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这篇文章主要介绍“如何理解Java的枚举与注解”,在日常操作中,相信很多人在如何理解Java的枚举与注解问题上存在疑惑,小编查阅了各式资料,整理出简单好用的操作方法,希望对大家解答”如何理解Java的枚举与注解”的疑惑有所帮助!接下来,请跟着小编一起来学习吧!
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JDK1.5之前需要自定义枚举类;JDK 1.5 新增的 enum 关键字用于定义枚举类。若枚举只有一个成员, 则可以作为一种单例模式的实现方式
枚举类对象的属性不应允许被改动, 所以应该使用 private final 修饰。
枚举类的使用 private final 修饰的属性应该在构造器中为其赋值。
若枚举类显式的定义了带参数的构造器, 则在列出枚举值时也必须对应的传入参数。
必须在枚举类的第一行声明枚举类对象。 JDK 1.5 中可以在 switch 表达式中使用Enum定义的枚举类的对象作为表达式, case 子句可以直接使用枚举值的名字, 无需添加枚举类作为限定。
枚举类和普通类的区别:
使用 enum 定义的枚举类默认继承了 java.lang.Enum 类
枚举类的构造器只能使用 private 访问控制符
枚举类的所有实例必须在枚举类中显式列出(, 分隔 ; 结尾). 列出的实例系统会自动添加 public static final 修饰
package org.springframework.http; import org.springframework.lang.Nullable; /** * Enumeration of HTTP status codes. * *The HTTP status code series can be retrieved via {@link #series()}. * * @author Arjen Poutsma * @author Sebastien Deleuze * @author Brian Clozel * @since 3.0 * @see HttpStatus.Series * @see HTTP Status Code Registry * @see List of HTTP status codes - Wikipedia */ public enum HttpStatus { // 1xx Informational /** * {@code 100 Continue}. * @see HTTP/1.1: Semantics and Content, section 6.2.1 */ CONTINUE(100, "Continue"), /** * {@code 101 Switching Protocols}. * @see HTTP/1.1: Semantics and Content, section 6.2.2 */ SWITCHING_PROTOCOLS(101, "Switching Protocols"), /** * {@code 102 Processing}. * @see WebDAV */ PROCESSING(102, "Processing"), /** * {@code 103 Checkpoint}. * @see A proposal for supporting * resumable POST/PUT HTTP requests in HTTP/1.0 */ CHECKPOINT(103, "Checkpoint"), // 2xx Success /** * {@code 200 OK}. * @see HTTP/1.1: Semantics and Content, section 6.3.1 */ OK(200, "OK"), /** * {@code 201 Created}. * @see HTTP/1.1: Semantics and Content, section 6.3.2 */ CREATED(201, "Created"), /** * {@code 202 Accepted}. * @see HTTP/1.1: Semantics and Content, section 6.3.3 */ ACCEPTED(202, "Accepted"), /** * {@code 203 Non-Authoritative Information}. * @see HTTP/1.1: Semantics and Content, section 6.3.4 */ NON_AUTHORITATIVE_INFORMATION(203, "Non-Authoritative Information"), /** * {@code 204 No Content}. * @see HTTP/1.1: Semantics and Content, section 6.3.5 */ NO_CONTENT(204, "No Content"), /** * {@code 205 Reset Content}. * @see HTTP/1.1: Semantics and Content, section 6.3.6 */ RESET_CONTENT(205, "Reset Content"), /** * {@code 206 Partial Content}. * @see HTTP/1.1: Range Requests, section 4.1 */ PARTIAL_CONTENT(206, "Partial Content"), /** * {@code 207 Multi-Status}. * @see WebDAV */ MULTI_STATUS(207, "Multi-Status"), /** * {@code 208 Already Reported}. * @see WebDAV Binding Extensions */ ALREADY_REPORTED(208, "Already Reported"), /** * {@code 226 IM Used}. * @see Delta encoding in HTTP */ IM_USED(226, "IM Used"), // 3xx Redirection /** * {@code 300 Multiple Choices}. * @see HTTP/1.1: Semantics and Content, section 6.4.1 */ MULTIPLE_CHOICES(300, "Multiple Choices"), /** * {@code 301 Moved Permanently}. * @see HTTP/1.1: Semantics and Content, section 6.4.2 */ MOVED_PERMANENTLY(301, "Moved Permanently"), /** * {@code 302 Found}. * @see HTTP/1.1: Semantics and Content, section 6.4.3 */ FOUND(302, "Found"), /** * {@code 302 Moved Temporarily}. * @see HTTP/1.0, section 9.3 * @deprecated in favor of {@link #FOUND} which will be returned from {@code HttpStatus.valueOf(302)} */ @Deprecated MOVED_TEMPORARILY(302, "Moved Temporarily"), /** * {@code 303 See Other}. * @see HTTP/1.1: Semantics and Content, section 6.4.4 */ SEE_OTHER(303, "See Other"), /** * {@code 304 Not Modified}. * @see HTTP/1.1: Conditional Requests, section 4.1 */ NOT_MODIFIED(304, "Not Modified"), /** * {@code 305 Use Proxy}. * @see HTTP/1.1: Semantics and Content, section 6.4.5 * @deprecated due to security concerns regarding in-band configuration of a proxy */ @Deprecated USE_PROXY(305, "Use Proxy"), /** * {@code 307 Temporary Redirect}. * @see HTTP/1.1: Semantics and Content, section 6.4.7 */ TEMPORARY_REDIRECT(307, "Temporary Redirect"), /** * {@code 308 Permanent Redirect}. * @see RFC 7238 */ PERMANENT_REDIRECT(308, "Permanent Redirect"), // --- 4xx Client Error --- /** * {@code 400 Bad Request}. * @see HTTP/1.1: Semantics and Content, section 6.5.1 */ BAD_REQUEST(400, "Bad Request"), /** * {@code 401 Unauthorized}. * @see HTTP/1.1: Authentication, section 3.1 */ UNAUTHORIZED(401, "Unauthorized"), /** * {@code 402 Payment Required}. * @see HTTP/1.1: Semantics and Content, section 6.5.2 */ PAYMENT_REQUIRED(402, "Payment Required"), /** * {@code 403 Forbidden}. * @see HTTP/1.1: Semantics and Content, section 6.5.3 */ FORBIDDEN(403, "Forbidden"), /** * {@code 404 Not Found}. * @see HTTP/1.1: Semantics and Content, section 6.5.4 */ NOT_FOUND(404, "Not Found"), /** * {@code 405 Method Not Allowed}. * @see HTTP/1.1: Semantics and Content, section 6.5.5 */ METHOD_NOT_ALLOWED(405, "Method Not Allowed"), /** * {@code 406 Not Acceptable}. * @see HTTP/1.1: Semantics and Content, section 6.5.6 */ NOT_ACCEPTABLE(406, "Not Acceptable"), /** * {@code 407 Proxy Authentication Required}. * @see HTTP/1.1: Authentication, section 3.2 */ PROXY_AUTHENTICATION_REQUIRED(407, "Proxy Authentication Required"), /** * {@code 408 Request Timeout}. * @see HTTP/1.1: Semantics and Content, section 6.5.7 */ REQUEST_TIMEOUT(408, "Request Timeout"), /** * {@code 409 Conflict}. * @see HTTP/1.1: Semantics and Content, section 6.5.8 */ CONFLICT(409, "Conflict"), /** * {@code 410 Gone}. * @see * HTTP/1.1: Semantics and Content, section 6.5.9 */ GONE(410, "Gone"), /** * {@code 411 Length Required}. * @see * HTTP/1.1: Semantics and Content, section 6.5.10 */ LENGTH_REQUIRED(411, "Length Required"), /** * {@code 412 Precondition failed}. * @see * HTTP/1.1: Conditional Requests, section 4.2 */ PRECONDITION_FAILED(412, "Precondition Failed"), /** * {@code 413 Payload Too Large}. * @since 4.1 * @see * HTTP/1.1: Semantics and Content, section 6.5.11 */ PAYLOAD_TOO_LARGE(413, "Payload Too Large"), /** * {@code 413 Request Entity Too Large}. * @see HTTP/1.1, section 10.4.14 * @deprecated in favor of {@link #PAYLOAD_TOO_LARGE} which will be * returned from {@code HttpStatus.valueOf(413)} */ @Deprecated REQUEST_ENTITY_TOO_LARGE(413, "Request Entity Too Large"), /** * {@code 414 URI Too Long}. * @since 4.1 * @see * HTTP/1.1: Semantics and Content, section 6.5.12 */ URI_TOO_LONG(414, "URI Too Long"), /** * {@code 414 Request-URI Too Long}. * @see HTTP/1.1, section 10.4.15 * @deprecated in favor of {@link #URI_TOO_LONG} which will be returned from {@code HttpStatus.valueOf(414)} */ @Deprecated REQUEST_URI_TOO_LONG(414, "Request-URI Too Long"), /** * {@code 415 Unsupported Media Type}. * @see * HTTP/1.1: Semantics and Content, section 6.5.13 */ UNSUPPORTED_MEDIA_TYPE(415, "Unsupported Media Type"), /** * {@code 416 Requested Range Not Satisfiable}. * @see HTTP/1.1: Range Requests, section 4.4 */ REQUESTED_RANGE_NOT_SATISFIABLE(416, "Requested range not satisfiable"), /** * {@code 417 