枚举在Java中如何内部表示？ [英] How are enums internally represented in Java?

问题描述

Java枚举是类。它们被编译为类。

Java enums are classes. They are compiled as classes.

下面的示例将如何编译？
什么是类版本？
exact 类的代码是什么？我想要实际的Java代码。

How will the following example be compiled? What is its "class version"? What is the exact class code? I want actual Java code.

public enum Ordinals {
    FIRST("st"),
    SECOND("nd"),
    THIRD("rd");
    private String notation;
    private Ordinals(String notation) {
        this.notation = notation;
    }
    public String getNotation() {
        return notation;
    }
}

推荐答案

每个 enum 类被编译为作为 java.lang.Enum 的子类的类。每个枚举常量在该类中成为 static final 常量。然后，使用声明顺序将带有所有枚举常量的数组 $ VALUES 创建。

Each enum class is compiled as a class being a subclass of java.lang.Enum. Each enum constant becomes a static final constant within that class. Then, an array $VALUES is created with all of the enum constants, in order of declaration.

您可以使用命令 javap -p -c Ordinals （在已编译的.class文件中）反汇编代码以了解详细信息。

You can disassemble the code using the command javap -p -c Ordinals (on the compiled .class file) to find out the details.

Compiled from "Ordinals.java"
public final class Ordinals extends java.lang.Enum<Ordinals> {
  public static final Ordinals FIRST;

  public static final Ordinals SECOND;

  public static final Ordinals THIRD;

  private java.lang.String notation; // your custom field

  private static final Ordinals[] $VALUES; // all enum constants

  public static Ordinals[] values(); // every enum class has this static method
    Code:
       0: getstatic     #1                  // Field $VALUES:[LOrdinals;
       3: invokevirtual #2                  // Method "[LOrdinals;".clone:()Ljava/lang/Object;
       6: checkcast     #3                  // class "[LOrdinals;"
       9: areturn       

  public static Ordinals valueOf(java.lang.String); // every enum class has this static method
    Code:
       0: ldc_w         #4                  // class Ordinals
       3: aload_0       
       4: invokestatic  #5                  // Method java/lang/Enum.valueOf:(Ljava/lang/Class;Ljava/lang/String;)Ljava/lang/Enum;
       7: checkcast     #4                  // class Ordinals
      10: areturn       

  private Ordinals(java.lang.String);
    Code:
       0: aload_0       
       1: aload_1       
       2: iload_2       
       3: invokespecial #6                  // Method java/lang/Enum."<init>":(Ljava/lang/String;I)V
       6: aload_0       
       7: aload_3       
       8: putfield      #7                  // Field notation:Ljava/lang/String;
      11: return        

  public java.lang.String getNotation();
    Code:
       0: aload_0       
       1: getfield      #7                  // Field notation:Ljava/lang/String;
       4: areturn       

  static {}; // fills the $VALUES array and initializes the static fields corresponding to the enum constants
    Code:
       0: new           #4                  // class Ordinals
       3: dup           
       4: ldc           #8                  // String FIRST
       6: iconst_0      
       7: ldc           #9                  // String st
       9: invokespecial #10                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
      12: putstatic     #11                 // Field FIRST:LOrdinals;
      15: new           #4                  // class Ordinals
      18: dup           
      19: ldc           #12                 // String SECOND
      21: iconst_1      
      22: ldc           #13                 // String nd
      24: invokespecial #10                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
      27: putstatic     #14                 // Field SECOND:LOrdinals;
      30: new           #4                  // class Ordinals
      33: dup           
      34: ldc           #15                 // String THIRD
      36: iconst_2      
      37: ldc           #16                 // String rd
      39: invokespecial #10                 // Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
      42: putstatic     #17                 // Field THIRD:LOrdinals;
      45: iconst_3      
      46: anewarray     #4                  // class Ordinals
      49: dup           
      50: iconst_0      
      51: getstatic     #11                 // Field FIRST:LOrdinals;
      54: aastore       
      55: dup           
      56: iconst_1      
      57: getstatic     #14                 // Field SECOND:LOrdinals;
      60: aastore       
      61: dup           
      62: iconst_2      
      63: getstatic     #17                 // Field THIRD:LOrdinals;
      66: aastore       
      67: putstatic     #1                  // Field $VALUES:[LOrdinals;
      70: return        
}

这将转换为Java

public final class Ordinals extends java.lang.Enum<Ordinals> {
  public static final Ordinals FIRST;

  public static final Ordinals SECOND;

  public static final Ordinals THIRD;

  private String notation;

  private static final Ordinals[] $VALUES;

  public static Ordinals[] values() {
      return $VALUES.clone();
  }

  public static Ordinals valueOf(String name) {
      return (Ordinals) Enum.valueOf(Ordinals.class, name);
  }

  private Ordinals(String name, int ordinal, String notation) {
      super(name, ordinal);
      this.notation = notation
  }

  static {
      FIRST = new Ordinals("FIRST", 0, "st");
      SECOND = new Ordinals("SECOND", 1, "nd");
      THIRD = new Ordinals("THIRD", 2, "rd");
      Ordinals[] $VALUES = new Ordinals[3];
      $VALUES[0] = FIRST;
      $VALUES[1] = SECOND;
      $VALUES[2] = THIRD;
      Ordinals.$VALUES = $VALUES;
  }
}

类版本与此完全无关-这取决于您使用的Java编译器的版本（或在编译器上的显式设置上强制其针对较旧的Java版本进行编译）。

Class version is totally unrelated to that - it depends on the version of the Java compiler that you used (or on the explicit setting on the compiler to force it to compile for an older Java version).

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