Annotation比起xml是各有优势，Annotation用起来就是简洁，少了大量的配置文件。配置文件则很容易的就看出类之间的关联，看xml配置文件即可。但是如果用Annotation，还得看java代码。至于选择使用什么就看自己实际的情况而定。这里讲解spring在用Annotation的时候，是怎么读取Annotation，又像IOC中注入BeanDefinition的。
如果是用Annotation，则只需下面一行即可：
<!-- 激活spring的注解 ,有了下面一行，这行可以省略 <context:annotation-config /> --> <context:component-scan base-package="com.zzx.study" />
- 怎么读取xml配置文件
我们直接看spring-context-4.0.0.M2.jar文件下的META-INF目录下的spring.handlers文件有如下一行，至于怎么到这行，可看DefaultNamespaceHandlerResolver类的解析：
http\://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandler
即如果在xml配置文件中，如果命名空间为http://www.springframework.org/schema/context则使用org.springframework.context.config.ContextNamespaceHandler这个类来解析，ContextNamespaceHandler类有init，是初始化针对不同的标签用不同的类来解析。例如上面的component-scan，则使用ComponentScanBeanDefinitionParser类解析。具体怎么到这些类类解析xml，可参考另外一篇博文http://992012.iteye.com/blog/1921633
@Override
public void init() {
registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
}
在ComponentScanBeanDefinitionParser类中使用到了ASM来获得Annotation，而不是使用java的反射。原因是java的反射效率相对ASM第三方字节码库来说低一点。所以使用到了ASM。如果想看ASM，可到这个官网上查看http://asm.ow2.org/。ComponentScanBeanDefinitionParser的parse方法如下：
@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE),
ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
// Actually scan for bean definitions and register them.
ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element);
//通过scanner找到符合要求的Annotation，通过ASM第三方库
Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages);
//向IOC容器注入BeanDefinition
registerComponents(parserContext.getReaderContext(), beanDefinitions, element);
return null;
}
在ClassPathBeanDefinitionScanner的doScan方法中，会调用到ClassPathScanningCandidateComponentProvider中的findCandidateComponents方法
public Set<BeanDefinition> findCandidateComponents(String basePackage) {
Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>();
try {
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + "/" + this.resourcePattern;
Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);
boolean traceEnabled = logger.isTraceEnabled();
boolean debugEnabled = logger.isDebugEnabled();
for (Resource resource : resources) {
if (traceEnabled) {
logger.trace("Scanning " + resource);
}
if (resource.isReadable()) {
try {
//通过ASM创建元数据reader
MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
//根据reader判断是否BeanDefinition
if (isCandidateComponent(metadataReader)) {
//这里创建了ScannedGenericBeanDefinition
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setResource(resource);
sbd.setSource(resource);
if (isCandidateComponent(sbd)) {
if (debugEnabled) {
logger.debug("Identified candidate component class: " + resource);
}
candidates.add(sbd);
}
else {
if (debugEnabled) {
logger.debug("Ignored because not a concrete top-level class: " + resource);
}
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because not matching any filter: " + resource);
}
}
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to read candidate component class: " + resource, ex);
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because not readable: " + resource);
}
}
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
}
return candidates;
}
如果是使用了Service，Repository同意是可以被spring认识到的，因为这两个类的类同样也有元数据@Component。
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Service {
/**
* The value may indicate a suggestion for a logical component name,
* to be turned into a Spring bean in case of an autodetected component.
* @return the suggested component name, if any
*/
String value() default "";
}
这里顺带讲解一下ASM。
@Test
public void test3() throws IOException{
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage("com.zzx.study.spring") + "/" + "**/*.class";
PathMatchingResourcePatternResolver pathResolver = new PathMatchingResourcePatternResolver();
Resource[] rr = pathResolver.getResources(packageSearchPath);
for(Resource r:rr){
if(r.getFilename().equals("UserDao.class")){
System.out.println(r.getFilename());
AnnotationMetadataReadingVisitor visitor = new AnnotationMetadataReadingVisitor(Thread.currentThread().getContextClassLoader());
InputStream is = new BufferedInputStream(r.getInputStream());
ClassReader classReader;
try {
//创建ASM的ClassReader
classReader = new ClassReader(is);
}catch (IllegalArgumentException ex) {
throw new NestedIOException("ASM ClassReader failed to parse class file - " +
"probably due to a new Java class file version that isn't supported yet: " + r, ex);
}
finally {
is.close();
}
//调用reader的接受方法，这个方法实际就是解析class字节码的实现，这里使用了Visitor模式
classReader.accept(visitor, ClassReader.SKIP_DEBUG);
for(String annotationType:visitor.getAnnotationTypes()){
System.out.println(annotationType);
}
System.out.println("hasAnnotation:"+visitor.hasAnnotation(Component.class.getName()));
System.out.println("hasMetaAnnotation:"+visitor.hasMetaAnnotation(Component.class.getName()));
}
}
}
用户可以实现自己的visitor，也就是具体的访问者了。
public class MyOwnClassVisitor extends ClassVisitor {
@Override
public AnnotationVisitor visitAnnotation(String desc, boolean visible) {
//解析过程中如果有annotation就会调用到我这个具体访问者的方法了，下面的Field和method类似
System.out.println("visitAnnotation:"+desc);
return super.visitAnnotation(desc, visible);
}
@Override
public FieldVisitor visitField(int access, String name, String desc,
String signature, Object value) {
System.out.println("visitField:"+name);
return super.visitField(access, name, desc, signature, value);
}
@Override
public MethodVisitor visitMethod(int access, String name, String desc,
String signature, String[] exceptions) {
System.out.println("visitMethod:"+name);
return super.visitMethod(access, name, desc, signature, exceptions);
}
public MyOwnClassVisitor(int api) {
super(api);
}
public static void main(String[] args) throws IOException {
MyOwnClassVisitor visitor = new MyOwnClassVisitor(
SpringAsmInfo.ASM_VERSION);
//创建reader
ClassReader reader=new ClassReader("com.zzx.study.spring.dao.UserDao");
// 调用accept解析字节码，第一个参数传用户具体的访问者
reader.accept(visitor, ClassReader.SKIP_DEBUG);
System.out.println(reader.getSuperName());
}
}