Expectation Failed}. * @see * HTTP/1.1: Semantics and Content, section 6.5.14 */ EXPECTATION_FAILED(417, "Expectation Failed"), /** * {@code 418 I'm a teapot}. * @see HTCPCP/1.0 */ I_AM_A_TEAPOT(418, "I'm a teapot"), /** * @deprecated See * * WebDAV Draft Changes */ @Deprecated INSUFFICIENT_SPACE_ON_RESOURCE(419, "Insufficient Space On Resource"), /** * @deprecated See * * WebDAV Draft Changes */ @Deprecated METHOD_FAILURE(420, "Method Failure"), /** * @deprecated * See * WebDAV Draft Changes */ @Deprecated DESTINATION_LOCKED(421, "Destination Locked"), /** * {@code 422 Unprocessable Entity}. * @see WebDAV */ UNPROCESSABLE_ENTITY(422, "Unprocessable Entity"), /** * {@code 423 Locked}. * @see WebDAV */ LOCKED(423, "Locked"), /** * {@code 424 Failed Dependency}. * @see WebDAV */ FAILED_DEPENDENCY(424, "Failed Dependency"), /** * {@code 425 Too Early}. * @since 5.2 * @see RFC 8470 */ TOO_EARLY(425, "Too Early"), /** * {@code 426 Upgrade Required}. * @see Upgrading to TLS Within HTTP/1.1 */ UPGRADE_REQUIRED(426, "Upgrade Required"), /** * {@code 428 Precondition Required}. * @see Additional HTTP Status Codes */ PRECONDITION_REQUIRED(428, "Precondition Required"), /** * {@code 429 Too Many Requests}. * @see Additional HTTP Status Codes */ TOO_MANY_REQUESTS(429, "Too Many Requests"), /** * {@code 431 Request Header Fields Too Large}. * @see Additional HTTP Status Codes */ REQUEST_HEADER_FIELDS_TOO_LARGE(431, "Request Header Fields Too Large"), /** * {@code 451 Unavailable For Legal Reasons}. * @see * An HTTP Status Code to Report Legal Obstacles * @since 4.3 */ UNAVAILABLE_FOR_LEGAL_REASONS(451, "Unavailable For Legal Reasons"), // --- 5xx Server Error --- /** * {@code 500 Internal Server Error}. * @see HTTP/1.1: Semantics and Content, section 6.6.1 */ INTERNAL_SERVER_ERROR(500, "Internal Server Error"), /** * {@code 501 Not Implemented}. * @see HTTP/1.1: Semantics and Content, section 6.6.2 */ NOT_IMPLEMENTED(501, "Not Implemented"), /** * {@code 502 Bad Gateway}. * @see HTTP/1.1: Semantics and Content, section 6.6.3 */ BAD_GATEWAY(502, "Bad Gateway"), /** * {@code 503 Service Unavailable}. * @see HTTP/1.1: Semantics and Content, section 6.6.4 */ SERVICE_UNAVAILABLE(503, "Service Unavailable"), /** * {@code 504 Gateway Timeout}. * @see HTTP/1.1: Semantics and Content, section 6.6.5 */ GATEWAY_TIMEOUT(504, "Gateway Timeout"), /** * {@code 505 HTTP Version Not Supported}. * @see HTTP/1.1: Semantics and Content, section 6.6.6 */ HTTP_VERSION_NOT_SUPPORTED(505, "HTTP Version not supported"), /** * {@code 506 Variant Also Negotiates} * @see Transparent Content Negotiation */ VARIANT_ALSO_NEGOTIATES(506, "Variant Also Negotiates"), /** * {@code 507 Insufficient Storage} * @see WebDAV */ INSUFFICIENT_STORAGE(507, "Insufficient Storage"), /** * {@code 508 Loop Detected} * @see WebDAV Binding Extensions */ LOOP_DETECTED(508, "Loop Detected"), /** * {@code 509 Bandwidth Limit Exceeded} */ BANDWIDTH_LIMIT_EXCEEDED(509, "Bandwidth Limit Exceeded"), /** * {@code 510 Not Extended} * @see HTTP Extension Framework */ NOT_EXTENDED(510, "Not Extended"), /** * {@code 511 Network Authentication Required}. * @see Additional HTTP Status Codes */ NETWORK_AUTHENTICATION_REQUIRED(511, "Network Authentication Required"); private final int value; private final String reasonPhrase; HttpStatus(int value, String reasonPhrase) { this.value = value; this.reasonPhrase = reasonPhrase; } /** * Return the integer value of this status