Java動態生成枚舉類型

原文：https://blog.csdn.net/qq_30038111/article/details/80816286

枚舉一般會用在一些狀態的表示、單例實現等場景上。有的時候可能希望將枚舉類的值放到配置文件、或者數據庫中，由項目啟動或者運行時動態確定枚舉值，這個時候就需要動態添加枚舉值。

使用動態添加枚舉值時建議在枚舉類上增加getEnum方法，增加對枚舉的緩沖，減少對枚舉類的改變，最好是在項目啟動時就把枚舉類初始化好

舉個栗子：

import xxx.DynamicEnumUtil;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.Map;

public enum  YesNoEnum {
    YES(1,"yes"),
    NO(0,"no");

    private Integer value;
    private String name;
    private static Map<Integer, YesNoEnum> enumMap = new HashMap<>();

    static{
        //  可以在這里加載枚舉的配置文件 比如從 properties  數據庫中加載
        //  加載完后 使用DynamicEnumUtil.addEnum 動態增加枚舉值
        //  然后正常使用枚舉即可
        EnumSet<YesNoEnum> set = EnumSet.allOf(YesNoEnum.class);
        for (YesNoEnum each: set ) {
            // 增加一個緩存 減少對枚舉的修改
            enumMap.put(each.value, each);
        }
    }

    YesNoEnum(Integer value ,String name){
        this.value = value;
        this.name = name;
    }

    public Integer getValue() {
        return value;
    }

    public String getName() {
        return name;
    }

    // 根據關鍵字段獲取枚舉值  可以在這里做一些修改 來達到動態添加的效果
    public YesNoEnum getEnum(Integer value){
        // 這里可以做一些修改  比如若從 enumMap 中沒有取得 則加載配置動態添加
        return enumMap.get(value);
    }

    //  動態添加方法  添加完后加入緩存 減少對枚舉的修改
    public YesNoEnum addEnum(String enumName, Integer value,String name){
        YesNoEnum yesNoEnum = DynamicEnumUtil.addEnum(YesNoEnum.class, enumName, new Class[]{Integer.class, String.class}, new Object[]{value, name});
        enumMap.put(value,yesNoEnum);
        return yesNoEnum;
    }
}

工具類如下:

import sun.reflect.ConstructorAccessor;
import sun.reflect.FieldAccessor;
import sun.reflect.ReflectionFactory;

import java.lang.reflect.AccessibleObject;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class DynamicEnumUtil {
 
   private static ReflectionFactory reflectionFactory = ReflectionFactory.getReflectionFactory();
 
   private static void setFailsafeFieldValue(Field field, Object target, Object value) throws NoSuchFieldException,
           IllegalAccessException {
 
       // let's make the field accessible
       field.setAccessible(true);
 
       // next we change the modifier in the Field instance to
       // not be final anymore, thus tricking reflection into
       // letting us modify the static final field
       Field modifiersField = Field.class.getDeclaredField("modifiers");
       modifiersField.setAccessible(true);
       int modifiers = modifiersField.getInt(field);
 
       // blank out the final bit in the modifiers int
       modifiers &= ~Modifier.FINAL;
       modifiersField.setInt(field, modifiers);
 
       FieldAccessor fa = reflectionFactory.newFieldAccessor(field, false);
       fa.set(target, value);
   }
 
   private static void blankField(Class<?> enumClass, String fieldName) throws NoSuchFieldException,
           IllegalAccessException {
       for (Field field : Class.class.getDeclaredFields()) {
           if (field.getName().contains(fieldName)) {
               AccessibleObject.setAccessible(new Field[] { field }, true);
               setFailsafeFieldValue(field, enumClass, null);
               break;
           }
       }
   }
 
   private static void cleanEnumCache(Class<?> enumClass) throws NoSuchFieldException, IllegalAccessException {
       blankField(enumClass, "enumConstantDirectory"); // Sun (Oracle?!?) JDK 1.5/6
       blankField(enumClass, "enumConstants"); // IBM JDK
   }
 
   private static ConstructorAccessor getConstructorAccessor(Class<?> enumClass, Class<?>[] additionalParameterTypes)
           throws NoSuchMethodException {
       Class<?>[] parameterTypes = new Class[additionalParameterTypes.length + 2];
       parameterTypes[0] = String.class;
       parameterTypes[1] = int.class;
       System.arraycopy(additionalParameterTypes, 0, parameterTypes, 2, additionalParameterTypes.length);
       return reflectionFactory.newConstructorAccessor(enumClass.getDeclaredConstructor(parameterTypes));
   }
 
   private static Object makeEnum(Class<?> enumClass, String value, int ordinal, Class<?>[] additionalTypes,
           Object[] additionalValues) throws Exception {
       Object[] parms = new Object[additionalValues.length + 2];
       parms[0] = value;
       parms[1] = Integer.valueOf(ordinal);
       System.arraycopy(additionalValues, 0, parms, 2, additionalValues.length);
//       parms[1] = parms[parms.length-1];
       return enumClass.cast(getConstructorAccessor(enumClass, additionalTypes).newInstance(parms));
   }
 
   /**
    * Add an enum instance to the enum class given as argument
    *
    * @param <T> the type of the enum (implicit)
    * @param enumType the class of the enum to be modified
    * @param enumName the name of the new enum instance to be added to the class.
    */
   @SuppressWarnings("unchecked")
   public static <T extends Enum<?>> T addEnum(Class<T> enumType, String enumName, Class<?>[] additionalTypes, Object[] additionalValues) {
 
       // 0. Sanity checks
       if (!Enum.class.isAssignableFrom(enumType)) {
           throw new RuntimeException("class " + enumType + " is not an instance of Enum");
       }
 
       // 1. Lookup "$VALUES" holder in enum class and get previous enum instances
       Field valuesField = null;
       Field[] fields = enumType.getDeclaredFields();
       for (Field field : fields) {
           if (field.getName().contains("$VALUES")) {
               valuesField = field;
               break;
           }
       }
       AccessibleObject.setAccessible(new Field[] { valuesField }, true);
 
       try {
 
           // 2. Copy it
           T[] previousValues = (T[]) valuesField.get(enumType);
           List<T> values = new ArrayList<T>(Arrays.asList(previousValues));
 
           // 3. build new enum
           T newValue = (T) makeEnum(enumType, enumName, values.size(), additionalTypes, additionalValues);
 
           // 4. add new value
           values.add(newValue);
 
           // 5. Set new values field
           setFailsafeFieldValue(valuesField, null, values.toArray((T[]) Array.newInstance(enumType, 0)));
 
           // 6. Clean enum cache
           cleanEnumCache(enumType);
           return newValue;
       } catch (Exception e) {
           e.printStackTrace();
           throw new RuntimeException(e.getMessage(), e);
       }
   }
 
 
   public static void main(String[] args) {

   }
   
 
}