深入理解Spring中的各种注解

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:

public enum ElementType {
    /** Class, interface (including annotation type), or enum declaration */
    TYPE,
    /** Field declaration (includes enum constants) */
    FIELD,
    /** Method declaration */
    METHOD,
    /** Formal parameter declaration */
    PARAMETER,
    /** Constructor declaration */
    CONSTRUCTOR,
    /** Local variable declaration */
    LOCAL_VARIABLE,
    /** Annotation type declaration */
    ANNOTATION_TYPE,
    /** Package declaration */
    PACKAGE,
    /**
    * Type parameter declaration
    * @since 1.8
    */
    TYPE_PARAMETER,
    /**
    * Use of a type
    * @since 1.8
    */
    TYPE_USE
}

分别表示该注解可以被使用的地方:1)类,接口,注解,enum; 2)属性域;3)方法;4)参数;5)构造函数;6)局部变量;7)注解类型;8)包

所以:

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.SOURCE)
public @interface Override {
}

表示 @Override 只能使用在方法上,保留在源码级别,被编译器处理,然后抛弃掉。

还有一个经常使用的元注解 @Documented :

内容版权声明:除非注明,否则皆为本站原创文章。

转载注明出处:https://www.heiqu.com/77b8c6da492095cbd674ad43a37ad80e.html