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JDK中枚举的底层实现

前提

上一篇文章复习介绍了JDK中注解的底层实现,跟注解一样比较常用,但是底层实现比较神秘的还有枚举类型。趁着国庆假期的最后两天,把JDK中枚举的底层实现也进行一次探究。

通过例子查找本质

在探究JDK注解的底层实现的时候,因为预先参考了不少资料,所以整个过程有点"未卜先知"的意味,这里尝试用未知的角度去看注解的底层实现。先定义一个手机操作系统类型枚举PhoneOsEnum:

package club.throwable.enumeration;public enum PhoneOsEnum { /** * 安卓 */ ANDROID(1, "android"), /** * ios */ IOS(2, "ios"); private final Integer type; private final String typeName; PhoneOsEnum(Integer type, String typeName) { this.type = type; this.typeName = typeName; } public Integer getType() { return type; } public String getTypeName() { return typeName; }}

这是一个很简单的枚举,接着使用JDK的反编译工具反编译出其字节码,执行下面的命令:

javap -c -v D:Projectsxjava-seedargetclassesclubhrowableenumerationPhoneOsEnum.class

然后就得到了关于PhoneOsEnum.class的很长的字节码,这里全部贴出来:

Classfile /D:/Projects/rxjava-seed/target/classes/club/throwable/enumeration/PhoneOsEnum.class Last modified 2018-10-6; size 1561 bytes MD5 checksum 6d3186042f54233219000927a2f196aa Compiled from "PhoneOsEnum.java"public final class club.throwable.enumeration.PhoneOsEnum extends java.lang.Enum<club.throwable.enumeration.PhoneOsEnum> minor version: 0 major version: 52 flags: ACC_PUBLIC, ACC_FINAL, ACC_SUPER, ACC_ENUMConstant pool: #1 = Fieldref #4.#49 // club/throwable/enumeration/PhoneOsEnum.$VALUES:[Lclub/throwable/enumeration/PhoneOsEnum; #2 = Methodref #50.#51 // "[Lclub/throwable/enumeration/PhoneOsEnum;".clone:()Ljava/lang/Object; #3 = Class #26 // "[Lclub/throwable/enumeration/PhoneOsEnum;" #4 = Class #52 // club/throwable/enumeration/PhoneOsEnum #5 = Methodref #17.#53 // java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; #6 = Methodref #17.#54 // java/lang/Enum."<init>":(Ljava/lang/String;I)V #7 = Fieldref #4.#55 // club/throwable/enumeration/PhoneOsEnum.type:Ljava/lang/Integer; #8 = Fieldref #4.#56 // club/throwable/enumeration/PhoneOsEnum.typeName:Ljava/lang/String; #9 = String #18 // ANDROID #10 = Methodref #57.#58 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer; #11 = String #59 // android #12 = Methodref #4.#60 // club/throwable/enumeration/PhoneOsEnum."<init>":(Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V #13 = Fieldref #4.#61 // club/throwable/enumeration/PhoneOsEnum.ANDROID:Lclub/throwable/enumeration/PhoneOsEnum; #14 = String #20 // IOS #15 = String #62 // ios #16 = Fieldref #4.#63 // club/throwable/enumeration/PhoneOsEnum.IOS:Lclub/throwable/enumeration/PhoneOsEnum; #17 = Class #64 // java/lang/Enum #18 = Utf8 ANDROID #19 = Utf8 Lclub/throwable/enumeration/PhoneOsEnum; #20 = Utf8 IOS #21 = Utf8 type #22 = Utf8 Ljava/lang/Integer; #23 = Utf8 typeName #24 = Utf8 Ljava/lang/String; #25 = Utf8 $VALUES #26 = Utf8 [Lclub/throwable/enumeration/PhoneOsEnum; #27 = Utf8 values #28 = Utf8 ()[Lclub/throwable/enumeration/PhoneOsEnum; #29 = Utf8 Code #30 = Utf8 LineNumberTable #31 = Utf8 valueOf #32 = Utf8 (Ljava/lang/String;)Lclub/throwable/enumeration/PhoneOsEnum; #33 = Utf8 LocalVariableTable #34 = Utf8 name #35 = Utf8 <init> #36 = Utf8 (Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V #37 = Utf8 this #38 = Utf8 Signature #39 = Utf8 (Ljava/lang/Integer;Ljava/lang/String;)V #40 = Utf8 getType #41 = Utf8 ()Ljava/lang/Integer; #42 = Utf8 getTypeName #43 = Utf8 ()Ljava/lang/String; #44 = Utf8 <clinit> #45 = Utf8 ()V #46 = Utf8 Ljava/lang/Enum<Lclub/throwable/enumeration/PhoneOsEnum;>; #47 = Utf8 SourceFile #48 = Utf8 PhoneOsEnum.java #49 = NameAndType #25:#26 // $VALUES:[Lclub/throwable/enumeration/PhoneOsEnum; #50 = Class #26 // "[Lclub/throwable/enumeration/PhoneOsEnum;" #51 = NameAndType #65:#66 // clone:()Ljava/lang/Object; #52 = Utf8 club/throwable/enumeration/PhoneOsEnum #53 = NameAndType #31:#67 // valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; #54 = NameAndType #35:#68 // "<init>":(Ljava/lang/String;I)V #55 = NameAndType #21:#22 // type:Ljava/lang/Integer; #56 = NameAndType #23:#24 // typeName:Ljava/lang/String; #57 = Class #69 // java/lang/Integer #58 = NameAndType #31:#70 // valueOf:(I)Ljava/lang/Integer; #59 = Utf8 android #60 = NameAndType #35:#36 // "<init>":(Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V #61 = NameAndType #18:#19 // ANDROID:Lclub/throwable/enumeration/PhoneOsEnum; #62 = Utf8 ios #63 = NameAndType #20:#19 // IOS:Lclub/throwable/enumeration/PhoneOsEnum; #64 = Utf8 java/lang/Enum #65 = Utf8 clone #66 = Utf8 ()Ljava/lang/Object; #67 = Utf8 (Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; #68 = Utf8 (Ljava/lang/String;I)V #69 = Utf8 java/lang/Integer #70 = Utf8 (I)Ljava/lang/Integer;{ public static final club.throwable.enumeration.PhoneOsEnum ANDROID; descriptor: Lclub/throwable/enumeration/PhoneOsEnum; flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM public static final club.throwable.enumeration.PhoneOsEnum IOS; descriptor: Lclub/throwable/enumeration/PhoneOsEnum; flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL, ACC_ENUM public static club.throwable.enumeration.PhoneOsEnum[] values(); descriptor: ()[Lclub/throwable/enumeration/PhoneOsEnum; flags: ACC_PUBLIC, ACC_STATIC Code: stack=1, locals=0, args_size=0 0: getstatic #1 // Field $VALUES:[Lclub/throwable/enumeration/PhoneOsEnum; 3: invokevirtual #2 // Method "[Lclub/throwable/enumeration/PhoneOsEnum;".clone:()Ljava/lang/Object; 6: checkcast #3 // class "[Lclub/throwable/enumeration/PhoneOsEnum;" 9: areturn LineNumberTable: line 9: 0 public static club.throwable.enumeration.PhoneOsEnum valueOf(java.lang.String); descriptor: (Ljava/lang/String;)Lclub/throwable/enumeration/PhoneOsEnum; flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=1, args_size=1 0: ldc #4 // class club/throwable/enumeration/PhoneOsEnum 2: aload_0 3: invokestatic #5 // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum; 6: checkcast #4 // class club/throwable/enumeration/PhoneOsEnum 9: areturn LineNumberTable: line 9: 0 LocalVariableTable: Start Length Slot Name Signature 0 10 0 name Ljava/lang/String; public java.lang.Integer getType(); descriptor: ()Ljava/lang/Integer; flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #7 // Field type:Ljava/lang/Integer; 4: areturn LineNumberTable: line 31: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lclub/throwable/enumeration/PhoneOsEnum; public java.lang.String getTypeName(); descriptor: ()Ljava/lang/String; flags: ACC_PUBLIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #8 // Field typeName:Ljava/lang/String; 4: areturn LineNumberTable: line 35: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this Lclub/throwable/enumeration/PhoneOsEnum; static {}; descriptor: ()V flags: ACC_STATIC Code: stack=6, locals=0, args_size=0 0: new #4 // class club/throwable/enumeration/PhoneOsEnum 3: dup 4: ldc #9 // String ANDROID 6: iconst_0 7: iconst_1 8: invokestatic #10 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 11: ldc #11 // String android 13: invokespecial #12 // Method "<init>":(Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V 16: putstatic #13 // Field ANDROID:Lclub/throwable/enumeration/PhoneOsEnum; 19: new #4 // class club/throwable/enumeration/PhoneOsEnum 22: dup 23: ldc #14 // String IOS 25: iconst_1 26: iconst_2 27: invokestatic #10 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 30: ldc #15 // String ios 32: invokespecial #12 // Method "<init>":(Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V 35: putstatic #16 // Field IOS:Lclub/throwable/enumeration/PhoneOsEnum; 38: iconst_2 39: anewarray #4 // class club/throwable/enumeration/PhoneOsEnum 42: dup 43: iconst_0 44: getstatic #13 // Field ANDROID:Lclub/throwable/enumeration/PhoneOsEnum; 47: aastore 48: dup 49: iconst_1 50: getstatic #16 // Field IOS:Lclub/throwable/enumeration/PhoneOsEnum; 53: aastore 54: putstatic #1 // Field $VALUES:[Lclub/throwable/enumeration/PhoneOsEnum; 57: return LineNumberTable: line 14: 0 line 19: 19 line 9: 38}Signature: #46 // Ljava/lang/Enum<Lclub/throwable/enumeration/PhoneOsEnum;>;SourceFile: "PhoneOsEnum.java"

