java 修改枚举类字段_枚举枚举和修改“最终静态”字段的方法

java 修改枚举类字段

在本新闻通讯中，该新闻通讯最初发表在Java专家的新闻通讯第161期中，我们研究了如何使用sun.reflect包中的反射类在Sun JDK中创建枚举实例。显然，这仅适用于Sun的JDK。如果需要在另一个JVM上执行此操作，则您可以自己完成。

所有这些都始于爱丁堡的Ken Dobson的电子邮件，该电子邮件向我指出了sun.reflect.ConstructorAccessor的方向，他声称可以将其用于构造枚举实例。我以前的方法（通讯＃141）在Java 6中不起作用。

我很好奇为什么Ken要构造枚举。这是他想使用它的方式：

public enum HumanState {HAPPY, SAD
}public class Human {public void sing(HumanState state) {switch (state) {case HAPPY:singHappySong();break;case SAD:singDirge();break;default:new IllegalStateException("Invalid State: " + state);}}private void singHappySong() {System.out.println("When you're happy and you know it ...");}private void singDirge() {System.out.println("Don't cry for me Argentina, ...");}
}

上面的代码需要进行单元测试。你发现错误了吗？如果没有，请使用细梳再次遍历代码以尝试找到它。当我第一次看到这个时，我也没有发现错误。

当我们产生这样的错误时，我们应该做的第一件事就是进行一个显示它的单元测试。但是，在这种情况下，我们无法使default情况发生，因为HumanState仅具有HAPPY和SAD枚举。

Ken的发现使我们可以使用sun.reflect包中的ConstructorAccessor类来创建枚举的实例。它涉及到以下内容：

Constructor cstr = clazz.getDeclaredConstructor(String.class, int.class
);
ReflectionFactory reflection =ReflectionFactory.getReflectionFactory();
Enum e =reflection.newConstructorAccessor(cstr).newInstance("BLA",3);

但是，如果仅执行此操作，则最终会出现ArrayIndexOutOfBoundsException，这在我们看到Java编译器如何将switch语句转换为字节代码时才有意义。以上面的Human类为例，下面是反编译后的代码（感谢Pavel Kouznetsov的JAD ）：

public class Human {public void sing(HumanState state) {static class _cls1 {static final int $SwitchMap$HumanState[] =new int[HumanState.values().length];static {try {$SwitchMap$HumanState[HumanState.HAPPY.ordinal()] = 1;} catch(NoSuchFieldError ex) { }try {$SwitchMap$HumanState[HumanState.SAD.ordinal()] = 2;} catch(NoSuchFieldError ex) { }}}switch(_cls1.$SwitchMap$HumanState[state.ordinal()]) {case 1:singHappySong();break;case 2:singDirge();break;default:new IllegalStateException("Invalid State: " + state);break;}}private void singHappySong() {System.out.println("When you're happy and you know it ...");}private void singDirge() {System.out.println("Don't cry for me Argentina, ...");}
}

您可以立即看到为什么要得到ArrayIndexOutOfBoundsException，这要归功于内部类_cls1。

我第一次尝试解决此问题并没有得到一个不错的解决方案。我试图在HumanState枚举中修改$ VALUES数组。但是，我只是摆脱了Java的保护性代码。您可以修改final字段，只要它们是非静态的即可。这种限制对我来说似乎是人为的，因此我着手寻找静态最终领域的圣杯。再次，它被隐藏在阳光反射的房间里。

设置“最终静态”字段

设置final static字段需要几件事。首先，我们需要使用法线反射获取Field对象。如果将其传递给FieldAccessor，我们将退出安全代码，因为我们正在处理静态的final字段。 其次，我们将Field对象实例内的修饰符字段值更改为非最终值。 第三，我们将经过修改的字段传递给sun.reflect包中的FieldAccessor并使用它进行设置。

这是我的ReflectionHelper类，可用于通过反射设置final static字段：

import sun.reflect.*;
import java.lang.reflect.*;public class ReflectionHelper {private static final String MODIFIERS_FIELD = "modifiers";private static final ReflectionFactory reflection =ReflectionFactory.getReflectionFactory();public static void setStaticFinalField(Field field, Object value)throws NoSuchFieldException, IllegalAccessException {// we mark the field to be publicfield.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 fieldField modifiersField =Field.class.getDeclaredField(MODIFIERS_FIELD);modifiersField.setAccessible(true);int modifiers = modifiersField.getInt(field);// blank out the final bit in the modifiers intmodifiers &= ~Modifier.FINAL;modifiersField.setInt(field, modifiers);FieldAccessor fa = reflection.newFieldAccessor(field, false);fa.set(null, value);}
}

