前言
接上一篇《从零开始造Spring05—实现spring注解-1》，今天我们接着学习spring注解。这是学习刘欣老师《从零开始造Spring》课程的学习笔记。上一篇我们实现了Bean的生成，这一篇我们将接着来实现Bean的注入，也叫依赖注入。
具体实现
数据结构
注： @Autowired 应用的地方有多处，此处我们只实现了应用于Field上。Member对象来自于java.lang.reflect 包，其相当于Field，Method，Constructor的子类。
关键代码
InjectionMetadata类
public class InjectionMetadata { private final Class<?> targetClass; private List<InjectionElement> injectionElementList; public InjectionMetadata(Class<?> targetClass, List<InjectionElement> injectionElementList) { this.targetClass = targetClass; this.injectionElementList = injectionElementList; } public List<InjectionElement> getInjectionElementList() { return injectionElementList; } /** * 核心的inject,在Bean生命周期的postProcessPropertyValues方法中被调用 * @param target */ public void inject(Object target) { if (injectionElementList == null || injectionElementList.isEmpty()) { return; } for (InjectionElement ele : injectionElementList) { ele.inject(target); } } }
AutowiredFieldElement 类
public class AutowiredFieldElement extends InjectionElement { boolean required; public AutowiredFieldElement(Field f, boolean required, AutowireCapableBeanFactory factory) { super(f,factory); this.required = required; } public Field getField() { return (Field) this.member; } @Override public void inject(Object target) { Field field = this.getField(); try { DependencyDescriptor desc = new DependencyDescriptor(field, this.required); //获取Bean的实例 Object value = factory.resolveDependency(desc); if (value != null) { //将field 置于可访问的 ReflectionUtils.makeAccessible(field); // 将被依赖的Bean注入到依赖的Bean中 field.set(target, value); } } catch (Throwable ex) { throw new BeanCreationException("Could not autowire field: " + field, ex); } } }
DependencyDescriptor 类是用来描述依赖的。相关类图如下：
这里扩展AutowireCapableBeanFactory 新接口主要是为了将resolveDependency方法隐藏于Spring内部，因为BeanFactory 主要是面向使用Spring的程序员。resolveDependency方法于他们没有实际用途。
关键代码：DefaultBeanFactory 类
@Override public Object resolveDependency(DependencyDescriptor descriptor) { Class<?> typeToMatch = descriptor.getDepencyType(); for (BeanDefinition bd : this.beanDefinitionMap.values()) { //确保BeanDefinition 有class对象 resolveBeanClass(bd); Class<?> beanClass = bd.getBeanClass(); if (typeToMatch.isAssignableFrom(beanClass)) { return getBean(bd.getID()); } } return null; } public void resolveBeanClass(BeanDefinition bd) { if (bd.hasBeanClass()) { return; } else { try { bd.resolveBeanClass(this.getBeanClassLoader()); } catch (ClassNotFoundException e) { throw new RuntimeException("can't load class:"+bd.getBeanClassName()); } } }
Bean的生命周期
Bean的生命周期共有四个阶段：
在此处我们只关注Bean的实例化和初始化阶段。
关键代码：AutowiredAnnotationProcessor 类
@Override public void postProcessPropertyValues(Object bean, String beanName) throws BeansException { InjectionMetadata metadata = buildAutowiringMetadata(bean.getClass()); try { metadata.inject(bean); } catch (Exception ex) { throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex); } } public InjectionMetadata buildAutowiringMetadata(Class<?> clazz) { LinkedList<InjectionElement> elements = new LinkedList<InjectionElement>(); Class<?> targetClass = clazz; LinkedList<InjectionElement> currentElements = new LinkedList<InjectionElement>(); for (Field field : targetClass.getDeclaredFields()) { Annotation ann = findAutowiredAnnotation(field); if (ann != null) { if (Modifier.isStatic(field.getModifiers())) { continue; } boolean required = determineRequiredStatus(ann); currentElements.add(new AutowiredFieldElement(field, required, beanFactory)); } } for (Method method : targetClass.getDeclaredMethods()) { //TODO 处理方法注入 } elements.addAll(0, currentElements); targetClass = targetClass.getSuperclass(); while (targetClass != null && targetClass != Object.class) ; return new InjectionMetadata(clazz, elements); }
那么怎么使用这些PostProcessor呢？
1. 创建PostProcessor
public AbstractApplicationContext(String configFile) { factory = new DefaultBeanFactory(); XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(factory); Resource resource = getResourceByPath(configFile); reader.loadBeanDefinitions(resource); factory.setBeanClassLoader(this.getBeanClassLoader()); registerBeanPostProcessors(factory); } protected void registerBeanPostProcessors(ConfigurableBeanFactory beanFactory) { AutowiredAnnotationProcessor postProcessor = new AutowiredAnnotationProcessor(); postProcessor.setBeanFactory(beanFactory); beanFactory.addBeanPostProcessor(postProcessor); }
1.加到BeanFactory
在ConfigurableBeanFactory上加入
2.在DefaultBeanFactory. populateBean()调用
DefaultBeanFactory 类实现了ConfigurableBeanFactory 接口
protected void populateBean(BeanDefinition bd, Object bean) { for (BeanPostProcessor processor : this.getBeanPostProcessors()) { if (processor instanceof InstantiationAwareBeanPostProcessor) { ((InstantiationAwareBeanPostProcessor)processor).postProcessPropertyValues(bean, bd.getID()); } } ..... }
