JDK中列舉的底層實現

摘要： 前提 上一篇文章複習介紹了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祕的還有列舉型別。趁著國慶假期的最後兩天，把JDK中列舉的底層實現也進行一次探究。 通過例子查詢本質 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"未卜先知...

前提

上一篇文章複習介紹了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祕的還有列舉型別。趁著國慶假期的最後兩天，把JDK中列舉的底層實現也進行一次探究。

通過例子查詢本質

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"未卜先知"的意味，這裡嘗試用未知的角度去看註解的底層實現。先定義一個手機作業系統型別列舉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;
}
}

這是一個很簡單的列舉，接著使用JDK的反編譯工具反編譯出其位元組碼，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的很長的位元組碼，這裡全部貼出來：

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.$VALUES:[Lclub/throwable/enumeration/PhoneOsEnum;
#2 = Methodref#50.#51// "[Lclub/throwable/enumeration/PhoneOsEnum;".clone:()Ljava/lang/Object;
#3 = Class#26// "[Lclub/throwable/enumeration/PhoneOsEnum;"
#4 = Class#52// club/throwable/enumeration/PhoneOsEnum
#5 = Methodref#17.#53// java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
#6 = Methodref#17.#54// java/lang/Enum."<init>":(Ljava/lang/String;I)V
#7 = Fieldref#4.#55// club/throwable/enumeration/PhoneOsEnum.type:Ljava/lang/Integer;
#8 = Fieldref#4.#56// club/throwable/enumeration/PhoneOsEnum.typeName:Ljava/lang/String;
#9 = String#18// ANDROID
#10 = Methodref#57.#58// java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#11 = String#59// android
#12 = Methodref#4.#60// club/throwable/enumeration/PhoneOsEnum."<init>":(Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V
#13 = Fieldref#4.#61// club/throwable/enumeration/PhoneOsEnum.ANDROID:Lclub/throwable/enumeration/PhoneOsEnum;
#14 = String#20// IOS
#15 = String#62// ios
#16 = Fieldref#4.#63// club/throwable/enumeration/PhoneOsEnum.IOS:Lclub/throwable/enumeration/PhoneOsEnum;
#17 = Class#64// java/lang/Enum
#18 = Utf8ANDROID
#19 = Utf8Lclub/throwable/enumeration/PhoneOsEnum;
#20 = Utf8IOS
#21 = Utf8type
#22 = Utf8Ljava/lang/Integer;
#23 = Utf8typeName
#24 = Utf8Ljava/lang/String;
#25 = Utf8$VALUES
#26 = Utf8[Lclub/throwable/enumeration/PhoneOsEnum;
#27 = Utf8values
#28 = Utf8()[Lclub/throwable/enumeration/PhoneOsEnum;
#29 = Utf8Code
#30 = Utf8LineNumberTable
#31 = Utf8valueOf
#32 = Utf8(Ljava/lang/String;)Lclub/throwable/enumeration/PhoneOsEnum;
#33 = Utf8LocalVariableTable
#34 = Utf8name
#35 = Utf8<init>
#36 = Utf8(Ljava/lang/String;ILjava/lang/Integer;Ljava/lang/String;)V
#37 = Utf8this
#38 = Utf8Signature
#39 = Utf8(Ljava/lang/Integer;Ljava/lang/String;)V
#40 = Utf8getType
#41 = Utf8()Ljava/lang/Integer;
#42 = Utf8getTypeName
#43 = Utf8()Ljava/lang/String;
#44 = Utf8<clinit>
#45 = Utf8()V
#46 = Utf8Ljava/lang/Enum<Lclub/throwable/enumeration/PhoneOsEnum;>;
#47 = Utf8SourceFile
#48 = Utf8PhoneOsEnum.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 = Utf8club/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 = Utf8android
#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 = Utf8ios
#63 = NameAndType#20:#19// IOS:Lclub/throwable/enumeration/PhoneOsEnum;
#64 = Utf8java/lang/Enum
#65 = Utf8clone
#66 = Utf8()Ljava/lang/Object;
#67 = Utf8(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
#68 = Utf8(Ljava/lang/String;I)V
#69 = Utf8java/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:
StartLengthSlotNameSignature
0100nameLjava/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:
StartLengthSlotNameSignature
050thisLclub/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:
StartLengthSlotNameSignature
050thisLclub/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。上面的位元組碼的可讀性相對比較低，直接翻譯為Java程式碼(當然我們不能宣告一個類直接繼承java.lang.Enum，這裡僅僅為了說明反編譯後的列舉類的原型)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數都是通過靜態程式碼塊宣告，這裡注意一點父類Enum例項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的列舉描述

國際慣例，先看一下JavaSE-8的語言規範中 ofollow,noindex" target="_blank">JLS-8.9 對列舉型別的定義和描述：

感覺有點似曾相識，總結一下重要內容有以下幾點：

{ClassModifier} enum Identifier [Superinterfaces] EnumBody

列舉的公共父類java.lang.Enum的原始碼如下(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，值得注意的幾點是：

• 1、 valueOf 方法依賴到的 Class<?>#enumConstantDirectory() ，這個方法首次呼叫完成之後，結果會快取在 Class<?>#enumConstantDirectory 變數中。
• 2、Enum實現了Serializable介面，但是 readObject 和 readObjectNoData 直接丟擲了InvalidObjectException異常，註釋說到是"防止預設的反序列化"，這一點有點不明不白，既然禁用反序列化為何要實現Serializable介面，這裡可能考慮到是否實現Serializable介面應該交給開發者決定。
• 3、Enum禁用克隆。

小結

JDK中列舉的底層實現就是使用了enum關鍵字宣告的列舉類編譯後最終會變成public final修飾同時實現了泛型介面java.lang.Enum並且指定泛型引數為自身的普通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，列舉型別的成員變數都是通過靜態程式碼塊宣告的。

(本文完 c-1-d e-20181006)