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spring自动注入AutowiredAnnotationBeanPostProcessor源码分析

时间:2024-8-4 06:36     作者:韩俊     分类: PHP


本篇内容介绍了“spring自动注入AutowiredAnnotationBeanPostProcessor源码分析”的有关知识,在实际案例的操作过程中,不少人都会遇到这样的困境,接下来就让小编带领大家学习一下如何处理这些情况吧!希望大家仔细阅读,能够学有所成!

    一、MergedBeanDefinitionPostProcessor

    1.1、postProcessMergedBeanDefinition

    在Bean属性赋值前,缓存属性字段上的@Autowired和@Value注解信息。

    public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
       //1.1.1 查询属性或方法上有@Value和@Autowired注解的元素
       InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
       //1.1.2 检查元数据信息
       metadata.checkConfigMembers(beanDefinition);
    }

    1.1.1 findAutowiringMetadata 查询属性或方法上有@Value和@Autowired注解的元素

    private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
       // Fall back to class name as cache key, for backwards compatibility with custom callers.
       //获取Bean名称作为缓存key
       String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
       // Quick check on the concurrent map first, with minimal locking.
       //使用双重检查机制获取缓存
       InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
       //判断是否有元数据
       if (InjectionMetadata.needsRefresh(metadata, clazz)) {
          synchronized (this.injectionMetadataCache) {
             metadata = this.injectionMetadataCache.get(cacheKey);
             if (InjectionMetadata.needsRefresh(metadata, clazz)) {
                if (metadata != null) {
                   metadata.clear(pvs);
                }
                //构建元数据
                metadata = buildAutowiringMetadata(clazz);
                this.injectionMetadataCache.put(cacheKey, metadata);
             }
          }
       }
       return metadata;
    }

    这个 do-while 循环是用来一步一步往父类上爬的(可以看到这个循环体的最后一行是获取父类,判断条件是判断是否爬到了 

    Object

    private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
        // 判断是不是候选者类,比如说类名是以 java.开头的则不是候选者类 Order的实现类也不是候选者类
        // 但是如果this.autowiredAnnotationTypes 中有以java.开头的注解就返回true了
       if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {
          return InjectionMetadata.EMPTY;
       }
       List<InjectionMetadata.InjectedElement> elements = new ArrayList<>();
       Class<?> targetClass = clazz;
       do {
          final List<InjectionMetadata.InjectedElement> currElements = new ArrayList<>();
          // 循环获取类上的属性,如果类属性上有@Value和@Autowired包装成AutowiredFieldElement放入结果集  
          ReflectionUtils.doWithLocalFields(targetClass, field -> {
             MergedAnnotation<?> ann = findAutowiredAnnotation(field);
             if (ann != null) {
                if (Modifier.isStatic(field.getModifiers())) {
                   if (logger.isInfoEnabled()) {
                      logger.info("Autowired annotation is not supported on static fields: " + field);
                   }
                   return;
                }
                boolean required = determineRequiredStatus(ann);
                currElements.add(new AutowiredFieldElement(field, required));
             }
          });
          // 循环获取类上的方法,如果类方法上有@Value和@Autowired包装成AutowiredMethodElement放入结果集
          ReflectionUtils.doWithLocalMethods(targetClass, method -> {
             Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
             if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
                return;
             }
             MergedAnnotation<?> ann = findAutowiredAnnotation(bridgedMethod);
             if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
                if (Modifier.isStatic(method.getModifiers())) {
                   if (logger.isInfoEnabled()) {
                      logger.info("Autowired annotation is not supported on static methods: " + method);
                   }
                   return;
                }
                if (method.getParameterCount() == 0) {
                   if (logger.isInfoEnabled()) {
                      logger.info("Autowired annotation should only be used on methods with parameters: " +
                            method);
                   }
                }
                boolean required = determineRequiredStatus(ann);
                PropertyDescriptor pd = BeanUtils.findPropertyForMethod(bridgedMethod, clazz);
                currElements.add(new AutowiredMethodElement(method, required, pd));
             }
          });
          elements.addAll(0, currElements);
          targetClass = targetClass.getSuperclass();
       }
       while (targetClass != null && targetClass != Object.class);
       return InjectionMetadata.forElements(elements, clazz);
    }

