简介
Spring 3.0之前，创建Bean可以通过xml配置文件与扫描特定包下面的类来将类注入到Spring IOC容器内。而在Spring 3.0之后提供了JavaConfig的方式，也就是将IOC容器里Bean的元信息以java代码的方式进行描述。我们可以通过@Configuration与@Bean这两个注解配合使用来将原来配置在xml文件里的bean通过java代码的方式进行描述
@Import注解提供了@Bean注解的功能，同时还有xml配置文件里
先看一下@Import注解的源码：
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {
/**
* {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}
* or regular component classes to import.
*/
Class<?>[] value();
}
从源码里可以看出@Import可以配合 Configuration , ImportSelector, ImportBeanDefinitionRegistrar 来使用，下面的or表示也可以把Import当成普通的Bean使用
@Import只允许放到类上面，不能放到方法上。下面我们来看具体的使用方式
普通使用方法
这种方式可以直接把类加入到Spring IOC容器
@Configuration
@Import(value={UserServiceImpl.class})
public class Config {
}
但是这种方式有一些问题，那就是只能使用类的无参构造方法来创建bean，对于有参数的构造方法就无能为力了
结合ImportBeanDefinitionRegistrar接口
ImportBeanDefinitionRegistrar接口的源码如下：
public interface ImportBeanDefinitionRegistrar {
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);
}
可以看到这个接口唯一的方法是有两个参数的
- AnnotationMetadata：通过这个参数可以拿到类的元数据信息
- BeanDefinitionRegistry：通过这个参数可以操作IOC容器
我们可以使用一个类来实现这个接口
public class UserServiceBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {
BeanDefinitionBuilder userService = BeanDefinitionBuilder.rootBeanDefinition(UserServiceImpl.class);
registry.registerBeanDefinition("userService", userService.getBeanDefinition());
}
}
可以看到我们在这个方法里面做一些特殊操作什么的都是可以的，相比较于普通的方式可是灵活了很多
接着我们在@Import注解引入的地方只需要修改为引入UserServiceBeanDefinitionRegistrar就可以了
@Configuration
@Import(value={UserServiceBeanDefinitionRegistrar.class})
public class Config {
}
结合ImportSelector接口
相比较与实现ImportBeanDefinitionRegistrar接口之后直接操作Bean容器来说，使用ImportSelector会更加优雅一些，只需要返回需要注入类的全限定名就可以了
ImportSelector接口的源码如下：
public interface ImportSelector {
String[] selectImports(AnnotationMetadata importingClassMetadata);
}
public class UserServiceImportSelect implements ImportSelector{
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
return new String[]{UserServiceImpl.class.getName()};
}
}
@Configuration()
@Import(value={UserServiceImportSelect.class})
public class Config {
}
相比较三种方式来说可以看到最后这种才是最优雅的方式
源码解析
首先我们就以第三种比较优雅的方式出发，使用Call Hierarchy看一下ImportSelector接口的selectImports方法调用链关系：
看过之前Spring源码解析文章的同学都知道，refresh方法是用来初始化容器上下文的。跟着这个调用链走下来到中间有一个类是ConfigurationClassPostProcessor，根据类名我们就可以猜到这个类应该是处理配置类（也就是标注@Configuration）的。那么从这开始看吧
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
List<BeanDefinitionHolder> configCandidates = new ArrayList<BeanDefinitionHolder>();
String[] candidateNames = registry.getBeanDefinitionNames();
for (String beanName : candidateNames) {
BeanDefinition beanDef = registry.getBeanDefinition(beanName);
if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
if (logger.isDebugEnabled()) {
logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
}
}
//查看是否是配置类
else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
}
}
//如果没有配置类就直接返回
if (configCandidates.isEmpty()) {
return;
}
//对这些配置类根据Order排序
Collections.sort(configCandidates, new Comparator<BeanDefinitionHolder>() {
@Override
public int compare(BeanDefinitionHolder bd1, BeanDefinitionHolder bd2) {
int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
return (i1 < i2) ? -1 : (i1 > i2) ? 1 : 0;
}
});
SingletonBeanRegistry sbr = null;
if (registry instanceof SingletonBeanRegistry) {
sbr = (SingletonBeanRegistry) registry;
if (!this.localBeanNameGeneratorSet && sbr.containsSingleton(CONFIGURATION_BEAN_NAME_GENERATOR)) {
BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
this.componentScanBeanNameGenerator = generator;
this.importBeanNameGenerator = generator;
}
}
//创建配置类的解析类
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<BeanDefinitionHolder>(configCandidates);
Set<ConfigurationClass> alreadyParsed = new HashSet<ConfigurationClass>(configCandidates.size());
do {
ConfigurationClassParser的parse方法进行解析，重点哈
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<ConfigurationClass>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// Read the model and create bean definitions based on its content
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
this.reader.loadBeanDefinitions(configClasses);
alreadyParsed.addAll(configClasses);
candidates.clear();
if (registry.getBeanDefinitionCount() > candidateNames.length) {
String[] newCandidateNames = registry.getBeanDefinitionNames();
Set<String> oldCandidateNames = new HashSet<String>(Arrays.asList(candidateNames));
Set<String> alreadyParsedClasses = new HashSet<String>();
for (ConfigurationClass configurationClass : alreadyParsed) {
alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
}
for (String candidateName : newCandidateNames) {
if (!oldCandidateNames.contains(candidateName)) {
BeanDefinition bd = registry.getBeanDefinition(candidateName);
if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
!alreadyParsedClasses.contains(bd.getBeanClassName())) {
candidates.add(new BeanDefinitionHolder(bd, candidateName));
}
}
}
candidateNames = newCandidateNames;
}
}
while (!candidates.isEmpty());
if (sbr != null) {
if (!sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
}
}
if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
}
}
现在该进入ConfigurationClassParser类了
public void parse(Set<BeanDefinitionHolder> configCandidates) {
this.deferredImportSelectors = new LinkedList<DeferredImportSelectorHolder>();
for (BeanDefinitionHolder holder : configCandidates) {
BeanDefinition bd = holder.getBeanDefinition();
try {
if (bd instanceof AnnotatedBeanDefinition) {
parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
}
else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
}
else {
parse(bd.getBeanClassName(), holder.getBeanName());
}
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
}
}
processDeferredImportSelectors();
}
