Enum.values() 从何而来？

发布于 2024-09-26 13:08:34 字数 489 浏览 7 评论 0原文

我正在查看文档和源代码，因为我想确定 value() 总是按照声明 Enum 值的顺序返回一个数组。事实证明，据我所知，它不在文档中。

我检查了 Enum 类的源代码，但没有运气（那里是一个相关的私有“getValues”方法）。

所以我猜测某些编译器/解释器-foo 已经在继续创建一个类，该类将 Enum 扩展为如下声明：

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？或者它实际上是一个在运行时调用的方法，如果是的话，它是在哪里定义的？

原文

I was looking through the documentation and source code, because I wanted to be certain that values() would always return an array in the order in which the Enum values are declared. Turns out, it's not in the documentation as far as I can tell.

I checked the source code for the Enum class, and no luck (there is a related, private "getValues" method).

So I'm guessing that some compiler/interpreter-foo is already going on to create a class that extends Enum out of a declaration like:

public static enum MyEnum

So is values() also statically translated into a hardcoded array during compilation? Or is it actually a method called at runtime, and if so, where is it defined?

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伴随着你 2024-10-03 13:08:34

values() 方法是 enum 类型定义的一部分。不要与 Enum 基类混淆。正式定义位于 JLS 第 8.9 节确实指定返回的顺序与声明它们的顺序相匹配。

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夏有森光若流苏 2024-10-03 13:08:34

正如您从下面通过反汇编枚举获得的字节码中看到的，枚举上的 values() 方法只是返回一个 private static 数组的副本，其中包含所有声明的枚举常量。该数组 ENUM$VALUES 填充在静态初始化块中。

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并标记为 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

}

As you can see from the bytecode below obtained by disassemlbing an enum, the values() method on the enums simply return a copy of the private static array which contains all the declared enum constant. This array ENUM$VALUES is filled in a static initilization block.

DaysOfTheWeek.java

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

DaysOfTheWeek.java disassembled

After the static block, the bytecode marked 0-92 initializes the enum constants, the bytecode marked 94-139 puts those constants in an array and the bytecode marked 140 assigns the array to the ENUM$VALUES static field of the class. The code in the values() method simply creates a copy of the array assigned to the ENUM$VALUES field by calling System.arraycopy on it and returns the copy.

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

}

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枕梦 2024-10-03 13:08:34

我将补充 Devon 的答案，即 values() 方法是由编译器根据定义添加的。
从枚举教程：

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

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梦太阳 2024-10-03 13:08:34

您提供的链接（其中有一个私有 getValues 方法）来自 Apache Harmony (版本 6，这是一个开源 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(ClassenumType, 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，这是 Apache 实现其 valueOf 的不同之处。

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

Your link you provided (where there's a private getValues method) is from Apache Harmony (version 6, which is an open source Java SE). They have a different implementation to Oracle's Enum class (which has no private getValues method).

The Oracle java (version 1.6.0-21 at the time of writing) has the valueOf(Class<T> enumType, String name). Here's its implementation:

 /**
     * 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);
    }

In essence, it's how Apache implemented their valueOf that's different.

Enum (or enum) have a public static method called values() that returns the enumerated values constants declared inside the enum. That's populated by the compiler.

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