Java复习笔记：注解

文章发布时间:

2021-05-13

最后更新时间:

2023-03-26

Java 内置注解

`@Override`

定义在：java.lang.Override 中

只能用于修辞方法，表示一个方法声明打算重写超类中的另一个方法。

`@Deprecated`

定义在：java.lang.Deprecated 中

可以用于修辞方法、属性、类，表示不推荐程序员使用的元素，通常意味着它存在危险或有更好的选择

`@SuppressWarnings`

定义在：java.lang.SuppressWarnings 中

用于抑制编译时的警告信息

@SuppressWarnings("all"): 抑制所有类型的警告
@SuppressWarnings(value={"unchecked", "rawtypes"}): 抑制所多种类型的警告
@SuppressWarnings("unchecked"): 抑制unchecked类型的警告

参数：

all: to suppress all warnings
boxing : to suppress warnings relative to boxing/unboxing operations
cast: to suppress warnings relative to cast operations
dep-ann: to suppress warnings relative to deprecated annotation
deprecation: to suppress warnings relative to deprecation
fallthrough: to suppress warnings relative to missing breaks in switch statements
finally : to suppress warnings relative to finally block that don’t return
hiding: to suppress warnings relative to locals that hide variable
incomplete-switch: to suppress warnings relative to missing entries in a switch statement (enum case)
nls: to suppress warnings relative to non-nls string literals
null: to suppress warnings relative to null analysis
rawtypes: to suppress warnings relative to un-specific types when using generics on class params
restriction: to suppress warnings relative to usage of discouraged or forbidden references
serial: to suppress warnings relative to missing serialVersionUID field for a serializable class
static-access: o suppress warnings relative to incorrect static access
synthetic-access : to suppress warnings relative to unoptimized access from inner classes
unchecked: to suppress warnings relative to unchecked operations
unqualified-field-access: to suppress warnings relative to field access unqualified
unused: to suppress warnings relative to unused code

元注解(meta-annotation)

用于注解其它注解的注解

1. `@Target`

用于描述注解的使用范围

example:

1	`@Target(value = ElementType.TYPE)`

可用{}来给value传入数组

1	`@Target(value = {ElementType.TYPE, ElementType.METHOD})`

value参数值为枚举类型，定义如下：

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
}

2. `@Retention`

表示需要什么级别保存该注解信息，用于描述注解的生命周期

取值：SOURCE < CLASS < RUNTIME

SOURCE: 保留到源码
CLASS: 保留到字节码
RUNTIME: 保留到虚拟机

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
}

3. `@Document`

表示该注解将被包含在javadoc中

4. `@Inherited`

表示子类可以继承父类中的该注解

自定义注解

反射(Reflection)

反射(Reflection)是Java被视为动态语言的关键，反射机制允许程序在执行期借助于Reflection API取得任何类型的内部信息，并能直接操作任意对象的内部属性及方法。

加载完类之后，在堆内存的方法区中就产生了一个Class类型的对象（一个类只有一个Class对象），这个对象就包含了完整的类的结构信息。我们可以通过这个对象看到类的结构。

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Java复习笔记：注解

Java 内置注解

@Override

@Deprecated

@SuppressWarnings