通过使用ReflectionHelper，我可以在枚举中设置$ VALUES数组以包含新的枚举。这行得通，只是我必须在首次加载Human类之前执行此操作。这会将竞争条件引入我们的测试用例中。单独进行每个测试都可以，但是总的来说它们可能会失败。这不是一个好方案！

重新连接枚举开关

下一个想法是重新连接实际的switch语句的$ SwitchMap $ HumanState字段。在匿名内部类中很容易找到此字段。您所需要的只是前缀$ SwitchMap $，后跟枚举类名称。如果枚举在一个类中切换了几次，则内部类仅创建一次。

我昨天写的其他解决方案之一检查了我们的switch语句是否正在处理所有可能的情况。将新类型引入系统后，这对于发现错误很有用。我放弃了该特定解决方案，但是您应该能够根据稍后将向您展示的EnumBuster重新创建该解决方案。

纪念品设计模式

我最近重新编写了我的设计模式课程（警告，该网站可能尚未建立最新的结构–请查询更多信息），以考虑Java的变化，丢弃一些过时的模式并介绍我以前排除的一些。 Memento是“新”模式之一，通常与撤消功能一起使用。我认为这是一个很好的模式，可以用来在我们努力测试不可能的案例的努力中消除对枚举的损害。

出版专家通讯给我某些自由。我不必解释我写的每一行。因此，事不宜迟，这里是我的EnumBuster类，它使您可以创建枚举，将它们添加到现有的values []中，从数组中删除枚举，同时保留您指定的任何类的switch语句。