    1.1.2 检查元数据信息

    检查是否有重复的元数据,去重处理,如一个属性上既有@Autowired注解,又有@Resource注解 。只使用一种方式进行注入,由于@Resource先进行解析,所以会选择@Resource的方式

    public void checkConfigMembers(RootBeanDefinition beanDefinition) {
       Set<InjectedElement> checkedElements = new LinkedHashSet<>(this.injectedElements.size());
       for (InjectedElement element : this.injectedElements) {
          Member member = element.getMember();
          if (!beanDefinition.isExternallyManagedConfigMember(member)) {
             beanDefinition.registerExternallyManagedConfigMember(member);
             checkedElements.add(element);
             if (logger.isTraceEnabled()) {
                logger.trace("Registered injected element on class [" + this.targetClass.getName() + "]: " + element);
             }
          }
       }
       this.checkedElements = checkedElements;
    }

    二、SmartInstantiationAwareBeanPostProcessor

    2.1、determineCandidateConstructors

    在bean实例化前选择@Autowired注解的构造函数,同时注入属性,从而完成自定义构造函数的选择。

    public Constructor<?>[] determineCandidateConstructors(Class<?> beanClass, final String beanName)
                throws BeanCreationException {
            // Let's check for lookup methods here...
            if (!this.lookupMethodsChecked.contains(beanName)) {
                try {
                    ReflectionUtils.doWithMethods(beanClass, method -> {
                        Lookup lookup = method.getAnnotation(Lookup.class);
                        if (lookup != null) {
                            Assert.state(this.beanFactory != null, "No BeanFactory available");
                            LookupOverride override = new LookupOverride(method, lookup.value());
                            try {
                                RootBeanDefinition mbd = (RootBeanDefinition)
                                        this.beanFactory.getMergedBeanDefinition(beanName);
                                mbd.getMethodOverrides().addOverride(override);
                            }
                            catch (NoSuchBeanDefinitionException ex) {
                                throw new BeanCreationException(beanName,
                                        "Cannot apply @Lookup to beans without corresponding bean definition");
                            }
                        }
                    });
                }
                catch (IllegalStateException ex) {
                    throw new BeanCreationException(beanName, "Lookup method resolution failed", ex);
                }
                this.lookupMethodsChecked.add(beanName);
            }
            // Quick check on the concurrent map first, with minimal locking.
            Constructor<?>[] candidateConstructors = this.candidateConstructorsCache.get(beanClass);
            if (candidateConstructors == null) {
                // Fully synchronized resolution now...
                synchronized (this.candidateConstructorsCache) {
                    candidateConstructors = this.candidateConstructorsCache.get(beanClass);
                    if (candidateConstructors == null) {
                        Constructor<?>[] rawCandidates;
                        try {
                            //反射获取所有构造函数
                            rawCandidates = beanClass.getDeclaredConstructors();
                        }
                        catch (Throwable ex) {
                            throw new BeanCreationException(beanName,
                                    "Resolution of declared constructors on bean Class [" + beanClass.getName() +
                                            "] from ClassLoader [" + beanClass.getClassLoader() + "] failed", ex);
                        }
                        //候选构造方法
                        List<Constructor<?>> candidates = new ArrayList<>(rawCandidates.length);
                        Constructor<?> requiredConstructor = null;
                        Constructor<?> defaultConstructor = null;
                        //这个貌似是 Kotlin 上用的, 不用管它
                        Constructor<?> primaryConstructor = BeanUtils.findPrimaryConstructor(beanClass);
                        int nonSyntheticConstructors = 0;
                        //遍历这些构造函数
                        for (Constructor<?> candidate : rawCandidates) {
                            //判断构造方法是否是合成的
                            if (!candidate.isSynthetic()) {
                                nonSyntheticConstructors++;
                            }
                            else if (primaryConstructor != null) {
                                continue;
                            }
                            //查看是否有 @Autowired 注解
                            //如果有多个构造方法, 可以通过标注 @Autowired 的方式来指定使用哪个构造方法
                            AnnotationAttributes ann = findAutowiredAnnotation(candidate);
                            if (ann == null) {
                                Class<?> userClass = ClassUtils.getUserClass(beanClass);
                                if (userClass != beanClass) {
                                    try {
                                        Constructor<?> superCtor =
                                                userClass.getDeclaredConstructor(candidate.getParameterTypes());