code. */ public int value() { return this.value; } /** * Return the reason phrase of this status code. */ public String getReasonPhrase() { return this.reasonPhrase; } /** * Return the HTTP status series of this status code. * @see HttpStatus.Series */ public Series series() { return Series.valueOf(this); } /** * Whether this status code is in the HTTP series * {@link org.springframework.http.HttpStatus.Series#INFORMATIONAL}. * This is a shortcut for checking the value of {@link #series()}. * @since 4.0 * @see #series() */ public boolean is1xxInformational() { return (series() == Series.INFORMATIONAL); } /** * Whether this status code is in the HTTP series * {@link org.springframework.http.HttpStatus.Series#SUCCESSFUL}. * This is a shortcut for checking the value of {@link #series()}. * @since 4.0 * @see #series() */ public boolean is2xxSuccessful() { return (series() == Series.SUCCESSFUL); } /** * Whether this status code is in the HTTP series * {@link org.springframework.http.HttpStatus.Series#REDIRECTION}. * This is a shortcut for checking the value of {@link #series()}. * @since 4.0 * @see #series() */ public boolean is3xxRedirection() { return (series() == Series.REDIRECTION); } /** * Whether this status code is in the HTTP series * {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR}. * This is a shortcut for checking the value of {@link #series()}. * @since 4.0 * @see #series() */ public boolean is4xxClientError() { return (series() == Series.CLIENT_ERROR); } /** * Whether this status code is in the HTTP series * {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}. * This is a shortcut for checking the value of {@link #series()}. * @since 4.0 * @see #series() */ public boolean is5xxServerError() { return (series() == Series.SERVER_ERROR); } /** * Whether this status code is in the HTTP series * {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR} or * {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}. * This is a shortcut for checking the value of {@link #series()}. * @since 5.0 * @see #is4xxClientError() * @see #is5xxServerError() */ public boolean isError() { return (is4xxClientError() || is5xxServerError()); } /** * Return a string representation of this status code. */ @Override public String toString() { return this.value + " " + name(); } /** * Return the enum constant of this type with the specified numeric value. * @param statusCode the numeric value of the enum to be returned * @return the enum constant with the specified numeric value * @throws IllegalArgumentException if this enum has no constant for the specified numeric value */ public static HttpStatus valueOf(int statusCode) { HttpStatus status = resolve(statusCode); if (status == null) { throw new IllegalArgumentException("No matching constant for [" + statusCode + "]"); } return status; } /** * Resolve the given status code to an {@code HttpStatus}, if possible. * @param statusCode the HTTP status code (potentially non-standard) * @return the corresponding {@code HttpStatus}, or {@code null} if not found * @since 5.0 */ @Nullable public static HttpStatus resolve(int statusCode) { for (HttpStatus status : values()) { if (status.value == statusCode) { return status; } } return null; } /** * Enumeration of HTTP status series. *
