JDK中枚舉的底層實現
阿新 • • 發佈：2018-10-07
ati cto 國慶 andro local 同時 父類 one constant
前提
上一篇文章復習介紹了JDK中註解的底層實現，跟註解一樣比較常用，但是底層實現比較神秘的還有枚舉類型。趁著國慶假期的最後兩天，把JDK中枚舉的底層實現也進行一次探究。
通過例子查找本質
在探究JDK註解的底層實現的時候，因為預先參考了不少資料，所以整個過程有點"未蔔先知"的意味，這裏嘗試用未知的角度去看註解的底層實現。先定義一個手機操作系統類型枚舉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; } }
這是一個很簡單的枚舉，接著使用JDK的反編譯工具反編譯出其字節碼，執行下面的命令：
javap -c -v D:\Projects\rxjava-seed\target\classes\club\throwable\enumeration\PhoneOsEnum.class然後就得到了關於PhoneOsEnum.class的很長的字節碼，這裏全部貼出來：
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_ENUM Constant 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"
先看類的簽名是public final class club.throwable.enumeration.PhoneOsEnum extends java.lang.Enum<club.throwable.enumeration.PhoneOsEnum>，它的父類是java.lang.Enum，父類的泛型就是自身club.throwable.enumeration.PhoneOsEnum。上面的字節碼的可讀性相對比較低，直接翻譯為Java代碼(當然我們不能聲明一個類直接繼承java.lang.Enum，這裏僅僅為了說明反編譯後的枚舉類的原型)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}; } }
概括來說就是成員變量都是通過靜態代碼塊聲明，這裏註意一點父類Enum實例化的時候需要覆蓋父類構造器protected Enum(String name, int ordinal)，其他方法的實現都是十分簡單。
JDK的枚舉描述
國際慣例，先看一下JavaSE-8的語言規範中JLS-8.9對枚舉類型的定義和描述：
感覺有點似曾相識，總結一下重要內容有以下幾點：
- 枚舉的聲明格式是：
{ClassModifier} enum Identifier [Superinterfaces] EnumBody，ClassModifier是修飾符，Identifier是枚舉的名稱可以類比為類名，枚舉類型可以實現接口。 - 枚舉類型不能使用abstract或者final修飾，否則會產生編譯錯誤。
- 枚舉類型的直接超類是java.lang.Enum。
- 枚舉類型除了枚舉常量定義之外沒有其他實例，也就是枚舉類型不能實例化。
- 枚舉類型禁用反射操作進行實例化(這個特性就是Effetive Java中推薦使用枚舉實現單例的原因)。
枚舉的公共父類java.lang.Enum的源碼如下(已經去掉全部註釋)：
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");
}
} 大部分方法都比較簡單，值得註意的幾點是：
- 1、
valueOf方法依賴到的Class<?>#enumConstantDirectory()，這個方法首次調用完成之後，結果會緩存在Class<?>#enumConstantDirectory變量中。 - 2、Enum實現了Serializable接口，但是
readObject和readObjectNoData直接拋出了InvalidObjectException異常，註釋說到是"防止默認的反序列化"，這一點有點不明不白，既然禁用反序列化為何要實現Serializable接口，這裏可能考慮到是否實現Serializable接口應該交給開發者決定。 - 3、Enum禁用克隆。
小結
JDK中枚舉的底層實現就是使用了enum關鍵字聲明的枚舉類編譯後最終會變成public final修飾同時實現了泛型接口java.lang.Enum並且指定泛型參數為自身的普通Java類，而成員屬性和方法實現相關都是在編譯完成後就已經成型的，枚舉類型的成員變量都是通過靜態代碼塊聲明的。
(本文完 c-1-d e-20181006)
JDK中枚舉的底層實現