Spring中的注解大概可以分为两大类：

1）spring的bean容器相关的注解，或者说bean工厂相关的注解；

2）springmvc相关的注解。

spring的bean容器相关的注解，先后有：@Required， @Autowired, @PostConstruct, @PreDestory，还有Spring3.0开始支持的JSR-330标准javax.inject.*中的注解(@Inject, @Named, @Qualifier, @Provider, @Scope, @Singleton).

springmvc相关的注解有：@Controller, @RequestMapping, @RequestParam， @ResponseBody等等。

要理解Spring中的注解，先要理解Java中的注解。

1. Java中的注解

?Java中1.5中开始引入注解，我们最熟悉的应该是：@Override, 它的定义如下：

/**
?* Indicates that a method declaration is intended to override a
?* method declaration in a supertype. If a method is annotated with
?* this annotation type compilers are required to generate an error
?* message unless at least one of the following conditions hold:
?* The method does override or implement a method declared in a
?* supertype.
?* The method has a signature that is override-equivalent to that of
?* any public method declared in Object.
?*
?* @author? Peter von der Ahé
?* @author? Joshua Bloch
?* @jls 9.6.1.4 @Override
?* @since 1.5
?*/
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

?

从注释，我们可以看出，@Override的作用是，提示编译器，使用了@Override注解的方法必须override父类或者java.lang.Object中的一个同名方法。我们看到@Override的定义中使用到了 @Target, @Retention，它们就是所谓的“元注解”——就是定义注解的注解，或者说注解注解的注解(晕了...)。我们看下@Retention

?

/**
?* Indicates how long annotations with the annotated type are to
?* be retained.? If no Retention annotation is present on
?* an annotation type declaration, the retention policy defaults to
?* RetentionPolicy.CLASS.
?*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
? ? /**
? ? * Returns the retention policy.
? ? * @return the retention policy
? ? */
? ? RetentionPolicy value();
}

?

@Retention用于提示注解被保留多长时间，有三种取值：

?

public enum RetentionPolicy {
? ? /**
? ? * Annotations are to be discarded by the compiler.
? ? */
? ? SOURCE,
? ? /**
? ? * Annotations are to be recorded in the class file by the compiler
? ? * but need not be retained by the VM at run time.? This is the default
? ? * behavior.
? ? */
? ? CLASS,
? ? /**
? ? * Annotations are to be recorded in the class file by the compiler and
? ? * retained by the VM at run time, so they may be read reflectively.
? ? *
? ? * @see java.lang.reflect.AnnotatedElement
? ? */
? ? RUNTIME
}

RetentionPolicy.SOURCE 保留在源码级别，被编译器抛弃(@Override就是此类)； RetentionPolicy.CLASS被编译器保留在编译后的类文件级别，但是被虚拟机丢弃；

RetentionPolicy.RUNTIME保留至运行时，可以被反射读取。

再看 @Target:

?

package java.lang.annotation;

/**
?* Indicates the contexts in which an annotation type is applicable. The
?* declaration contexts and type contexts in which an annotation type may be
?* applicable are specified in JLS 9.6.4.1, and denoted in source code by enum
?* constants of java.lang.annotation.ElementType
?* @since 1.5
?* @jls 9.6.4.1 @Target
?* @jls 9.7.4 Where Annotations May Appear
?*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Target {
? ? /**
? ? * Returns an array of the kinds of elements an annotation type
? ? * can be applied to.
? ? * @return an array of the kinds of elements an annotation type
? ? * can be applied to
? ? */
? ? ElementType[] value();
}

?

?@Target用于提示该注解使用的地方，取值有：

?