内容

工具类

Java语言

提取方法

static  {
    Fizz = new FizzBuzz("Fizz", 0);
    //  0: new           #2                  // class FizzBuzz
    //  3: dup
    //  4: ldc           #22                 // String Fizz
    //  6: iconst_0
    //  7: invokespecial #24                 // Method "<init>":(Ljava/lang/String;I)V
    // 10: putstatic     #25                 // Field Fizz:LFizzBuzz;
    Buzz = new FizzBuzz("Buzz", 1);
    // 13: new           #2                  // class FizzBuzz
    // 16: dup
    // 17: ldc           #28                 // String Buzz
    // 19: iconst_1
    // 20: invokespecial #24                 // Method "<init>":(Ljava/lang/String;I)V
    // 23: putstatic     #30                 // Field Buzz:LFizzBuzz;
    FizzBuzz = new FizzBuzz("FizzBuzz", 2);
    // 26: new           #2                  // class FizzBuzz
    // 29: dup
    // 30: ldc           #32                 // String FizzBuzz
    // 32: iconst_2
    // 33: invokespecial #24                 // Method "<init>":(Ljava/lang/String;I)V
    // 36: putstatic     #33                 // Field FizzBuzz:LFizzBuzz;

    $VALUES = new FizzBuzz[] {
    // 39: iconst_3
    // 40: anewarray     #2                  // class FizzBuzz
        Fizz, 
    // 43: dup
    // 44: iconst_0
    // 45: getstatic     #25                 // Field Fizz:LFizzBuzz;
    // 48: aastore
        Buzz, 
    // 49: dup
    // 50: iconst_1
    // 51: getstatic     #30                 // Field Buzz:LFizzBuzz;
    // 54: aastore
        FizzBuzz
    // 55: dup
    // 56: iconst_2
    // 57: getstatic     #33                 // Field FizzBuzz:LFizzBuzz;
    // 60: aastore
    };
    // 61: putstatic     #1                  // Field $VALUES:[LFizzBuzz;
    // 64: return
}

$VALUES = createValues();

static  {
    FizzBuzz[] localValues = createValues();

    int index = 0;
    Fizz = localValues[index++];
    Buzz = localValues[index++];
    FizzBuzz = localValues[index++];

    $VALUES = localValues;
}

private static FizzBuzz[] createValues() {
    return new FizzBuzz[] {
        new FizzBuzz("Fizz", 0), 
        new FizzBuzz("Buzz", 1), 
        new FizzBuzz("FizzBuzz", 2)
    };
}

private static int valueIndex;

static  {
    $VALUES = createValues();

    valueIndex = 0;
    Fizz = nextValue();
    Buzz = nextValue();
    FizzBuzz = nextValue();
}

private static FizzBuzz nextValue() {
    return $VALUES[valueIndex++];
}

动态类文件常量

从行为上讲，可以通过对以下参数执行其bootstrap方法来解析CONSTANT_Dynamic常量：

1. 本地查找对象，
2. 代表常量名称部分的字符串，
3. 代表期望的常量类型的Class，以及
4. 任何剩余的引导程序参数。

As with invokedynamic, multiple threads can race to resolve, but a unique winner will be chosen and any other contending answers discarded.

private FizzBuzz(MethodHandles.Lookup lookup, String name, Class<?> enumClass, int ordinal) {
    super(name, ordinal);
}

static  {
    Fizz = JVM_ldc(FizzBuzz::new, "Fizz", 0);
    Buzz = JVM_ldc(FizzBuzz::new, "Buzz", 1);
    FizzBuzz = JVM_ldc(FizzBuzz::new, "FizzBuzz", 2);

    $VALUES = createValues();
}

private static FizzBuzz[] createValues(MethodHandles.Lookup lookup, String name, Class<?> clazz, FizzBuzz... elements) {
    return elements;
}

BootstrapMethods：
  ...
  1：＃54 REF_invokeStatic FizzBu​​zz.createValues：（Ljava / lang / invoke / MethodHandles $ Lookup; Ljava / lang / String; Ljava / lang / Class; [LFizzBu​​zz;）[LFizzBu​​zz;
    方法参数：
      ＃1＃0：嘶嘶声：LFizzBu​​zz；
      ＃2＃0：嗡嗡声：LFizzBu​​zz；
      ＃3＃0：FizzBu​​zz：LFizzBu​​zz；