import sun.reflect.*;import java.lang.reflect.*;
import java.util.*;public class EnumBuster<E extends Enum<E>> {private static final Class[] EMPTY_CLASS_ARRAY =new Class[0];private static final Object[] EMPTY_OBJECT_ARRAY =new Object[0];private static final String VALUES_FIELD = "$VALUES";private static final String ORDINAL_FIELD = "ordinal";private final ReflectionFactory reflection =ReflectionFactory.getReflectionFactory();private final Class<E> clazz;private final Collection<Field> switchFields;private final Deque<Memento> undoStack =new LinkedList<Memento>();/*** Construct an EnumBuster for the given enum class and keep* the switch statements of the classes specified in* switchUsers in sync with the enum values.*/public EnumBuster(Class<E> clazz, Class... switchUsers) {try {this.clazz = clazz;switchFields = findRelatedSwitchFields(switchUsers);} catch (Exception e) {throw new IllegalArgumentException("Could not create the class", e);}}/*** Make a new enum instance, without adding it to the values* array and using the default ordinal of 0.*/public E make(String value) {return make(value, 0,EMPTY_CLASS_ARRAY, EMPTY_OBJECT_ARRAY);}/*** Make a new enum instance with the given ordinal.*/public E make(String value, int ordinal) {return make(value, ordinal,EMPTY_CLASS_ARRAY, EMPTY_OBJECT_ARRAY);}/*** Make a new enum instance with the given value, ordinal and* additional parameters.  The additionalTypes is used to match* the constructor accurately.*/public E make(String value, int ordinal,Class[] additionalTypes, Object[] additional) {try {undoStack.push(new Memento());ConstructorAccessor ca = findConstructorAccessor(additionalTypes, clazz);return constructEnum(clazz, ca, value,ordinal, additional);} catch (Exception e) {throw new IllegalArgumentException("Could not create enum", e);}}/*** This method adds the given enum into the array* inside the enum class.  If the enum already* contains that particular value, then the value* is overwritten with our enum.  Otherwise it is* added at the end of the array.** In addition, if there is a constant field in the* enum class pointing to an enum with our value,* then we replace that with our enum instance.** The ordinal is either set to the existing position* or to the last value.** Warning: This should probably never be called,* since it can cause permanent changes to the enum* values.  Use only in extreme conditions.** @param e the enum to add*/public void addByValue(E e) {try {undoStack.push(new Memento());Field valuesField = findValuesField();// we get the current Enum[]E[] values = values();for (int i = 0; i < values.length; i++) {E value = values[i];if (value.name().equals(e.name())) {setOrdinal(e, value.ordinal());values[i] = e;replaceConstant(e);return;}}// we did not find it in the existing array, thus// append it to the arrayE[] newValues =Arrays.copyOf(values, values.length + 1);newValues[newValues.length - 1] = e;ReflectionHelper.setStaticFinalField(valuesField, newValues);int ordinal = newValues.length - 1;setOrdinal(e, ordinal);addSwitchCase();} catch (Exception ex) {throw new IllegalArgumentException("Could not set the enum", ex);}}/*** We delete the enum from the values array and set the* constant pointer to null.** @param e the enum to delete from the type.* @return true if the enum was found and deleted;*         false otherwise*/public boolean deleteByValue(E e) {if (e == null) throw new NullPointerException();try {undoStack.push(new Memento());// we get the current E[]E[] values = values();for (int i = 0; i < values.length; i++) {E value = values[i];if (value.name().equals(e.name())) {E[] newValues =Arrays.copyOf(values, values.length - 1);System.arraycopy(values, i + 1, newValues, i,values.length - i - 1);for (int j = i; j < newValues.length; j++) {setOrdinal(newValues[j], j);}Field valuesField = findValuesField();ReflectionHelper.setStaticFinalField(valuesField, newValues);removeSwitchCase(i);blankOutConstant(e);return true;}}} catch (Exception ex) {throw new IllegalArgumentException("Could not set the enum", ex);}return false;}/*** Undo the state right back to the beginning when the* EnumBuster was created.*/public void restore() {while (undo()) {//}}/*** Undo the previous operation.*/public boolean undo() {try {Memento memento = undoStack.poll();if (memento == null) return false;memento.undo();return true;} catch (Exception e) {throw new IllegalStateException("Could not undo", e);}}private ConstructorAccessor findConstructorAccessor(Class[] additionalParameterTypes,Class<E> clazz) 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);Constructor<E> cstr = clazz.getDeclaredConstructor(parameterTypes);return reflection.newConstructorAccessor(cstr);}private E constructEnum(Class<E> clazz,ConstructorAccessor ca,String value, int ordinal,Object[] additional)throws Exception {Object[] parms = new Object[additional.length + 2];parms[0] = value;parms[1] = ordinal;System.arraycopy(additional, 0, parms, 2, additional.length);return clazz.cast(ca.newInstance(parms));}/*** The only time we ever add a new enum is at the end.* Thus all we need to do is expand the switch map arrays* by one empty slot.*/private void addSwitchCase() {try {for (Field switchField : switchFields) {int[] switches = (int[]) switchField.get(null);switches = Arrays.copyOf(switches, switches.length + 1);ReflectionHelper.setStaticFinalField(switchField, switches);}} catch (Exception e) {throw new IllegalArgumentException("Could not fix switch", e);}}private void replaceConstant(E e)throws IllegalAccessException, NoSuchFieldException {Field[] fields = clazz.getDeclaredFields();for (Field field : fields) {if (field.getName().equals(e.name())) {ReflectionHelper.setStaticFinalField(field, e);}}}private void blankOutConstant(E e)throws IllegalAccessException, NoSuchFieldException {Field[] fields = clazz.getDeclaredFields();for (Field field : fields) {if (field.getName().equals(e.name())) {ReflectionHelper.setStaticFinalField(field, null);}}}private void setOrdinal(E e, int ordinal)throws NoSuchFieldException, IllegalAccessException {Field ordinalField = Enum.class.getDeclaredField(ORDINAL_FIELD);ordinalField.setAccessible(true);ordinalField.set(e, ordinal);}/*** Method to find the values field, set it to be accessible,* and return it.** @return the values array field for the enum.* @throws NoSuchFieldException if the field could not be found*/private Field findValuesField()throws NoSuchFieldException {// first we find the static final array that holds// the values in the enum classField valuesField = clazz.getDeclaredField(VALUES_FIELD);// we mark it to be publicvaluesField.setAccessible(true);return valuesField;}private Collection<Field> findRelatedSwitchFields(Class[] switchUsers) {Collection<Field> result = new ArrayList<Field>();try {for (Class switchUser : switchUsers) {Class[] clazzes = switchUser.getDeclaredClasses();for (Class suspect : clazzes) {Field[] fields = suspect.getDeclaredFields();for (Field field : fields) {if (field.getName().startsWith("$SwitchMap$" +clazz.getSimpleName())) {field.setAccessible(true);result.add(field);}}}}} catch (Exception e) {throw new IllegalArgumentException("Could not fix switch", e);}return  result;}private void removeSwitchCase(int ordinal) {try {for (Field switchField : switchFields) {int[] switches = (int[]) switchField.get(null);int[] newSwitches = Arrays.copyOf(switches, switches.length - 1);System.arraycopy(switches, ordinal + 1, newSwitches,ordinal, switches.length - ordinal - 1);ReflectionHelper.setStaticFinalField(switchField, newSwitches);}} catch (Exception e) {throw new IllegalArgumentException("Could not fix switch", e);}}@SuppressWarnings("unchecked")private E[] values()throws NoSuchFieldException, IllegalAccessException {Field valuesField = findValuesField();return (E[]) valuesField.get(null);}private class Memento {private final E[] values;private final Map<Field, int[]> savedSwitchFieldValues =new HashMap<Field, int[]>();private Memento() throws IllegalAccessException {try {values = values().clone();for (Field switchField : switchFields) {int[] switchArray = (int[]) switchField.get(null);savedSwitchFieldValues.put(switchField,switchArray.clone());}} catch (Exception e) {throw new IllegalArgumentException("Could not create the class", e);}}private void undo() throwsNoSuchFieldException, IllegalAccessException {Field valuesField = findValuesField();ReflectionHelper.setStaticFinalField(valuesField, values);for (int i = 0; i < values.length; i++) {setOrdinal(values[i], i);}// reset all of the constants defined inside the enumMap<String, E> valuesMap =new HashMap<String, E>();for (E e : values) {valuesMap.put(e.name(), e);}Field[] constantEnumFields = clazz.getDeclaredFields();for (Field constantEnumField : constantEnumFields) {E en = valuesMap.get(constantEnumField.getName());if (en != null) {ReflectionHelper.setStaticFinalField(constantEnumField, en);}}for (Map.Entry<Field, int[]> entry :savedSwitchFieldValues.entrySet()) {Field field = entry.getKey();int[] mappings = entry.getValue();ReflectionHelper.setStaticFinalField(field, mappings);}}}
}

