使用值（）创建枚举常量的最终Java类数组 [英] Creating a final Java class array of enum constants with values( )

问题描述

在Java枚举类中，我想创建一个 final static 数组，其中包含 values（）的类。当我沿着以下行执行此操作时，生成的数组 null 。

  public enum Name {
 E1（stuff），E2（stuff）; 
 private static final Name [] values = Name.values（）; 
 
私人名称（stuff）{更多东西; } 
} 
  

我也尝试通过调用显式类setter方法，但是这给出了 java.lang.ExceptionInInitializerError 异常。

我理解问题是由一些浅依赖引起的上面代码中的 stuff 使用其他类，这些类本身依赖于枚举类。

tl; dr：你想要的是什么？这样做是不可能的 - 枚举类型的静态字段在所有构造函数调用完成后才会初始化。

---

请考虑这个例子：

  public enum Name {
 E1（hello），E2 ）; 
 
 private static final Name [] values = values（）; 
 
 private Name（String val）{
 System.out.println（val =+ val）; 
 dump（）; 
} 
 
 protected void dump（）{
 System.out.println（this =+ this +，values =+ values）; 
} 
} 
  

请注意， c $ c> dump 方法是它是一个编译时错误（ Java语言规范第8.9.2节）尝试从内部引用 value Name 的构造函数。使用此测试工具：

  public class Main {
 public static void main（String ... args）throws Exception {
 System.out.println（Name.values（））; 
} 
} 
  

我们获得

  $ java Main 
 val = hello 
 this = E1，values = null 
 val = world 
 this = E2 ，values = null 
 [LName; @ 35960f05 
  

---

使用 javap 解析 Name 类，我们看到以下内容：

  private static final Name [] $ VALUES; 
 
 public static Name [] values（）; 
代码：
 0：getstatic＃1; // Field $ VALUES：[LName; 
 3：invokevirtual＃2; //方法[LName;。clone :(）Ljava / lang / Object; 
 6：checkcast＃3; // class[LName; 
 9：areturn 
  

编译器创建一个私有字段 $ VALUES 方法实现为 {return（Name []）$ VALUES。 clone（）} 。那么如何 $ VALUES 得到初始化？

  static {}; 
代码：
 0：new＃4; // class Name 
 3：dup 
 4：ldc＃19; // String E1 
 6：iconst_0 
 7：ldc＃20; // String hello 
 9：invokespecial＃21; //方法< init>:( Ljava / lang / String; ILjava / lang / String;）V 
 12：putstatic＃ //字段E1：LName; 
 15：new＃4; //类名
 18：dup 
 19：ldc＃23; // String E2 
 21：iconst_1 
 22：ldc＃24; // String world 
 24：invokespecial＃21; //方法< init>:( Ljava / lang / String; ILjava / lang / String;）V 
 27：putstatic＃ //字段E2：LName; 
 30：iconst_2 
 31：anewarray＃4; //类名
 34：dup 
 35：iconst_0 
 36：getstatic＃22; //字段E1：LName; 
 39：aastore 
 40：dup 
 41：iconst_1 
 42：getstatic＃25; //字段E2：LName; 
 45：aastore 
 46：putstatic＃1; // Field $ VALUES：[LName; 
 49：invokestatic＃26; //方法值：（）[LName; 
 52：putstatic＃18; //字段值：[LName; 
 55：return 
 
} 
  

初始化基本上是：

  //编译器生成的初始化代码
 E1 = new Name（hello） ; 
 E2 = new Name（world）; 
 $ VALUES = new Name [] {E1，E2}; 
 
 //值字段的静态初始值设定项
 values = Name.values（）; 
  

因此在执行构造函数调用期间， values 字段将为null，并且 values（）方法将抛出NullPointerException（将被包装在ExceptionInInitializerError中）。

Inside a Java enumerated class, I'd like to create a final static array containing the values() of the class. When I do this along the following lines, the resulting array is null.

public enum Name {
    E1( stuff ), E2( stuff );
    private static final Name[] values = Name.values();

    private Name( stuff ) { more stuff; }
}

I've also tried doing this by calling an explicit class setter method, but this gave an java.lang.ExceptionInInitializerError exception.

I understand the problem is caused by some shallow dependencies as the stuff in the previous code uses other classes, which themselves depend on the enumerated class.

Is there a tested and proven technique to achieve what I need?

解决方案

tl;dr: what you're trying to do isn't possible - static fields of an enum type don't get initialized until after all the constructor calls have completed.

---

Consider this example:

public enum Name {
  E1("hello"), E2("world");

  private static final Name[] values = values();

  private Name(String val) {
    System.out.println("val = " + val);
    dump();
  }

  protected void dump() {
    System.out.println("this = " + this + ", values = " + values);
  }
}

Note that the reason for the existence of the dump method is that it is a compile-time error (Java Language Spec section 8.9.2) to try and reference the value field from inside the constructor of Name. With this test harness:

public class Main {
  public static void main(String... args) throws Exception {
    System.out.println(Name.values());
  }
}

we get

$ java Main
val = hello
this = E1, values = null
val = world
this = E2, values = null
[LName;@35960f05

---

Decompiling the Name class with javap we see the following:

private static final Name[] $VALUES;

public static Name[] values();
  Code:
   0:   getstatic   #1; //Field $VALUES:[LName;
   3:   invokevirtual   #2; //Method "[LName;".clone:()Ljava/lang/Object;
   6:   checkcast   #3; //class "[LName;"
   9:   areturn

The compiler creates a private field $VALUES holding the value array, and the values() method is implemented as { return (Name[])$VALUES.clone() }. So how does $VALUES get initialized?

static {};
  Code:
   0:   new #4; //class Name
   3:   dup
   4:   ldc #19; //String E1
   6:   iconst_0
   7:   ldc #20; //String hello
   9:   invokespecial   #21; //Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
   12:  putstatic   #22; //Field E1:LName;
   15:  new #4; //class Name
   18:  dup
   19:  ldc #23; //String E2
   21:  iconst_1
   22:  ldc #24; //String world
   24:  invokespecial   #21; //Method "<init>":(Ljava/lang/String;ILjava/lang/String;)V
   27:  putstatic   #25; //Field E2:LName;
   30:  iconst_2
   31:  anewarray   #4; //class Name
   34:  dup
   35:  iconst_0
   36:  getstatic   #22; //Field E1:LName;
   39:  aastore
   40:  dup
   41:  iconst_1
   42:  getstatic   #25; //Field E2:LName;
   45:  aastore
   46:  putstatic   #1; //Field $VALUES:[LName;
   49:  invokestatic    #26; //Method values:()[LName;
   52:  putstatic   #18; //Field values:[LName;
   55:  return

}

What we see here is that the initialization essentially does:

// compiler-generated initialization code
E1 = new Name("hello");
E2 = new Name("world");
$VALUES = new Name[] {E1, E2};

// static initializer of the values field
values = Name.values();

so during the execution of the constructor calls, the values field will be null and the values() method will throw a NullPointerException (which will get wrapped in an ExceptionInInitializerError).

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