突如其来的困难

• COUNT类型记录CONSTANT_Dynamic-我们的动态常数
• COUNT类型记录CONSTANT_NameAndType-指向枚举元素名称及其类型的链接对。每个人的类型都是相同的，这是我们枚举的类类型。
• COUNT类型记录CONSTANT_Utf8-直接列出枚举元素的名称
• COUNT类型的记录CONSTANT_Integer-作为参数值传递给构造函数的枚举元素的序列号ordinal
• 当前和基类的名称，属性，方法签名以及其他令人厌烦的实现细节。那些感兴趣的人可以查看生成器的源代码。

int elementCount = elementNames.size();

int baseConDy = 1;
int baseNameAndType = baseConDy + elementCount;
int baseUtf8 = baseNameAndType + elementCount;
int baseInteger = baseUtf8 + elementCount;
int indexThisClass = baseInteger + elementCount;
int indexThisClassUtf8 = indexThisClass + 1;
int indexSuperClass = indexThisClassUtf8 + 1;
int indexSuperClassUtf8 = indexSuperClass + 1;
int indexBootstrapMethodsUtf8 = indexSuperClassUtf8 + 1;
int indexConDyDescriptorUtf8 = indexBootstrapMethodsUtf8 + 1;
int indexBootstrapMethodHandle = indexConDyDescriptorUtf8 + 1;
int indexBootstrapMethodRef = indexBootstrapMethodHandle + 1;
int indexBootstrapMethodNameAndType = indexBootstrapMethodRef + 1;
int indexBootstrapMethodName = indexBootstrapMethodNameAndType + 1;
int indexBootstrapMethodDescriptor = indexBootstrapMethodName + 1;

int constantPoolSize = indexBootstrapMethodDescriptor + 1;

// Class file header
u4(CLASS_FILE_SIGNATURE);
u4(version);

// Constant pool
u2(constantPoolSize);

// N * CONSTANT_Dynamic
for (int i = 0; i < elementCount; i++) {
    u1u2u2(CONSTANT_Dynamic, i, baseNameAndType + i);
}

// N * CONSTANT_NameAndType
for (int i = 0; i < elementCount; i++) {
    u1u2u2(CONSTANT_NameAndType, baseUtf8 + i, indexConDyDescriptorUtf8);
}

// N * CONSTANT_Utf8
//noinspection ForLoopReplaceableByForEach
for (int i = 0; i < elementCount; i++) {
    u1(CONSTANT_Utf8);
    utf8(elementNames.get(i));
}

// N * CONSTANT_Integer
for (int i = 0; i < elementCount; i++) {
    u1(CONSTANT_Integer);
    u4(i);
}

// ThisClass
u1(CONSTANT_Class);
u2(indexThisClassUtf8);

// ThisClassUtf8
u1(CONSTANT_Utf8);
utf8(enumClassName);

// SuperClass
u1(CONSTANT_Class);
u2(indexSuperClassUtf8);

// SuperClassUtf8
u1(CONSTANT_Utf8);
utf8(JAVA_LANG_ENUM);

// BootstrapMethodsUtf8
u1(CONSTANT_Utf8);
utf8(ATTRIBUTE_NAME_BOOTSTRAP_METHODS);

// ConDyDescriptorUtf8
u1(CONSTANT_Utf8);
utf8(binaryEnumClassName);

// BootstrapMethodHandle
u1(CONSTANT_MethodHandle);
u1(REF_newInvokeSpecial);
u2(indexBootstrapMethodRef);

// BootstrapMethodRef
u1u2u2(CONSTANT_Methodref, indexThisClass, indexBootstrapMethodNameAndType);

// BootstrapMethodNameAndType
u1u2u2(CONSTANT_NameAndType, indexBootstrapMethodName, indexBootstrapMethodDescriptor);

// BootstrapMethodName
u1(CONSTANT_Utf8);
utf8(BOOTSTRAP_METHOD_NAME);

// BootstrapMethodDescriptor
u1(CONSTANT_Utf8);
utf8(BOOTSTRAP_METHOD_DESCRIPTOR);

u2(access);
u2(indexThisClass);
u2(indexSuperClass);

// Interfaces count
u2(0);
// Fields count
u2(0);
// Methods count
u2(0);
// Attributes count
u2(1);

// BootstrapMethods attribute
u2(indexBootstrapMethodsUtf8);
// BootstrapMethods attribute size
u4(2 /* num_bootstrap_methods */ + 6 * elementCount);
// Bootstrap method count
u2(elementCount);

for (int i = 0; i < elementCount; i++) {
    // bootstrap_method_ref
    u2(indexBootstrapMethodHandle);
    // num_bootstrap_arguments
    u2(1);
    // bootstrap_arguments[1]
    u2(baseInteger + i);
}

private ClassReader getBootstrapClassReader(int version, int access, String enumClassName, List<String> elementNames) {
    byte[] bootstrapClassBytes = new ConDyBootstrapClassGenerator(
        version,
        access,
        enumClassName,
        elementNames
    )
    .generate();

    if (bootstrapClassBytes == null) {
        return null;
    } else {
        return new ClassReader(bootstrapClassBytes);
    }
}

光明的未来

public class DiscoverConstantValueAttribute {

    public static final String STRING = "Habrahabr, world!";

    public static final Object OBJECT = new Object();

}

static {
    OBJECT = new Object();
    //  0: new           #2                  // class java/lang/Object
    //  3: dup
    //  4: invokespecial #1                  // Method java/lang/Object."<init>":()V
    //  7: putstatic     #7                  // Field OBJECT:Ljava/lang/Object;
    // 10: return
}

public static final java.lang.String STRING;
  descriptor: Ljava/lang/String;
  flags: (0x0019) ACC_PUBLIC, ACC_STATIC, ACC_FINAL
  ConstantValue: String Habrahabr, world!