该类很长，可能仍然存在一些错误。我是从旧金山到纽约的途中写的。这是我们可以使用它来测试人类班级的方法：

import junit.framework.TestCase;public class HumanTest extends TestCase {public void testSingingAddingEnum() {EnumBuster<HumanState> buster =new EnumBuster<HumanState>(HumanState.class,Human.class);try {Human heinz = new Human();heinz.sing(HumanState.HAPPY);heinz.sing(HumanState.SAD);HumanState MELLOW = buster.make("MELLOW");buster.addByValue(MELLOW);System.out.println(Arrays.toString(HumanState.values()));try {heinz.sing(MELLOW);fail("Should have caused an IllegalStateException");}catch (IllegalStateException success) { }}finally {System.out.println("Restoring HumanState");buster.restore();System.out.println(Arrays.toString(HumanState.values()));}}
}

现在，此单元测试在前面显示的Human.java文件中显示了错误。我们忘记添加throw关键字！

When you're happy and you know it ...
Don't cry for me Argentina, ...
[HAPPY, SAD, MELLOW]
Restoring HumanState
[HAPPY, SAD]AssertionFailedError: Should have caused an IllegalStateExceptionat HumanTest.testSingingAddingEnum(HumanTest.java:23)

EnumBuster类可以做的更多。我们可以使用它删除不需要的枚举。如果我们指定switch语句属于哪些类，则将同时维护这些类。另外，我们可以还原到初始状态。很多功能！

我注销之前的最后一个测试用例，我们将测试类添加到switch类中以进行维护。

import junit.framework.TestCase;public class EnumSwitchTest extends TestCase {public void testSingingDeletingEnum() {EnumBuster<HumanState> buster =new EnumBuster<HumanState>(HumanState.class,EnumSwitchTest.class);try {for (HumanState state : HumanState.values()) {switch (state) {case HAPPY:case SAD:break;default:fail("Unknown state");}}buster.deleteByValue(HumanState.HAPPY);for (HumanState state : HumanState.values()) {switch (state) {case SAD:break;case HAPPY:default:fail("Unknown state");}}buster.undo();buster.deleteByValue(HumanState.SAD);for (HumanState state : HumanState.values()) {switch (state) {case HAPPY:break;case SAD:default:fail("Unknown state");}}buster.deleteByValue(HumanState.HAPPY);for (HumanState state : HumanState.values()) {switch (state) {case HAPPY:case SAD:default:fail("Unknown state");}}} finally {buster.restore();}}
}

EnumBuster甚至保留常量，因此，如果从values（）中删除一个枚举，它将清空最终的静态字段。如果重新添加，它将设置为新值。

肯·多布森（Ken Dobson）的想法以一种我不知道有可能的方式进行反思，真是太有趣了。（任何Sun工程师都读过这篇文章，请不要在Java的未来版本中插入这些漏洞！）

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关于Heinz M. Kabutz博士

自2000年以来，我一直为Java专家社区写作。这很有趣。当您与可能会喜欢的人分享本文时，会更加有趣。如果他们前往www.javaspecialists.eu并将自己添加到列表中，他们可以每个月获得新鲜的东西。

中继：这篇文章是Java Advent Calendar的一部分，并根据Creative Commons 3.0 Attribution许可获得许可。 如果您喜欢它，请通过共享，发推，FB，G +等方式传播信息！ 想为博客写文章吗？ 我们正在寻找能够填补所有24个职位的贡献者，并希望能为您贡献力量！ 联系Attila Balazs贡献力量！

参考资料：来自Java日历日历博客的JCG合作伙伴 Attila-Mihaly Balazs的“枚举枚举和修改“最终静态”字段” 。