spring源码分析之@Conditional

根源在AnnotationConfigApplicationContext和AnnotationConfigWebApplicationContext，以AnnotationConfigApplicationContext为例：

1.构造方法

    /**
     * Create a new AnnotationConfigApplicationContext, deriving bean definitions
     * from the given annotated classes and automatically refreshing the context.
     * @param annotatedClasses one or more annotated classes,
     * e.g. {@link Configuration @Configuration} classes
     */
    public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
        this();
        register(annotatedClasses);
        refresh();
    }

    /**
     * Create a new AnnotationConfigApplicationContext, scanning for bean definitions
     * in the given packages and automatically refreshing the context.
     * @param basePackages the packages to check for annotated classes
     */
    public AnnotationConfigApplicationContext(String... basePackages) {
        this();
        scan(basePackages);
        refresh();
    }

一种是注解类方式，一种是扫描方式，殊途同归。以注解类来分析：

    /**
     * Register one or more annotated classes to be processed.
     * <p>Note that {@link #refresh()} must be called in order for the context
     * to fully process the new classes.
     * @param annotatedClasses one or more annotated classes,
     * e.g. {@link Configuration @Configuration} classes
     * @see #scan(String...)
     * @see #refresh()
     */
    public void register(Class<?>... annotatedClasses) {
        Assert.notEmpty(annotatedClasses, "At least one annotated class must be specified");
        this.reader.register(annotatedClasses);
    }

2. 实现方法AnnotatedBeanDefinitionReader

    public void register(Class<?>... annotatedClasses) {
        for (Class<?> annotatedClass : annotatedClasses) {
            registerBean(annotatedClass);
        }
    }

具体逻辑还是在该类内部：

    @SuppressWarnings("unchecked")
    public void registerBean(Class<?> annotatedClass, Class<? extends Annotation>... qualifiers) {
        registerBean(annotatedClass, null, qualifiers);
    }

    @SuppressWarnings("unchecked")
    public void registerBean(Class<?> annotatedClass, String name, Class<? extends Annotation>... qualifiers) {
        AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
        if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
            return;
        }

        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));
                }
            }
        }

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

3.分析逻辑

3.1 ConditionEvaluator

    /**
     * Determine if an item should be skipped based on {@code @Conditional} annotations.
     * @param metadata the meta data
     * @param phase the phase of the call
     * @return if the item should be skipped
     */
    public boolean shouldSkip(AnnotatedTypeMetadata metadata, 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);
        }

        List<Condition> conditions = new ArrayList<>();
        for (String[] conditionClasses : getConditionClasses(metadata)) {
            for (String conditionClass : conditionClasses) {
                Condition condition = getCondition(conditionClass, this.context.getClassLoader());
                conditions.add(condition);
            }
        }

        AnnotationAwareOrderComparator.sort(conditions);

        for (Condition condition : conditions) {
            ConfigurationPhase requiredPhase = null;
            if (condition instanceof ConfigurationCondition) {
                requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
            }
            if (requiredPhase == null || requiredPhase == phase) {
                if (!condition.matches(this.context, metadata)) {
                    return true;
                }
            }
        }

        

3.2 大鱼AnnotationConfigUtils来了

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));
                }
            }
        }

通用注解（lazy、primary、DependsOn、role、description）实现：

    static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd, AnnotatedTypeMetadata metadata) {
        if (metadata.isAnnotated(Lazy.class.getName())) {
            abd.setLazyInit(attributesFor(metadata, Lazy.class).getBoolean("value"));
        }
        else if (abd.getMetadata() != metadata && abd.getMetadata().isAnnotated(Lazy.class.getName())) {
            abd.setLazyInit(attributesFor(abd.getMetadata(), Lazy.class).getBoolean("value"));
        }

        if (metadata.isAnnotated(Primary.class.getName())) {
            abd.setPrimary(true);
        }
        if (metadata.isAnnotated(DependsOn.class.getName())) {
            abd.setDependsOn(attributesFor(metadata, DependsOn.class).getStringArray("value"));
        }

        if (abd instanceof AbstractBeanDefinition) {
            AbstractBeanDefinition absBd = (AbstractBeanDefinition) abd;
            if (metadata.isAnnotated(Role.class.getName())) {
                absBd.setRole(attributesFor(metadata, Role.class).getNumber("value").intValue());
            }
            if (metadata.isAnnotated(Description.class.getName())) {
                absBd.setDescription(attributesFor(metadata, Description.class).getString("value"));
            }
        }
    }

