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);
}
加到
BeanFactory
在ConfigurableBeanFactory
上加入在
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());
}
}
.....
}