Spring Bean的解析 BeanDefinition 与 RuntimeBeanReference

Myth丶恋晨 2021-08-26 17:00 337阅读 0赞

# 1.BeanDefinitionHolder #

持有name和aliases,为注册做准备

/**
     * Holder for a BeanDefinition with name and aliases.
     * Can be registered as a placeholder for an inner bean.
     */
    public class BeanDefinitionHolder implements BeanMetadataElement {
        private final BeanDefinition beanDefinition;
    
        private final String beanName;
    
        @Nullable
        private final String[] aliases;
    }

# 2. BeanDefinitionReaderUtils #

配合注册时配合BeanDefinitionHolder来注册

public class BeanDefinitionReaderUtils {
        public static void registerBeanDefinition(
                BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
                throws BeanDefinitionStoreException {
    
            // Register bean definition under primary name.
            String beanName = definitionHolder.getBeanName();
            registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
    
            // Register aliases for bean name, if any.
            String[] aliases = definitionHolder.getAliases();
            if (aliases != null) {
                for (String alias : aliases) {
                    registry.registerAlias(beanName, alias);
                }
            }
        }
    }

# 3.BeanDefinitionCustomizer #

提供一个接口定制化BeanDefinition

public interface BeanDefinitionCustomizer {
    
        /**
         * Customize the given bean definition.
         */
        void customize(BeanDefinition bd);
    }

示例:

@Test
        public void beanClassWithSimpleProperty() {
            String[] dependsOn = new String[] { "A", "B", "C" };
            BeanDefinitionBuilder bdb = BeanDefinitionBuilder.rootBeanDefinition(TestBean.class);
            bdb.setScope(BeanDefinition.SCOPE_PROTOTYPE);
            bdb.addPropertyReference("age", "15");
            for (int i = 0; i < dependsOn.length; i++) {
                bdb.addDependsOn(dependsOn[i]);
            }
    
            bdb.applyCustomizers(new BeanDefinitionCustomizer()
            {
    
                @Override
                public void customize(BeanDefinition bd) {
                    bdb.addPropertyReference("age", "16");
                }
            });
    
            RootBeanDefinition rbd = (RootBeanDefinition) bdb.getBeanDefinition();
            assertFalse(rbd.isSingleton());
            assertEquals(TestBean.class, rbd.getBeanClass());
            assertTrue("Depends on was added", Arrays.equals(dependsOn, rbd.getDependsOn()));
            assertTrue(rbd.getPropertyValues().contains("age"));
        }

# 4.BeanDefinitionValueResolver #

对BeanDefinition中的propertyValues进行解析

1. RuntimeBeanReference

在Spring的解析段，其实容器中是没有依赖的Bean的实例的因此，那么这是这个被依赖的Bean如何在BeanDefinition中表示呢？  
答案就是RuntimeBeanReference，在解析到依赖的Bean的时侯，解析器会依据依赖bean的name创建一个RuntimeBeanReference对像，将这个对像放入BeanDefinition的MutablePropertyValues中。

转换成代码

//我们知道foo.bar.xxx 被解析为一个beanDefiniton，假设为xxxBeanDefinition
    reference = new RuntimeBeanReference("otherBeanName");
    xxxBeanDefinition.getPropertyValues().addPropertyValue("referBeanName", reference);

在创建Bean时，需要将依赖解析成真正的在Spring容器中存在的Bean。这是在getBean时由AbstractAutowireCapableBeanFactory在applyPropertyValues方法中通过BeanDefinitionValueResolver来实现的。BeanDefinitionValueResolver将真正的依赖bean和referBeanName关联起来。

参考:  
[https://blog.csdn.net/jerryai1/article/details/52980239][https_blog.csdn.net_jerryai1_article_details_52980239]

到了BeanDefinitionValueResolver会对不同的value类型进行解析,最终还是调用了beanFactory的getBean方法获取

public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
            // We must check each value to see whether it requires a runtime reference
            // to another bean to be resolved.
            if (value instanceof RuntimeBeanReference) {
                RuntimeBeanReference ref = (RuntimeBeanReference) value;
                return resolveReference(argName, ref);
            }
    ...
    }
    
        private Object resolveReference(Object argName, RuntimeBeanReference ref) {
            try {
                Object bean;
                String refName = ref.getBeanName();
                refName = String.valueOf(doEvaluate(refName));
                if (ref.isToParent()) {
                    if (this.beanFactory.getParentBeanFactory() == null) {
                        throw new BeanCreationException(
                                this.beanDefinition.getResourceDescription(), this.beanName,
                                "Can't resolve reference to bean '" + refName +
                                "' in parent factory: no parent factory available");
                    }
                    bean = this.beanFactory.getParentBeanFactory().getBean(refName);
                }
                else {
                    bean = this.beanFactory.getBean(refName);
                    this.beanFactory.registerDependentBean(refName, this.beanName);
                }
                if (bean instanceof NullBean) {
                    bean = null;
                }
                return bean;
            }
            catch (BeansException ex) {
                throw new BeanCreationException(
                        this.beanDefinition.getResourceDescription(), this.beanName,
                        "Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
            }
        }

测试代码

@Test
        public void propertyReferenceTest() {
            DefaultListableBeanFactory beanFactory=new DefaultListableBeanFactory();
    
            {
                BeanDefinitionBuilder bdb = BeanDefinitionBuilder.rootBeanDefinition(Colour.class);
                beanFactory.registerBeanDefinition("colour",bdb.getBeanDefinition());
            }
    
            {
                BeanDefinitionBuilder bdb = BeanDefinitionBuilder.rootBeanDefinition(TestBean.class);
                bdb.addPropertyReference("favouriteColour","colour");
                beanFactory.registerBeanDefinition("testBean",bdb.getBeanDefinition());
            }
    
            TestBean bean=beanFactory.getBean(TestBean.class);
        }

2.innerBean

innerBean最终会解析成BeanDefinitionHolder

<bean id="hasInnerBeansForConstructor" class="org.springframework.tests.sample.beans.TestBean">
            <constructor-arg>
                <bean id="innerBean" class="org.springframework.tests.sample.beans.TestBean" destroy-method="destroy">
                    <constructor-arg><value>inner1</value></constructor-arg>
                    <constructor-arg type="int"><value>6</value></constructor-arg>
                </bean>
            </constructor-arg>
        </bean>

![format_png][]

参考:  
[https://www.cnblogs.com/EasonJim/p/6883719.html][https_www.cnblogs.com_EasonJim_p_6883719.html]

[https_blog.csdn.net_jerryai1_article_details_52980239]: https://blog.csdn.net/jerryai1/article/details/52980239
[format_png]: /images/20210527/f2c463c02cd04419be83cd73801b40c4.png
[https_www.cnblogs.com_EasonJim_p_6883719.html]: https://www.cnblogs.com/EasonJim/p/6883719.html