3.3 钓上大鱼，处理注解的相关processor在这里注册：

/**
     * Register all relevant annotation post processors in the given registry.
     * @param registry the registry to operate on
     * @param source the configuration source element (already extracted)
     * that this registration was triggered from. May be {@code null}.
     * @return a Set of BeanDefinitionHolders, containing all bean definitions
     * that have actually been registered by this call
     */
    public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
            BeanDefinitionRegistry registry, Object source) {

        DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
        if (beanFactory != null) {
            if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
                beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
            }
            if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
                beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
            }
        }

        Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(4);

        if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
        }

        if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
        }

        if (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
        }

        // Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
        if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
        }

        // Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
        if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition();
            try {
                def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
                        AnnotationConfigUtils.class.getClassLoader()));
            }
            catch (ClassNotFoundException ex) {
                throw new IllegalStateException(
                        "Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
            }
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
        }

        if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
        }
        if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
            RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
            def.setSource(source);
            beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
        }

        return beanDefs;
    }

    private static BeanDefinitionHolder registerPostProcessor(
            BeanDefinitionRegistry registry, RootBeanDefinition definition, String beanName) {

        definition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        registry.registerBeanDefinition(beanName, definition);
        return new BeanDefinitionHolder(definition, beanName);
    }

3.3.1 ConfigurationClassPostProcessor

/**
 * {@link BeanFactoryPostProcessor} used for bootstrapping processing of
 * {@link Configuration @Configuration} classes.
 *
 * <p>Registered by default when using {@code <context:annotation-config/>} or
 * {@code <context:component-scan/>}. Otherwise, may be declared manually as
 * with any other BeanFactoryPostProcessor.
 *
 * <p>This post processor is {@link Ordered#HIGHEST_PRECEDENCE} as it is important
 * that any {@link Bean} methods declared in Configuration classes have their
 * respective bean definitions registered before any other BeanFactoryPostProcessor
 * executes.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @author Phillip Webb
 * @since 3.0
 */
public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
        PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
}

3.3.2 AutowiredAnnotationBeanPostProcessor

/**
 * {@link org.springframework.beans.factory.config.BeanPostProcessor} implementation
 * that autowires annotated fields, setter methods and arbitrary config methods.
 * Such members to be injected are detected through a Java 5 annotation: by default,
 * Spring's {@link Autowired @Autowired} and {@link Value @Value} annotations.
 *
 * <p>Also supports JSR-330's {@link javax.inject.Inject @Inject} annotation,
 * if available, as a direct alternative to Spring's own {@code @Autowired}.
 *
 * <p>Only one constructor (at max) of any given bean class may carry this
 * annotation with the 'required' parameter set to {@code true},
 * indicating <i>the</i> constructor to autowire when used as a Spring bean.
 * If multiple <i>non-required</i> constructors carry the annotation, they
 * will be considered as candidates for autowiring. The constructor with
 * the greatest number of dependencies that can be satisfied by matching
 * beans in the Spring container will be chosen. If none of the candidates
 * can be satisfied, then a default constructor (if present) will be used.
 * An annotated constructor does not have to be public.
 *
 * <p>Fields are injected right after construction of a bean, before any
 * config methods are invoked. Such a config field does not have to be public.
 *
 * <p>Config methods may have an arbitrary name and any number of arguments; each of
 * those arguments will be autowired with a matching bean in the Spring container.
 * Bean property setter methods are effectively just a special case of such a
 * general config method. Config methods do not have to be public.
 *
 * <p>Note: A default AutowiredAnnotationBeanPostProcessor will be registered
 * by the "context:annotation-config" and "context:component-scan" XML tags.
 * Remove or turn off the default annotation configuration there if you intend
 * to specify a custom AutowiredAnnotationBeanPostProcessor bean definition.
 * <p><b>NOTE:</b> Annotation injection will be performed <i>before</i> XML injection;
 * thus the latter configuration will override the former for properties wired through
 * both approaches.
 *
 * <p>In addition to regular injection points as discussed above, this post-processor
 * also handles Spring's {@link Lookup @Lookup} annotation which identifies lookup
 * methods to be replaced by the container at runtime. This is essentially a type-safe
 * version of {@code getBean(Class, args)} and {@code getBean(String, args)},
 * See {@link Lookup @Lookup's javadoc} for details.
 *
 * @author Juergen Hoeller
 * @author Mark Fisher
 * @author Stephane Nicoll
 * @since 2.5
 * @see #setAutowiredAnnotationType
 * @see Autowired
 * @see Value
 */
public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
        implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
}