ConstantValue属性表示常量表达式的值（JLS§15.28），其用法如下：

• 如果在field_info结构的access_flags项中设置了ACC_STATIC标志，则将field_info结构表示的字段分配为其ConstantValue属性表示的值，作为声明该字段的类或接口的初始化的一部分（第5.5节）。这发生在调用该类或接口的类或接口初始化方法之前（第2.9.2节）。
• 否则，Java虚拟机必须静默忽略该属性。

将来的工作

可能的将来扩展包括：

...

• 将动态常量附加到静态字段的ConstantValue属性

不安全

import java.lang.reflect.Field;
import sun.misc.Unsafe;

public enum FizzBuzz {

    private static final FizzBuzz[] $VALUES;

    public static final FizzBuzz Fizz;
    public static final FizzBuzz Buzz;
    public static final FizzBuzz FizzBuzz;

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

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

    private FizzBuzz(String name, int ordinal) {
        super(name, ordinal);
    }

    private static FizzBuzz[] createValues() {
        return new FizzBuzz[] {
            Fizz,
            Buzz,
            FizzBuzz
        }
    }

    static  {
        Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe) unsafeField.get(null);

        String[] fieldNames = "Fizz,Buzz,FizzBuzz".split(",");
        for(int i = 0; i < fieldNames.length; i++) {
            String fieldName = fieldNames[i];
            Field field = FizzBuzz.class.getDeclaredField(fieldName);
            long fieldOffset = unsafe.staticFieldOffset(field);
            unsafe.putObject(FizzBuzz.class, fieldOffset, new FizzBuzz(fieldName, i));
        }

        $VALUES = createValues();
    }

}

public final int ordinal（）

返回此枚举常量的序数（其在枚举声明中的位置，其中初始常量的序数为零）。

static  {
    Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
    unsafeField.setAccessible(true);
    Unsafe unsafe = (Unsafe) unsafeField.get(null);

    String[] fieldNames = "Fizz,Buzz,FizzBuzz".split(",");
    FizzBuzz[] localValues = new FizzBuzz[fieldNames.length];
    for(int i = 0; i < fieldNames.length; i++) {
        String fieldName = fieldNames[i];
        Field field = FizzBuzz.class.getDeclaredField(fieldName);
        long fieldOffset = unsafe.staticFieldOffset(field);
        unsafe.putObject(
            FizzBuzz.class,
            fieldOffset,
            (localValues[i] = new FizzBuzz(fieldName, i))
        );
    }

    $VALUES = localValues;
}

static  {
    Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
    unsafeField.setAccessible(true);
    Unsafe unsafe = (Unsafe) unsafeField.get(null);

    String[] fieldNames = "Fizz,Buzz,FizzBuzz".split(",");
    Field[] fields = FizzBuzz.class.getDeclaredFields();
    HashMap<String, Field> cache = new HashMap<>(fields.length);

    for(Field field : fields) {
        cache.put(field.getName(), field);
    }

    for (int i = 0; i < fieldNames.length; i++) {
        String fieldName = fieldNames[i];
        Field field = cache.get(fieldName);
        long fieldOffset = unsafe.staticFieldOffset(field);
        unsafe.putObject(
            FizzBuzz.class,
            fieldOffset,
            (localValues[i] = new FizzBuzz(fieldName, i))
        );
    }    

    $VALUES = localValues;
}

检查效果

public class TestFew {

    public static void main(String... args) {
        for(String arg : args) {
            System.out.print(arg + " : ");

            try {
                UnsafeHugeEnum value = UnsafeHugeEnum.valueOf(arg);

                doSwitch(value);
            } catch(Throwable e) {
                e.printStackTrace(System.out);
            }
        }
    }

    private static void doSwitch(UnsafeHugeEnum value) {
        switch(value) {
            case VALUE_00001:
                System.out.println("First");
                break;
            case VALUE_31415:
                System.out.println("(int) (10_000 * Math.PI)");
                break;
            case VALUE_65410:
                System.out.println("Last");
                break;
            default:
                System.out.println("Unexpected value: " + value);
                break;
        }
    }