Retrievable via {@link HttpStatus#series()}. */ public enum Series { INFORMATIONAL(1), SUCCESSFUL(2), REDIRECTION(3), CLIENT_ERROR(4), SERVER_ERROR(5); private final int value; Series(int value) { this.value = value; } /** * Return the integer value of this status series. Ranges from 1 to 5. */ public int value() { return this.value; } /** * Return the enum constant of this type with the corresponding series. * @param status a standard HTTP status enum value * @return the enum constant of this type with the corresponding series * @throws IllegalArgumentException if this enum has no corresponding constant */ public static Series valueOf(HttpStatus status) { return valueOf(status.value); } /** * Return the enum constant of this type with the corresponding series. * @param statusCode the HTTP status code (potentially non-standard) * @return the enum constant of this type with the corresponding series * @throws IllegalArgumentException if this enum has no corresponding constant */ public static Series valueOf(int statusCode) { Series series = resolve(statusCode); if (series == null) { throw new IllegalArgumentException("No matching constant for [" + statusCode + "]"); } return series; } /** * Resolve the given status code to an {@code HttpStatus.Series}, if possible. * @param statusCode the HTTP status code (potentially non-standard) * @return the corresponding {@code Series}, or {@code null} if not found * @since 5.1.3 */ @Nullable public static Series resolve(int statusCode) { int seriesCode = statusCode / 100; for (Series series : values()) { if (series.value == seriesCode) { return series; } } return null; } } }
枚举类的主要方法:
values()方法:返回枚举类型的对象数组。该方法可以很方便地遍历所有的枚举值。
valueOf(String str):可以把一个字符串转为对应的枚举类对象。要求字符串必须是枚举类对象的“名字”。如不是,会有运行时异常。
和普通 Java 类一样,枚举类可以实现一个或多个接口。若需要每个枚举值在调用实现的接口方法呈现出不同的行为方式, 则可以让每个枚举值分别来实现该方法。
从 JDK 5.0 开始, Java 增加了对元数据(MetaData) 的支持, 也就是 Annotation(注解)。Annotation 其实就是代码里的特殊标记, 这些标记可以在编译, 类加载, 运行时被读取, 并执行相应的处理. 通过使用 Annotation, 程序员可以在不改变原有逻辑的情况下, 在源文件中嵌入一些补充信息。Annotation 可以像修饰符一样被使用, 可用于修饰包,类, 构造器, 方法, 成员变量, 参数, 局部变量的声明, 这些信息被保存在 Annotation 的 “name=value” 对中。Annotation 能被用来为程序元素(类, 方法, 成员变量等) 设置元数据。
使用 Annotation 时要在其前面增加 @ 符号, 并把该 Annotation 当成一个修饰符使用。用于修饰它支持的程序元素
三个基本的 Annotation:
@Override: 限定重写父类方法, 该注释只能用于方法
@Deprecated: 用于表示某个程序元素(类, 方法等)已过时
@SuppressWarnings: 抑制编译器警告
定义新的 Annotation 类型使用 @interface 关键字。
Annotation 的成员变量在 Annotation 定义中以无参数方法的形式来声明. 其方法名和返回值定义了该成员的名字和类型。
可以在定义 Annotation 的成员变量时为其指定初始值, 指定成员变量的初始值可使用 default 关键字
public @interface MyAnnotation{ String name() default "相逢何必曾相识"; }
没有成员定义的 Annotation 称为标记; 包含成员变量的 Annotation 称为元数据 Annotation。
JDK 5.0 在 java.lang.reflect 包下新增了 AnnotatedElement 接口, 该接口代表程序中可以接受注解的程序元素。
当一个 Annotation 类型被定义为运行时 Annotation 后, 该注释才是运行时可见, 当 class 文件被载入时保存在 class 文件中的 Annotation 才会被虚拟机读取。
程序可以调用 AnnotationElement 对象的如下方法来访问 Annotation 信息。
JDK 的元 Annotation 用于修饰其他 Annotation 定义;JDK5.0提供了专门在注解上的注解类型,分别是:
Retention
Target
Documented
Inherited
@Retention: 只能用于修饰一个 Annotation 定义, 用于指定该 Annotation 可以保留多长时间, @Rentention 包含一个 RetentionPolicy 类型的成员变量, 使用 @Rentention 时必须为该 value 成员变量指定值:
RetentionPolicy.SOURCE: 编译器直接丢弃这种策略的注释
RetentionPolicy.CLASS: 编译器将把注释记录在 class 文件中. 当运行 Java 程序时, JVM 不会保留注解。 这是默认值
RetentionPolicy.RUNTIME:编译器将把注释记录在 class 文件中. 当运行 Java 程序时, JVM 会保留注释. 程序可以通过反射获取该注释
@Target: 用于修饰 Annotation 定义, 用于指定被修饰的 Annotation 能用于修饰哪些程序元素. @Target 也包含一个名为 value 的成员变量.
@Documented: 用于指定被该元 Annotation 修饰的 Annotation 类将被 javadoc 工具提取成文档.
定义为Documented的注解必须设置Retention值为RUNTIME。
@Inherited: 被它修饰的 Annotation 将具有继承性.如果某个类使用了被 @Inherited 修饰的 Annotation, 则其子类将自动具有该注解
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