3.3.3 RequiredAnnotationBeanPostProcessor

/**
 * {@link org.springframework.beans.factory.config.BeanPostProcessor} implementation
 * that enforces required JavaBean properties to have been configured.
 * Required bean properties are detected through a Java 5 annotation:
 * by default, Spring's {@link Required} annotation.
 *
 * <p>The motivation for the existence of this BeanPostProcessor is to allow
 * developers to annotate the setter properties of their own classes with an
 * arbitrary JDK 1.5 annotation to indicate that the container must check
 * for the configuration of a dependency injected value. This neatly pushes
 * responsibility for such checking onto the container (where it arguably belongs),
 * and obviates the need (<b>in part</b>) for a developer to code a method that
 * simply checks that all required properties have actually been set.
 *
 * <p>Please note that an 'init' method may still need to implemented (and may
 * still be desirable), because all that this class does is enforce that a
 * 'required' property has actually been configured with a value. It does
 * <b>not</b> check anything else... In particular, it does not check that a
 * configured value is not {@code null}.
 *
 * <p>Note: A default RequiredAnnotationBeanPostProcessor will be registered
 * by the "context:annotation-config" and "context:component-scan" XML tags.
 * Remove or turn off the default annotation configuration there if you intend
 * to specify a custom RequiredAnnotationBeanPostProcessor bean definition.
 *
 * @author Rob Harrop
 * @author Juergen Hoeller
 * @since 2.0
 * @see #setRequiredAnnotationType
 * @see Required
 */
public class RequiredAnnotationBeanPostProcessor extends InstantiationAwareBeanPostProcessorAdapter
        implements MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware {
}

