1. 注解
注解是一种特殊的注释, 是一个修饰符, 本质上不参与程序的执行.
注解的特殊在于可以被编译器和 JVM 识别, 如果想让它参与必须通过反射方式.
2. 分类
2.1. 标记型注解
标记型注解不需要传递参数.
@Override
作用 : 提醒编译器, 帮助我们作方法覆盖条件的检查
/** Copyright (c) 2003, 2013, Oracle and/or its affiliates. All rights reserved.* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.*/package java.lang;import java.lang.annotation.*;/*** 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:** <ul><li>* The method does override or implement a method declared in a* supertype.* </li><li>* The method has a signature that is override-equivalent to that of* any public method declared in {@linkplain Object}.* </li></ul>** @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 {}
@Deprecated
作用 : 提醒使用者, 它所修饰的目标过期, 不建议使用.
package java.lang;
import java.lang.annotation.*;
import static java.lang.annotation.ElementType.*;
/**
* A program element annotated @Deprecated is one that programmers
* are discouraged from using, typically because it is dangerous,
* or because a better alternative exists. Compilers warn when a
* deprecated program element is used or overridden in non-deprecated code.
*
* @author Neal Gafter
* @since 1.5
* @jls 9.6.3.6 @Deprecated
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
public @interface Deprecated {
}
@SuppressWarnings
作用 : 提醒编译器, 它修饰的目标中的编译器警告忽略掉. 必须传参
/**
* Indicates that the named compiler warnings should be suppressed in the
* annotated element (and in all program elements contained in the annotated
* element). Note that the set of warnings suppressed in a given element is
* a superset of the warnings suppressed in all containing elements. For
* example, if you annotate a class to suppress one warning and annotate a
* method to suppress another, both warnings will be suppressed in the method.
*
* <p>As a matter of style, programmers should always use this annotation
* on the most deeply nested element where it is effective. If you want to
* suppress a warning in a particular method, you should annotate that
* method rather than its class.
*
* @author Josh Bloch
* @since 1.5
* @jls 4.8 Raw Types
* @jls 4.12.2 Variables of Reference Type
* @jls 5.1.9 Unchecked Conversion
* @jls 5.5.2 Checked Casts and Unchecked Casts
* @jls 9.6.3.5 @SuppressWarnings
*/
@Target({TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
@Retention(RetentionPolicy.SOURCE)
public @interface SuppressWarnings {
/**
* The set of warnings that are to be suppressed by the compiler in the
* annotated element. Duplicate names are permitted. The second and
* successive occurrences of a name are ignored. The presence of
* unrecognized warning names is <i>not</i> an error: Compilers must
* ignore any warning names they do not recognize. They are, however,
* free to emit a warning if an annotation contains an unrecognized
* warning name.
*
* <p> The string {@code "unchecked"} is used to suppress
* unchecked warnings. Compiler vendors should document the
* additional warning names they support in conjunction with this
* annotation type. They are encouraged to cooperate to ensure
* that the same names work across multiple compilers.
* @return the set of warnings to be suppressed
*/
String[] value();
}
2.2. 元注解
注解的注解, 作用是约束注解.
@Target
作用: 是限定自定义注解可以用在什么地方.
package java.lang.annotation;
/**
* Indicates that annotations with a type are to be documented by javadoc
* and similar tools by default. This type should be used to annotate the
* declarations of types whose annotations affect the use of annotated
* elements by their clients. If a type declaration is annotated with
* Documented, its annotations become part of the public API
* of the annotated elements.
*
* @author Joshua Bloch
* @since 1.5
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}
需要传一个数组, 是一个类型数组, ElementType[]
ElementType 是一枚举, 里面有: TYPE, FIELD, METHOD, PARAMETER
@Retention
作用: 是限定自定义注解可以停留在什么时期.
package java.lang.annotation;
/**
* 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
* {@code RetentionPolicy.CLASS}.
*
* <p>A Retention meta-annotation has effect only if the
* meta-annotated type is used directly for annotation. It has no
* effect if the meta-annotated type is used as a member type in
* another annotation type.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.3.2 @Retention
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Retention {
/**
* Returns the retention policy.
* @return the retention policy
*/
RetentionPolicy value();
}
有 3 个级别: RetentionPolicy.SOURCE 源码, RetentionPolicy.CLASS 类文件中保留, RetentionPolicy.RUNTIME 运行时,可反射.
@Document
作用: 标识类型是否要被收入javadoc
package java.lang.annotation;
/**
* Indicates that annotations with a type are to be documented by javadoc
* and similar tools by default. This type should be used to annotate the
* declarations of types whose annotations affect the use of annotated
* elements by their clients. If a type declaration is annotated with
* Documented, its annotations become part of the public API
* of the annotated elements.
*
* @author Joshua Bloch
* @since 1.5
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Documented {
}
@Inherited
作用: 使用此注解声明出来的自定义注解, 在使用此自定义注解时, 如果注解在类上面时, 子类会自动继承此注解, 否则的话子类不会继承此注解. 这里一定要记住, 使用 Inherited 声明出来的注解, 只有在类上使用时才会有效, 对方法, 属性等其他无效.
**
package java.lang.annotation;
/**
* Indicates that an annotation type is automatically inherited. If
* an Inherited meta-annotation is present on an annotation type
* declaration, and the user queries the annotation type on a class
* declaration, and the class declaration has no annotation for this type,
* then the class's superclass will automatically be queried for the
* annotation type. This process will be repeated until an annotation for this
* type is found, or the top of the class hierarchy (Object)
* is reached. If no superclass has an annotation for this type, then
* the query will indicate that the class in question has no such annotation.
*
* <p>Note that this meta-annotation type has no effect if the annotated
* type is used to annotate anything other than a class. Note also
* that this meta-annotation only causes annotations to be inherited
* from superclasses; annotations on implemented interfaces have no
* effect.
*
* @author Joshua Bloch
* @since 1.5
* @jls 9.6.3.3 @Inherited
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.ANNOTATION_TYPE)
public @interface Inherited {
}
3. 自定义注解
写法像接口, 是一个标记型注解.
package core.annotationu.work;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* <p>MyAnnm 自定义注解</p>
*
* <p>包含 1 个 String 型value属性, 1 个 int 型 num 属性
* 并且此注解只能修饰类和属性, 此注解也可以通过反射处理.</p>
*
* @author Zhang Chao
* @version java_day18
* @date 2020/4/13 8:54 上午
*/
@Target({ElementType.TYPE, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnn {
String value();
int num() default 10;
}
自定义注解的使用:
@MyAnn(value = "Amos", num = 100)
class A {
@MyAnn(value = "Hello")
int n = 1;
}
Rerefences
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