先看类的签名是public final class club.throwable.enumeration.PhoneOsEnum extends java.lang.Enum<club.throwable.enumeration.PhoneOsEnum>,它的父类是java.lang.Enum,父类的泛型就是自身club.throwable.enumeration.PhoneOsEnum。上面的字节码的可读性相对比较低,直接翻译为Java代码(当然我们不能声明一个类直接继承java.lang.Enum,这里仅仅为了说明反编译后的枚举类的原型)如下:

public final class PhoneOsEnumeration extends Enum<PhoneOsEnumeration> { public PhoneOsEnumeration(String name, int ordinal, Integer type, String typeName) { super(name, ordinal); this.type = type; this.typeName = typeName; } public Integer getType() { return type; } public String getTypeName() { return typeName; } public static PhoneOsEnumeration[] values() { return $VALUES.clone(); } public static PhoneOsEnumeration valueOf(String name) { return Enum.valueOf(PhoneOsEnumeration.class, name); } private final Integer type; private final String typeName; private static final PhoneOsEnumeration ANDROID; private static final PhoneOsEnumeration IOS; private static final PhoneOsEnumeration[] $VALUES; static { ANDROID = new PhoneOsEnumeration("ANDROID", 0, 1, "android"); IOS = new PhoneOsEnumeration("IOS", 1, 2, "ios"); $VALUES = new PhoneOsEnumeration[]{ANDROID, IOS}; }}

概括来说就是成员变量都是通过静态代码块声明,这里注意一点父类Enum实例化的时候需要覆盖父类构造器protected Enum(String name, int ordinal),其他方法的实现都是十分简单。

JDK的枚举描述

国际惯例,先看一下JavaSE-8的语言规范中JLS-8.9对枚举类型的定义和描述:

感觉有点似曾相识,总结一下重要内容有以下几点:

枚举的声明格式是:{ClassModifier} enum Identifier [Superinterfaces] EnumBody,ClassModifier是修饰符,Identifier是枚举的名称可以类比为类名,枚举类型可以实现接口。枚举类型不能使用abstract或者final修饰,否则会产生编译错误。枚举类型的直接超类是java.lang.Enum。枚举类型除了枚举常量定义之外没有其他实例,也就是枚举类型不能实例化。枚举类型禁用反射操作进行实例化(这个特性就是Effetive Java中推荐使用枚举实现单例的原因)。

枚举的公共父类java.lang.Enum的源码如下(已经去掉全部注释):

public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable { private final String name; public final String name() { return name; } private final int ordinal; public final int ordinal() { return ordinal; } protected Enum(String name, int ordinal) { this.name = name; this.ordinal = ordinal; } public String toString() { return name; } public final boolean equals(Object other) { return this==other; } public final int hashCode() { return super.hashCode(); } protected final Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException(); } public final int compareTo(E o) { Enum<?> other = (Enum<?>)o; Enum<E> self = this; if (self.getClass() != other.getClass() && // optimization self.getDeclaringClass() != other.getDeclaringClass()) throw new ClassCastException(); return self.ordinal - other.ordinal; } public final Class<E> getDeclaringClass() { Class<?> clazz = getClass(); Class<?> zuper = clazz.getSuperclass(); return (zuper == Enum.class) ? (Class<E>)clazz : (Class<E>)zuper; } public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name) { T result = enumType.enumConstantDirectory().get(name); if (result != null) return result; if (name == null) throw new NullPointerException("Name is null"); throw new IllegalArgumentException( "No enum constant " + enumType.getCanonicalName() + "." + name); } protected final void finalize() { } private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { throw new InvalidObjectException("can"t deserialize enum"); } private void readObjectNoData() throws ObjectStreamException { throw new InvalidObjectException("can"t deserialize enum"); } }

大部分方法都比较简单,值得注意的几点是:

1、valueOf方法依赖到的Class<?>#enumConstantDirectory(),这个方法首次调用完成之后,结果会缓存在Class<?>#enumConstantDirectory变量中。2、Enum实现了Serializable接口,但是readObjectreadObjectNoData直接抛出了InvalidObjectException异常,注释说到是"防止默认的反序列化",这一点有点不明不白,既然禁用反序列化为何要实现Serializable接口,这里可能考虑到是否实现Serializable接口应该交给开发者决定。3、Enum禁用克隆。

小结

JDK中枚举的底层实现就是使用了enum关键字声明的枚举类编译后最终会变成public final修饰同时实现了泛型接口java.lang.Enum并且指定泛型参数为自身的普通Java类,而成员属性和方法实现相关都是在编译完成后就已经成型的,枚举类型的成员变量都是通过静态代码块声明的。

(本文完 c-1-d e-20181006)

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