}

$ java TestFew VALUE_00001 VALUE_00400 VALUE_31415 VALUE_65410
VALUE_00001 : First
VALUE_00400 : Unexpected value: VALUE_00400
VALUE_31415 : (int) (10_000 * Math.PI)
VALUE_65410 : Last

switch(value) {
    case VALUE_00001:
    case VALUE_00002:
        ...
    case VALUE_65410:
        System.out.println("One of known values: " + value);
        break;
    default:
        System.out.println("Unexpected value: " + value);
        break;
}

$ javac -fullversion
javac full version "14.0.1+7"

$ javac TestAll.java
TestAll.java:18: error: code too large for try statement
        switch(value) {
        ^
TestAll.java:65433: error: too many constants
                break;
                ^
TestAll.java:17: error: code too large
    private static void doSwitch(UnsafeHugeEnum value) {
                        ^
TestAll.java:1: error: too many constants
public class TestAll {
       ^
4 errors

• Testfew.java
• TestAll.java

Javac和开关

// Determine whether to issue a tableswitch or a lookupswitch
// instruction.
long table_space_cost = 4 + ((long) hi - lo + 1); // words
long table_time_cost = 3; // comparisons
long lookup_space_cost = 3 + 2 * (long) nlabels;
long lookup_time_cost = nlabels;
int opcode =
    nlabels > 0 &&
    table_space_cost + 3 * table_time_cost <=
    lookup_space_cost + 3 * lookup_time_cost
    ?
    tableswitch : lookupswitch;

TestAll.java:18: error: code too large for try statement
        switch(value) {
        ^

/** This map gives a translation table to be used for enum
 *  switches.
 *
 *  <p>For each enum that appears as the type of a switch
 *  expression, we maintain an EnumMapping to assist in the
 *  translation, as exemplified by the following example:
 *
 *  <p>we translate
 *  <pre>
 *          switch(colorExpression) {
 *          case red: stmt1;
 *          case green: stmt2;
 *          }
 *  </pre>
 *  into
 *  <pre>
 *          switch(Outer$0.$EnumMap$Color[colorExpression.ordinal()]) {
 *          case 1: stmt1;
 *          case 2: stmt2
 *          }
 *  </pre>
 *  with the auxiliary table initialized as follows:
 *  <pre>
 *          class Outer$0 {
 *              synthetic final int[] $EnumMap$Color = new int[Color.values().length];
 *              static {
 *                  try { $EnumMap$Color[red.ordinal()] = 1; } catch (NoSuchFieldError ex) {}
 *                  try { $EnumMap$Color[green.ordinal()] = 2; } catch (NoSuchFieldError ex) {}
 *              }
 *          }
 *  </pre>
 *  class EnumMapping provides mapping data and support methods for this translation.
 */

try {
    $SwitchMap$UnsafeHugeEnum[UnsafeHugeEnum.VALUE_00001.ordinal()] = 1;
    //  9: getstatic     #2                  // Field $SwitchMap$UnsafeHugeEnum:[I
    // 12: getstatic     #3                  // Field UnsafeHugeEnum.VALUE_00001:LUnsafeHugeEnum;
    // 15: invokevirtual #4                  // Method UnsafeHugeEnum.ordinal:()I
    // 18: iconst_1
    // 19: iastore
}
// 20: goto          24
catch(NoSuchFieldError e) { }
// 23: astore_0

结论

Test.java:3: warning: Unsafe is internal proprietary API and may be removed in a future release
import sun.misc.Unsafe;
               ^

附加材料

Huge enumeration generator

    https://github.com/Maccimo/HugeEnumGeneratorArticle

Additional information (in Russian):

    https://habr.com/ru/post/483392/
    https://habr.com/ru/post/501870/

Usage:
    java -jar HugeEnumGen.jar [ <options> ] <enum name>

    <enum name>
        An enumeration class name.
        Should be a valid Java identifier. May contain package name.

Options:

    -d <directory>
        Output directory path.
        Current working directory by default.

    -e <item list file>
        Path to UTF8-encoded text file with list of enumeration item names.
        Item names will be autogenerated if absent.
        Mutually exclusive with the -c option.

    -c <count>
        Count of autogenerated enumeration item names.
        Mutually exclusive with the -e option.
        Default value: Algorithm-depended

    -a <algorithm>
        Enumeration generation algorithm.
        Supported algorithms:
          ConDy          - Employ Constant Dynamic (JEP 309) for enum elements initialization
          ExtractMethod  - Extract enum elements initialization code to separate method
          Unsafe         - Employ sun.misc.Unsafe for enum elements initialization

        Default algorithm: ExtractMethod

    -h / -?
        Show this help page.

Example:

    java -jar HugeEnumGen.jar -d ./bin -c 2020 com.habr.maccimo.HugeEnum2020