusions =
Collections.newSetFromMap(new ConcurrentHashMap<>(16));
/** List of suppressed Exceptions, available for associating related causes. */
@Nullable
private Set<Exception> suppressedExceptions;
/** Flag that indicates whether we're currently within destroySingletons. */
private boolean singletonsCurrentlyInDestruction = false;
/** Disposable bean instances: bean name to disposable instance. */
private final Map<String, Object> disposableBeans = new LinkedHashMap<>();
/** Map between containing bean names: bean name to Set of bean names that the bean contains. */
private final Map<String, Set<String>> containedBeanMap = new ConcurrentHashMap<>(16);
/** Map between dependent bean names: bean name to Set of dependent bean names. */
private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap<>(64);
/** Map between depending bean names: bean name to Set of bean names for the bean's dependencies. */
private final Map<String, Set<String>> dependenciesForBeanMap = new ConcurrentHashMap<>(64);
}
说明：
- DefaultSingletonBeanRegistry负责Spring所有单例对象的存储。
- Map<String, Object> singletonObjects 指单例对象的cache。
- Map<String, ObjectFactory<?>> singletonFactories 指单例对象工厂的cache。
- Map<String, Object> earlySingletonObjects 指提前曝光的单例对象的cache。
- 上述三个cache构成了三级缓存，Spring就用这三级缓存巧妙的解决了循环依赖问题。
- 优先级顺序 singletonFactories < earlySingletonObjects < singletonObjects 。
不同scope的Bean创建过程
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 检查父工厂的Bean Definition是否存在
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 先初始化Bean的依赖信息
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// 处理单例类型的Bean对象
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 处理Prototype的Bean对象
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
// 处理其他scope的Bean对象
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
// 不同scope类型的Bean保存在不同的scope对象当中
Object scopedInstance = scope.get(beanName, () -> {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
}
}
}
catch (BeansException ex) {
}
}
}
说明：
- 处理流程处理scope类型为Singleton、Prototype和其他类型的Bean。
- Singleton类型的Bean通过DefaultSingletonBeanRegistry的存储来解决。
- Prototype类型的Bean通过每次请求重新创建完成。
- 其他类型的Bean通过scopes获取Scope，通过scope.get获取保存的Bean实例。