//下面三种方法用于处理不同的BeanDefinition 类型，但最终都是使用的processConfigurationClass方法
protected final void parse(String className, String beanName) throws IOException {
MetadataReader reader = this.metadataReaderFactory.getMetadataReader(className);
processConfigurationClass(new ConfigurationClass(reader, beanName));
}
protected final void parse(Class<?> clazz, String beanName) throws IOException {
processConfigurationClass(new ConfigurationClass(clazz, beanName));
}
protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
processConfigurationClass(new ConfigurationClass(metadata, beanName));
}
可以看到配置类可能会是三种形式的存在，这三种形式的Bean在操作上有着部分不一样，但是大部分又是一样，所以Spring用这种模式来处理。不得不感叹人家设计的真好
接着往下看
protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
return;
}
ConfigurationClass existingClass = this.configurationClasses.get(configClass);
//在这里处理Configuration重复import
//如果同一个配置类被处理两次，两次都属于被import的则合并导入类，返回。如果配置类不是被导入的，则移除旧使用新的配置类
if (existingClass != null) {
if (configClass.isImported()) {
if (existingClass.isImported()) {
existingClass.mergeImportedBy(configClass);
}
return;
}
else {
this.configurationClasses.remove(configClass);
for (Iterator<ConfigurationClass> it = this.knownSuperclasses.values().iterator(); it.hasNext();) {
if (configClass.equals(it.next())) {
it.remove();
}
}
}
}
SourceClass sourceClass = asSourceClass(configClass);
do {
//接着往下看吧
sourceClass = doProcessConfigurationClass(configClass, sourceClass);
}
while (sourceClass != null);
this.configurationClasses.put(configClass, configClass);
}
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
throws IOException {
// 处理递归类
processMemberClasses(configClass, sourceClass);
// 处理@PropertySource注解
for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), PropertySources.class,
org.springframework.context.annotation.PropertySource.class)) {
if (this.environment instanceof ConfigurableEnvironment) {
processPropertySource(propertySource);
}
else {
logger.warn("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
"]. Reason: Environment must implement ConfigurableEnvironment");
}
}
// 处理 @ComponentScan 注解
Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
if (!componentScans.isEmpty() &&
!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
for (AnnotationAttributes componentScan : componentScans) {
// The config class is annotated with @ComponentScan -> perform the scan immediately
Set<BeanDefinitionHolder> scannedBeanDefinitions =
this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
// Check the set of scanned definitions for any further config classes and parse recursively if needed
for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
if (bdCand == null) {
bdCand = holder.getBeanDefinition();
}
if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
parse(bdCand.getBeanClassName(), holder.getBeanName());
}
}
}
}
//处理Import注解，这个是咱们的菜
processImports(configClass, sourceClass, getImports(sourceClass), true);
// 处理@ImportResource 注解
if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) {
AnnotationAttributes importResource =
AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
String[] resources = importResource.getStringArray("locations");
Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
for (String resource : resources) {
String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
configClass.addImportedResource(resolvedResource, readerClass);
}
}
//处理包含@Bean注解的方法
Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
// 处理普通方法
processInterfaces(configClass, sourceClass);
if (sourceClass.getMetadata().hasSuperClass()) {
String superclass = sourceClass.getMetadata().getSuperClassName();
if (!superclass.startsWith("java") && !this.knownSuperclasses.containsKey(superclass)) {
this.knownSuperclasses.put(superclass, configClass);
// Superclass found, return its annotation metadata and recurse
return sourceClass.getSuperClass();
}
}
return null;
}
看到这里好像突然发现了新大陆呀，原来我们经常见的@Bean、@ImportResource 、@Import、@ComponentScan、@PropertySource都是在这里处理的呀
咱们的重点还是放在@Import上，对其他几个注解感兴趣的同学可以自行研究一下
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
if (importCandidates.isEmpty()) {
return;
}
if (checkForCircularImports && isChainedImportOnStack(configClass)) {
this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
}
else {
this.importStack.push(configClass);
try {
for (SourceClass candidate : importCandidates) {
//如果实现了ImportSelector接口
if (candidate.isAssignable(ImportSelector.class)) {
Class<?> candidateClass = candidate.loadClass();
ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
ParserStrategyUtils.invokeAwareMethods(
selector, this.environment, this.resourceLoader, this.registry);
if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) {
this.deferredImportSelectors.add(
new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
}
else {
String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
processImports(configClass, currentSourceClass, importSourceClasses, false);
}
}
//如果实现了ImportBeanDefinitionRegistrar接口
else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
Class<?> candidateClass = candidate.loadClass();
ImportBeanDefinitionRegistrar registrar =
BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
ParserStrategyUtils.invokeAwareMethods(
registrar, this.environment, this.resourceLoader, this.registry);
configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
}
else {
//将import当成Configuration来使用就是我们的第一种应用的方式
this.importStack.registerImport(
currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
processConfigurationClass(candidate.asConfigClass(configClass));
}
}
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to process import candidates for configuration class [" +
configClass.getMetadata().getClassName() + "]", ex);
}
finally {
this.importStack.pop();
}
}
}