3.3.4 CommonAnnotationBeanPostProcessor

/**
 * {@link org.springframework.beans.factory.config.BeanPostProcessor} implementation
 * that supports common Java annotations out of the box, in particular the JSR-250
 * annotations in the {@code javax.annotation} package. These common Java
 * annotations are supported in many Java EE 5 technologies (e.g. JSF 1.2),
 * as well as in Java 6's JAX-WS.
 *
 * <p>This post-processor includes support for the {@link javax.annotation.PostConstruct}
 * and {@link javax.annotation.PreDestroy} annotations - as init annotation
 * and destroy annotation, respectively - through inheriting from
 * {@link InitDestroyAnnotationBeanPostProcessor} with pre-configured annotation types.
 *
 * <p>The central element is the {@link javax.annotation.Resource} annotation
 * for annotation-driven injection of named beans, by default from the containing
 * Spring BeanFactory, with only {@code mappedName} references resolved in JNDI.
 * The {@link #setAlwaysUseJndiLookup "alwaysUseJndiLookup" flag} enforces JNDI lookups
 * equivalent to standard Java EE 5 resource injection for {@code name} references
 * and default names as well. The target beans can be simple POJOs, with no special
 * requirements other than the type having to match.
 *
 * <p>The JAX-WS {@link javax.xml.ws.WebServiceRef} annotation is supported too,
 * analogous to {@link javax.annotation.Resource} but with the capability of creating
 * specific JAX-WS service endpoints. This may either point to an explicitly defined
 * resource by name or operate on a locally specified JAX-WS service class. Finally,
 * this post-processor also supports the EJB 3 {@link javax.ejb.EJB} annotation,
 * analogous to {@link javax.annotation.Resource} as well, with the capability to
 * specify both a local bean name and a global JNDI name for fallback retrieval.
 * The target beans can be plain POJOs as well as EJB 3 Session Beans in this case.
 *
 * <p>The common annotations supported by this post-processor are available in
 * Java 6 (JDK 1.6) as well as in Java EE 5/6 (which provides a standalone jar for
 * its common annotations as well, allowing for use in any Java 5 based application).
 *
 * <p>For default usage, resolving resource names as Spring bean names,
 * simply define the following in your application context:
 *
 * <pre class="code">
 * &lt;bean class="org.springframework.context.annotation.CommonAnnotationBeanPostProcessor"/&gt;</pre>
 *
 * For direct JNDI access, resolving resource names as JNDI resource references
 * within the Java EE application's "java:comp/env/" namespace, use the following:
 *
 * <pre class="code">
 * &lt;bean class="org.springframework.context.annotation.CommonAnnotationBeanPostProcessor"&gt;
 *   &lt;property name="alwaysUseJndiLookup" value="true"/&gt;
 * &lt;/bean&gt;</pre>
 *
 * {@code mappedName} references will always be resolved in JNDI,
 * allowing for global JNDI names (including "java:" prefix) as well. The
 * "alwaysUseJndiLookup" flag just affects {@code name} references and
 * default names (inferred from the field name / property name).
 *
 * <p><b>NOTE:</b> A default CommonAnnotationBeanPostProcessor will be registered
 * by the "context:annotation-config" and "context:component-scan" XML tags.
 * Remove or turn off the default annotation configuration there if you intend
 * to specify a custom CommonAnnotationBeanPostProcessor bean definition!
 * <p><b>NOTE:</b> Annotation injection will be performed <i>before</i> XML injection; thus
 * the latter configuration will override the former for properties wired through
 * both approaches.
 *
 * @author Juergen Hoeller
 * @since 2.5
 * @see #setAlwaysUseJndiLookup
 * @see #setResourceFactory
 * @see org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor
 * @see org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor
 */
@SuppressWarnings("serial")
public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBeanPostProcessor
        implements InstantiationAwareBeanPostProcessor, BeanFactoryAware, Serializable {
}

