JDK中枚舉的底層實現

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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。

通過例子尋找本質

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Crowdsourced Security Testing"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_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代碼(當然我們不能聲明一個類直接繼承java.lang.Enum，這裡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數都是通過靜態代碼塊聲明，這裡注意一點父類Enum執行個體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，枚舉類型可以實現介面。
• 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執行個體，也就是枚舉類型不能執行個體化。
• 枚舉類型禁用反射操作進行執行個體化(這個特性就是Effetive Java中推薦使用枚舉實現單例的原因)。

枚舉的公用父類java.lang.Enum的源碼如下(已經去掉全部注釋)：

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()，這個方法首次調用完成之後，結果會緩衝在Class<?>#enumConstantDirectory變數中。
• 2、Enum實現了Serializable介面，但是readObject和readObjectNoData直接拋出了InvalidObjectException異常，注釋說到是"防止預設的還原序列化"，這一點有點不明不白，既然禁用還原序列化為何要實現Serializable介面，這裡可能考慮到是否實現Serializable介面應該交給開發人員決定。
• 3、Enum禁用複製。

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介面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，枚舉類型的成員變數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