                                        ann = findAutowiredAnnotation(superCtor);
                                    }
                                    catch (NoSuchMethodException ex) {
                                        // Simply proceed, no equivalent superclass constructor found...
                                    }
                                }
                            }
                            //有 @Autowired 的情况
                            if (ann != null) {
                                if (requiredConstructor != null) {
                                    throw new BeanCreationException(beanName,
                                            "Invalid autowire-marked constructor: " + candidate +
                                                    ". Found constructor with 'required' Autowired annotation already: " +
                                                    requiredConstructor);
                                }
                                boolean required = determineRequiredStatus(ann);
                                if (required) {
                                    if (!candidates.isEmpty()) {
                                        throw new BeanCreationException(beanName,
                                                "Invalid autowire-marked constructors: " + candidates +
                                                        ". Found constructor with 'required' Autowired annotation: " +
                                                        candidate);
                                    }
                                    requiredConstructor = candidate;
                                }
                                candidates.add(candidate);
                            }
                            //无参构造函数的情况
                            else if (candidate.getParameterCount() == 0) {
                                //构造函数没有参数, 则设置为默认的构造函数
                                defaultConstructor = candidate;
                            }
                        }
                        //到这里, 已经循环完了所有的构造方法
                        //候选者不为空时
                        if (!candidates.isEmpty()) {
                            // Add default constructor to list of optional constructors, as fallback.
                            if (requiredConstructor == null) {
                                if (defaultConstructor != null) {
                                    candidates.add(defaultConstructor);
                                }
                                else if (candidates.size() == 1 && logger.isInfoEnabled()) {
                                    logger.info("Inconsistent constructor declaration on bean with name '" + beanName +
                                            "': single autowire-marked constructor flagged as optional - " +
                                            "this constructor is effectively required since there is no " +
                                            "default constructor to fall back to: " + candidates.get(0));
                                }
                            }
                            candidateConstructors = candidates.toArray(new Constructor<?>[0]);
                        }
                        //类的构造方法只有1个, 且该构造方法有多个参数
                        else if (rawCandidates.length == 1 && rawCandidates[0].getParameterCount() > 0) {
                            candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
                        }
                        //这里不会进, 因为 primaryConstructor = null
                        else if (nonSyntheticConstructors == 2 && primaryConstructor != null &&
                                defaultConstructor != null && !primaryConstructor.equals(defaultConstructor)) {
                            candidateConstructors = new Constructor<?>[] {primaryConstructor, defaultConstructor};
                        }
                        //这里也不会进, 因为 primaryConstructor = null
                        else if (nonSyntheticConstructors == 1 && primaryConstructor != null) {
                            candidateConstructors = new Constructor<?>[] {primaryConstructor};
                        }
                        else {
                            //如果方法进了这里, 就是没找到合适的构造方法
                            //1. 类定义了多个构造方法, 且没有 @Autowired , 则有可能会进这里
                            candidateConstructors = new Constructor<?>[0];
                        }
                        this.candidateConstructorsCache.put(beanClass, candidateConstructors);
                    }
                }
            }
       //这里如果没找到, 则会返回 null, 而不会返回空数组
            return (candidateConstructors.length > 0 ? candidateConstructors : null);
        }

    遍历构造方法:

      只有一个无参构造方法, 则返回null

      只有一个有参构造方法, 则返回这个构造方法

      有多个构造方法且没有@Autowired, 此时spring则会蒙圈了, 不知道使用哪一个了。这里的后置处理器智能选择构造方法后置处理器。当选择不了的时候, 干脆返回 null

      有多个构造方法, 且在其中一个方法上标注了 @Autowired , 则会返回这个标注的构造方法

      有多个构造方法, 且在多个方法上标注了@Autowired, 则spring会抛出异常, Spring会认为, 你指定了几个给我, 是不是你弄错了

    注意:

    这地方有个问题需要注意一下, 如果你写了多个构造方法, 且没有写 无参构造方法, 那么此处返回null, 

    在回到 createBeanInstance 方法中, 如果不能走 autowireConstructor(), 而走到 instantiateBean() 中去的话, 会报错的,因为类已经没有无参构造函数了。

    标签: php php教程

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