前言
本次介绍下@Value这个注解的底层实现
正文
本次介绍下@Value的底层实现
底层是先解析这个里面配置的表达式解析，再进行依赖注入
表达式解析
进入
org.springframework.context.expression.StandardBeanExpressionResolver#evaluate
@Override
@Nullable
public Object evaluate(@Nullable String value, BeanExpressionContext evalContext) throws BeansException {
if (!StringUtils.hasLength(value)) {
return value;
}
try {
// 从缓存中取关于这个值的表达式对象
Expression expr = this.expressionCache.get(value);
if (expr == null) {
expr = this.expressionParser.parseExpression(value, this.beanExpressionParserContext);
this.expressionCache.put(value, expr);
}
StandardEvaluationContext sec = this.evaluationCache.get(evalContext);
if (sec == null) {
sec = new StandardEvaluationContext();
sec.setRootObject(evalContext);
sec.addPropertyAccessor(new BeanExpressionContextAccessor());
sec.addPropertyAccessor(new BeanFactoryAccessor());
sec.addPropertyAccessor(new MapAccessor());
sec.addPropertyAccessor(new EnvironmentAccessor());
sec.setBeanResolver(new BeanFactoryResolver(evalContext.getBeanFactory()));
sec.setTypeLocator(new StandardTypeLocator(evalContext.getBeanFactory().getBeanClassLoader()));
ConversionService conversionService = evalContext.getBeanFactory().getConversionService();
if (conversionService != null) {
sec.setTypeConverter(new StandardTypeConverter(conversionService));
}
customizeEvaluationContext(sec);
this.evaluationCache.put(evalContext, sec);
}
return expr.getValue(sec);
}
catch (Throwable ex) {
throw new BeanExpressionException("Expression parsing failed", ex);
}
}
expr = this.expressionParser.parseExpression(value, this.beanExpressionParserContext);
进入
org.springframework.expression.common.TemplateAwareExpressionParser#parseExpression(java.lang.String, org.springframework.expression.ParserContext)
@Override
public Expression parseExpression(String expressionString, @Nullable ParserContext context) throws ParseException {
if (context != null && context.isTemplate()) {
return parseTemplate(expressionString, context);
}
else {
// 表达式解析
return doParseExpression(expressionString, context);
}
}
这一行
// 表达式解析 return doParseExpression(expressionString, context);
进入
org.springframework.expression.spel.standard.SpelExpressionParser#doParseExpression
@Override
protected SpelExpression doParseExpression(String expressionString, @Nullable ParserContext context) throws ParseException {
return new InternalSpelExpressionParser(this.configuration).doParseExpression(expressionString, context);
}
进入
org.springframework.expression.spel.standard.InternalSpelExpressionParser#doParseExpression
@Override
protected SpelExpression doParseExpression(String expressionString, @Nullable ParserContext context)
throws ParseException {
try {
this.expressionString = expressionString;
Tokenizer tokenizer = new Tokenizer(expressionString);
this.tokenStream = tokenizer.process();
this.tokenStreamLength = this.tokenStream.size();
this.tokenStreamPointer = 0;
this.constructedNodes.clear();
SpelNodeImpl ast = eatExpression();
Assert.state(ast != null, "No node");
Token t = peekToken();
if (t != null) {
throw new SpelParseException(t.startPos, SpelMessage.MORE_INPUT, toString(nextToken()));
}
Assert.isTrue(this.constructedNodes.isEmpty(), "At least one node expected");
return new SpelExpression(expressionString, ast, this.configuration);
}
catch (InternalParseException ex) {
throw ex.getCause();
}
}
这一行
// token解析 this.tokenStream = tokenizer.process();
返回
org.springframework.context.expression.StandardBeanExpressionResolver#evaluate
// 设置类型转换器 sec.setTypeConverter(new StandardTypeConverter(conversionService));
依赖注入
进入
org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition
@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {、
// 找到自动注入的元数据
InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
metadata.checkConfigMembers(beanDefinition);
}
进入
org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#findAutowiringMetadata
private InjectionMetadata findAutowiringMetadata(String beanName, Class<?> clazz, @Nullable PropertyValues pvs) {
// Fall back to class name as cache key, for backwards compatibility with custom callers.返回到类名作为缓存键，以便向后兼容自定义调用程序。
String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());
// Quick check on the concurrent map first, with minimal locking.
InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
synchronized (this.injectionMetadataCache) {
metadata = this.injectionMetadataCache.get(cacheKey);
if (InjectionMetadata.needsRefresh(metadata, clazz)) {
if (metadata != null) {
metadata.clear(pvs);
}
// 构建自动准入的元数据
metadata = buildAutowiringMetadata(clazz);
this.injectionMetadataCache.put(cacheKey, metadata);
}
}
}
return metadata;
}
这一行
// 构建自动准入的元数据 metadata = buildAutowiringMetadata(clazz);
进入
org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#buildAutowiringMetadata
private InjectionMetadata buildAutowiringMetadata(final Class<?> clazz) {
LinkedList<InjectionMetadata.InjectedElement> elements = new LinkedList<>();
Class<?> targetClass = clazz;
do {
final LinkedList<InjectionMetadata.InjectedElement> currElements = new LinkedList<>();
ReflectionUtils.doWithLocalFields(targetClass, field -> {
// 查找依赖注入注解的属性值
AnnotationAttributes ann = findAutowiredAnnotation(field);
if (ann != null) {
if (Modifier.isStatic(field.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static fields: " + field);
}
return;
}
// 判断required的值
boolean required = determineRequiredStatus(ann);
currElements.add(new AutowiredFieldElement(field, required));
}
});
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
Method bridgedMethod = BridgeMethodResolver.findBridgedMethod(method);
if (!BridgeMethodResolver.isVisibilityBridgeMethodPair(method, bridgedMethod)) {
return;
}
AnnotationAttributes ann = findAutowiredAnnotation(bridgedMethod);
if (ann != null && method.equals(ClassUtils.getMostSpecificMethod(method, clazz))) {
if (Modifier.isStatic(method.getModifiers())) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation is not supported on static methods: " + method);
}
return;
}
if (method.getParameterCount() == 0) {
if (logger.isWarnEnabled()) {
logger.warn("Autowired annotation should only be used on methods with parameters: " +
method);
}
}
boolean required = determineRequiredStatus(ann);
// 查找方法的属性
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 new InjectionMetadata(clazz, elements);
}
解析完的自动注入元数据会存储到本地缓存中，bean初始化时会调用，这部分的逻辑在bean初始化、依赖注入的部分解析过了
到这里，spring ioc源码解析完了
最后
本次介绍到这里，以上内容仅供参考。
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