3.3.5 PersistenceAnnotationBeanPostProcessor

/**
 * BeanPostProcessor that processes {@link javax.persistence.PersistenceUnit}
 * and {@link javax.persistence.PersistenceContext} annotations, for injection of
 * the corresponding JPA resources {@link javax.persistence.EntityManagerFactory}
 * and {@link javax.persistence.EntityManager}. Any such annotated fields or methods
 * in any Spring-managed object will automatically be injected.
 *
 * <p>This post-processor will inject sub-interfaces of {@code EntityManagerFactory}
 * and {@code EntityManager} if the annotated fields or methods are declared as such.
 * The actual type will be verified early, with the exception of a shared ("transactional")
 * {@code EntityManager} reference, where type mismatches might be detected as late
 * as on the first actual invocation.
 *
 * <p>Note: In the present implementation, PersistenceAnnotationBeanPostProcessor
 * only supports {@code @PersistenceUnit} and {@code @PersistenceContext}
 * with the "unitName" attribute, or no attribute at all (for the default unit).
 * If those annotations are present with the "name" attribute at the class level,
 * they will simply be ignored, since those only serve as deployment hint
 * (as per the Java EE specification).
 *
 * <p>This post-processor can either obtain EntityManagerFactory beans defined
 * in the Spring application context (the default), or obtain EntityManagerFactory
 * references from JNDI ("persistence unit references"). In the bean case,
 * the persistence unit name will be matched against the actual deployed unit,
 * with the bean name used as fallback unit name if no deployed name found.
 * Typically, Spring's {@link org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean}
 * will be used for setting up such EntityManagerFactory beans. Alternatively,
 * such beans may also be obtained from JNDI, e.g. using the {@code jee:jndi-lookup}
 * XML configuration element (with the bean name matching the requested unit name).
 * In both cases, the post-processor definition will look as simple as this:
 *
 * <pre class="code">
 * &lt;bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/&gt;</pre>
 *
 * In the JNDI case, specify the corresponding JNDI names in this post-processor's
 * {@link #setPersistenceUnits "persistenceUnits" map}, typically with matching
 * {@code persistence-unit-ref} entries in the Java EE deployment descriptor.
 * By default, those names are considered as resource references (according to the
 * Java EE resource-ref convention), located underneath the "java:comp/env/" namespace.
 * For example:
 *
 * <pre class="code">
 * &lt;bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"&gt;
 *   &lt;property name="persistenceUnits"&gt;
 *     &lt;map/gt;
 *       &lt;entry key="unit1" value="persistence/unit1"/&gt;
 *       &lt;entry key="unit2" value="persistence/unit2"/&gt;
 *     &lt;/map/gt;
 *   &lt;/property&gt;
 * &lt;/bean&gt;</pre>
 *
 * In this case, the specified persistence units will always be resolved in JNDI
 * rather than as Spring-defined beans. The entire persistence unit deployment,
 * including the weaving of persistent classes, is then up to the Java EE server.
 * Persistence contexts (i.e. EntityManager references) will be built based on
 * those server-provided EntityManagerFactory references, using Spring's own
 * transaction synchronization facilities for transactional EntityManager handling
 * (typically with Spring's {@code @Transactional} annotation for demarcation
 * and {@link org.springframework.transaction.jta.JtaTransactionManager} as backend).
 *
 * <p>If you prefer the Java EE server's own EntityManager handling, specify entries
 * in this post-processor's {@link #setPersistenceContexts "persistenceContexts" map}
 * (or {@link #setExtendedPersistenceContexts "extendedPersistenceContexts" map},
 * typically with matching {@code persistence-context-ref} entries in the
 * Java EE deployment descriptor. For example:
 *
 * <pre class="code">
 * &lt;bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"&gt;
 *   &lt;property name="persistenceContexts"&gt;
 *     &lt;map/gt;
 *       &lt;entry key="unit1" value="persistence/context1"/&gt;
 *       &lt;entry key="unit2" value="persistence/context2"/&gt;
 *     &lt;/map/gt;
 *   &lt;/property&gt;
 * &lt;/bean&gt;</pre>
 *
 * If the application only obtains EntityManager references in the first place,
 * this is all you need to specify. If you need EntityManagerFactory references
 * as well, specify entries for both "persistenceUnits" and "persistenceContexts",
 * pointing to matching JNDI locations.
 *
 * <p><b>NOTE: In general, do not inject EXTENDED EntityManagers into STATELESS beans,
 * i.e. do not use {@code @PersistenceContext} with type {@code EXTENDED} in
 * Spring beans defined with scope 'singleton' (Spring's default scope).</b>
 * Extended EntityManagers are <i>not</i> thread-safe, hence they must not be used
 * in concurrently accessed beans (which Spring-managed singletons usually are).
 *
 * <p>Note: A default PersistenceAnnotationBeanPostProcessor will be registered
 * by the "context:annotation-config" and "context:component-scan" XML tags.
 * Remove or turn off the default annotation configuration there if you intend
 * to specify a custom PersistenceAnnotationBeanPostProcessor bean definition.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 2.0
 * @see javax.persistence.PersistenceUnit
 * @see javax.persistence.PersistenceContext
 */
@SuppressWarnings("serial")
public class PersistenceAnnotationBeanPostProcessor
        implements InstantiationAwareBeanPostProcessor, DestructionAwareBeanPostProcessor,
        MergedBeanDefinitionPostProcessor, PriorityOrdered, BeanFactoryAware, Serializable {

3.3.6 EventListenerMethodProcessor

/**
 * Register {@link EventListener} annotated method as individual {@link ApplicationListener}
 * instances.
 *
 * @author Stephane Nicoll
 * @author Juergen Hoeller
 * @since 4.2
 */
public class EventListenerMethodProcessor implements SmartInitializingSingleton, ApplicationContextAware {
}

3.3.7 DefaultEventListenerFactory

/**
 * Default {@link EventListenerFactory} implementation that supports the
 * regular {@link EventListener} annotation.
 * <p>Used as "catch-all" implementation by default.
 *
 * @author Stephane Nicoll
 * @since 4.2
 */
public class DefaultEventListenerFactory implements EventListenerFactory, Ordered {
}

扯远了，就此打住!