Enum.values() 从何而来？

Question

我正在查看文档和源代码，因为我想确定 values() 将始终按照 Enum 值的声明顺序返回一个数组。结果，据我所知，它不是in the documentation。

我检查了 Enum 类的源代码，但没有运气 (there is a related, private "getValues" method)。

所以我猜测某些编译器/解释器 foo 已经在创建一个类，该类将 Enum 从如下声明中扩展出来：

public static enum MyEnum

那么 values() 在编译期间是否也静态转换为硬编码数组？或者它实际上是在运行时调用的方法，如果是，它定义在哪里？

Answer 1

values() 方法是enum 类型定义的一部分。不要与Enum 基类混淆。正式定义在 Section 8.9 of the JLS 中，它指定返回的顺序与声明它们的顺序相匹配。

Answer 2

从下面通过反汇编枚举获得的字节码可以看出，枚举上的values() 方法只是返回包含所有声明的枚举常量的private static 数组的副本。这个数组ENUM$VALUES 填充在一个静态初始化块中。

DaysOfTheWeek.java

public enum DaysOfTheWeek {
    MONDAY,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
}

DaysOfTheWeek.java 反汇编

在静态块之后，标记为 0-92 的字节码初始化枚举常量，标记为 94-139 的字节码将这些常量放入数组中，标记为 140 的字节码将数组分配给类的 ENUM$VALUES 静态字段。 values() 方法中的代码只是通过调用System.arraycopy 来创建分配给ENUM$VALUES 字段的数组的副本并返回该副本。

Compiled from "DaysOfTheWeek.java"
public final class DaysOfTheWeek extends java.lang.Enum{
public static final DaysOfTheWeek MONDAY;

public static final DaysOfTheWeek TUESDAY;

public static final DaysOfTheWeek WEDNESDAY;

public static final DaysOfTheWeek THURSDAY;

public static final DaysOfTheWeek FRIDAY;

public static final DaysOfTheWeek SATURDAY;

public static final DaysOfTheWeek SUNDAY;

static {};
  Code:
   0:   new #1; //class DaysOfTheWeek
   3:   dup
   4:   ldc #18; //String MONDAY
   6:   iconst_0
   7:   invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   10:  putstatic   #23; //Field MONDAY:LDaysOfTheWeek;
   13:  new #1; //class DaysOfTheWeek
   16:  dup
   17:  ldc #25; //String TUESDAY
   19:  iconst_1
   20:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   23:  putstatic   #26; //Field TUESDAY:LDaysOfTheWeek;
   26:  new #1; //class DaysOfTheWeek
   29:  dup
   30:  ldc #28; //String WEDNESDAY
   32:  iconst_2
   33:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   36:  putstatic   #29; //Field WEDNESDAY:LDaysOfTheWeek;
   39:  new #1; //class DaysOfTheWeek
   42:  dup
   43:  ldc #31; //String THURSDAY
   45:  iconst_3
   46:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   49:  putstatic   #32; //Field THURSDAY:LDaysOfTheWeek;
   52:  new #1; //class DaysOfTheWeek
   55:  dup
   56:  ldc #34; //String FRIDAY
   58:  iconst_4
   59:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   62:  putstatic   #35; //Field FRIDAY:LDaysOfTheWeek;
   65:  new #1; //class DaysOfTheWeek
   68:  dup
   69:  ldc #37; //String SATURDAY
   71:  iconst_5
   72:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   75:  putstatic   #38; //Field SATURDAY:LDaysOfTheWeek;
   78:  new #1; //class DaysOfTheWeek
   81:  dup
   82:  ldc #40; //String SUNDAY
   84:  bipush  6
   86:  invokespecial   #19; //Method "<init>":(Ljava/lang/String;I)V
   89:  putstatic   #41; //Field SUNDAY:LDaysOfTheWeek;
   92:  bipush  7
   94:  anewarray   #1; //class DaysOfTheWeek
   97:  dup
   98:  iconst_0
   99:  getstatic   #23; //Field MONDAY:LDaysOfTheWeek;
   102: aastore
   103: dup
   104: iconst_1
   105: getstatic   #26; //Field TUESDAY:LDaysOfTheWeek;
   108: aastore
   109: dup
   110: iconst_2
   111: getstatic   #29; //Field WEDNESDAY:LDaysOfTheWeek;
   114: aastore
   115: dup
   116: iconst_3
   117: getstatic   #32; //Field THURSDAY:LDaysOfTheWeek;
   120: aastore
   121: dup
   122: iconst_4
   123: getstatic   #35; //Field FRIDAY:LDaysOfTheWeek;
   126: aastore
   127: dup
   128: iconst_5
   129: getstatic   #38; //Field SATURDAY:LDaysOfTheWeek;
   132: aastore
   133: dup
   134: bipush  6
   136: getstatic   #41; //Field SUNDAY:LDaysOfTheWeek;
   139: aastore
   140: putstatic   #43; //Field ENUM$VALUES:[LDaysOfTheWeek;
   143: return

public static DaysOfTheWeek[] values();
  Code:
   0:   getstatic   #43; //Field ENUM$VALUES:[LDaysOfTheWeek;
   3:   dup
   4:   astore_0
   5:   iconst_0
   6:   aload_0
   7:   arraylength
   8:   dup
   9:   istore_1
   10:  anewarray   #1; //class DaysOfTheWeek
   13:  dup
   14:  astore_2
   15:  iconst_0
   16:  iload_1
   17:  invokestatic    #51; //Method java/lang/System.arraycopy:(Ljava/lang/Object;ILjava/lang/Object;II)V
   20:  aload_2
   21:  areturn

public static DaysOfTheWeek valueOf(java.lang.String);
  Code:
   0:   ldc #1; //class DaysOfTheWeek
   2:   aload_0
   3:   invokestatic    #59; //Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
   6:   checkcast   #1; //class DaysOfTheWeek
   9:   areturn

}

Answer 3

我将在 Devon 的回答中添加 values() 方法是由编译器根据定义添加的。
来自枚举tutorial：

编译器会自动添加一些 创建时的特殊方法 枚举。例如，它们有一个静态 返回数组的 values 方法 包含所有的值 按照声明的顺序枚举。

Answer 4

您提供的链接（其中有一个私有 getValues 方法）来自 Apache Harmony（版本 6，它是一个开源 Java SE）。它们与 Oracle 的 Enum 类（没有私有 getValues 方法）有不同的实现。

Oracle java（撰写本文时版本为 1.6.0-21）具有 valueOf(Class<T> enumType, String name)。这是它的实现：

 /**
     * Returns the enum constant of the specified enum type with the
     * specified name.  The name must match exactly an identifier used
     * to declare an enum constant in this type.  (Extraneous whitespace
     * characters are not permitted.) 
     *
     * @param enumType the <tt>Class</tt> object of the enum type from which
     *      to return a constant
     * @param name the name of the constant to return
     * @return the enum constant of the specified enum type with the
     *      specified name
     * @throws IllegalArgumentException if the specified enum type has
     *         no constant with the specified name, or the specified
     *         class object does not represent an enum type
     * @throws NullPointerException if <tt>enumType</tt> or <tt>name</tt>
     *         is null
     * @since 1.5
     */
    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 const " + enumType +"." + name);
    }

本质上，Apache 实现 valueOf 的方式不同。

---

Enum（或enum）有一个名为values() 的公共静态方法，它返回enum 中声明的枚举值常量。这是由编译器填充的。