Examining Enums
反射提供了三个特定于枚举的 API：
-
- 指示此类是否表示枚举类型
-
- 按声明 Sequences 检索由枚举定义的枚举常量列表
-
java.lang.reflect.Field.isEnumConstant()
- 指示此字段是否表示枚举类型的元素
有时有必要动态地获取枚举常量列表。在非反射代码中，这是通过在枚举上调用隐式声明的静态方法values()来实现的。如果枚举类型的实例不可用，则获取可能值列表的唯一方法是调用Class.getEnumConstants()，因为无法实例化枚举类型。
给定完全限定的名称，EnumConstants示例显示了如何使用Class.getEnumConstants()检索枚举中的常量的有序列表。
import java.util.Arrays;
import static java.lang.System.out;
enum Eon { HADEAN, ARCHAEAN, PROTEROZOIC, PHANEROZOIC }
public class EnumConstants {
public static void main(String... args) {
try {
Class<?> c = (args.length == 0 ? Eon.class : Class.forName(args[0]));
out.format("Enum name: %s%nEnum constants: %s%n",
c.getName(), Arrays.asList(c.getEnumConstants()));
if (c == Eon.class)
out.format(" Eon.values(): %s%n",
Arrays.asList(Eon.values()));
// production code should handle this exception more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
}
输出 samples 如下。用户 Importing 以斜体显示。
$ java EnumConstants java.lang.annotation.RetentionPolicy
Enum name: java.lang.annotation.RetentionPolicy
Enum constants: [SOURCE, CLASS, RUNTIME]
$ java EnumConstants java.util.concurrent.TimeUnit
Enum name: java.util.concurrent.TimeUnit
Enum constants: [NANOSECONDS, MICROSECONDS,
MILLISECONDS, SECONDS,
MINUTES, HOURS, DAYS]
此示例还显示Class.getEnumConstants()返回的值与通过枚举类型调用values()返回的值相同。
$ java EnumConstants
Enum name: Eon
Enum constants: [HADEAN, ARCHAEAN,
PROTEROZOIC, PHANEROZOIC]
Eon.values(): [HADEAN, ARCHAEAN,
PROTEROZOIC, PHANEROZOIC]
由于枚举是类，因此可以使用此跟踪的Fields，Methods和Constructors部分中描述的相同反射 API 获取其他信息。 EnumSpy代码说明了如何使用这些 API 来获取有关枚举声明的其他信息。该示例使用Class.isEnum()限制检查的类集。它还使用Field.isEnumConstant()将枚举常量与枚举声明中的其他字段区分开(并非所有字段都是枚举常量)。
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Member;
import java.util.List;
import java.util.ArrayList;
import static java.lang.System.out;
public class EnumSpy {
private static final String fmt = " %11s: %s %s%n";
public static void main(String... args) {
try {
Class<?> c = Class.forName(args[0]);
if (!c.isEnum()) {
out.format("%s is not an enum type%n", c);
return;
}
out.format("Class: %s%n", c);
Field[] flds = c.getDeclaredFields();
List<Field> cst = new ArrayList<Field>(); // enum constants
List<Field> mbr = new ArrayList<Field>(); // member fields
for (Field f : flds) {
if (f.isEnumConstant())
cst.add(f);
else
mbr.add(f);
}
if (!cst.isEmpty())
print(cst, "Constant");
if (!mbr.isEmpty())
print(mbr, "Field");
Constructor[] ctors = c.getDeclaredConstructors();
for (Constructor ctor : ctors) {
out.format(fmt, "Constructor", ctor.toGenericString(),
synthetic(ctor));
}
Method[] mths = c.getDeclaredMethods();
for (Method m : mths) {
out.format(fmt, "Method", m.toGenericString(),
synthetic(m));
}
// production code should handle this exception more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
private static void print(List<Field> lst, String s) {
for (Field f : lst) {
out.format(fmt, s, f.toGenericString(), synthetic(f));
}
}
private static String synthetic(Member m) {
return (m.isSynthetic() ? "[ synthetic ]" : "");
}
}
$ java EnumSpy java.lang.annotation.RetentionPolicy
Class: class java.lang.annotation.RetentionPolicy
Constant: public static final java.lang.annotation.RetentionPolicy
java.lang.annotation.RetentionPolicy.SOURCE
Constant: public static final java.lang.annotation.RetentionPolicy
java.lang.annotation.RetentionPolicy.CLASS
Constant: public static final java.lang.annotation.RetentionPolicy
java.lang.annotation.RetentionPolicy.RUNTIME
Field: private static final java.lang.annotation.RetentionPolicy[]
java.lang.annotation.RetentionPolicy. [ synthetic ]
Constructor: private java.lang.annotation.RetentionPolicy()
Method: public static java.lang.annotation.RetentionPolicy[]
java.lang.annotation.RetentionPolicy.values()
Method: public static java.lang.annotation.RetentionPolicy
java.lang.annotation.RetentionPolicy.valueOf(java.lang.String)
输出显示java.lang.annotation.RetentionPolicy的声明仅包含三个枚举常量。枚举常量公开为public static final字段。字段，构造函数和方法是编译器生成的。 $VALUES字段与values()方法的实现有关。
Note:
由于各种原因，包括对枚举类型的演化的支持，枚举常量的声明 Sequences 很重要。 Class.getFields()和Class.getDeclaredFields()不保证返回值的 Sequences 与声明的源代码中的 Sequences 匹配。如果应用程序需要 Order，请使用Class.getEnumConstants()。
java.util.concurrent.TimeUnit的输出显示可能有更复杂的枚举。此类包含几种方法以及声明为static final的其他字段，这些字段不是枚举常量。
$ java EnumSpy java.util.concurrent.TimeUnit
Class: class java.util.concurrent.TimeUnit
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.NANOSECONDS
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.MICROSECONDS
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.MILLISECONDS
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.SECONDS
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.MINUTES
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.HOURS
Constant: public static final java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.DAYS
Field: static final long java.util.concurrent.TimeUnit.C0
Field: static final long java.util.concurrent.TimeUnit.C1
Field: static final long java.util.concurrent.TimeUnit.C2
Field: static final long java.util.concurrent.TimeUnit.C3
Field: static final long java.util.concurrent.TimeUnit.C4
Field: static final long java.util.concurrent.TimeUnit.C5
Field: static final long java.util.concurrent.TimeUnit.C6
Field: static final long java.util.concurrent.TimeUnit.MAX
Field: private static final java.util.concurrent.TimeUnit[]
java.util.concurrent.TimeUnit. [ synthetic ]
Constructor: private java.util.concurrent.TimeUnit()
Constructor: java.util.concurrent.TimeUnit
(java.lang.String,int,java.util.concurrent.TimeUnit)
[ synthetic ]
Method: public static java.util.concurrent.TimeUnit
java.util.concurrent.TimeUnit.valueOf(java.lang.String)
Method: public static java.util.concurrent.TimeUnit[]
java.util.concurrent.TimeUnit.values()
Method: public void java.util.concurrent.TimeUnit.sleep(long)
throws java.lang.InterruptedException
Method: public long java.util.concurrent.TimeUnit.toNanos(long)
Method: public long java.util.concurrent.TimeUnit.convert
(long,java.util.concurrent.TimeUnit)
Method: abstract int java.util.concurrent.TimeUnit.excessNanos
(long,long)
Method: public void java.util.concurrent.TimeUnit.timedJoin
(java.lang.Thread,long) throws java.lang.InterruptedException
Method: public void java.util.concurrent.TimeUnit.timedWait
(java.lang.Object,long) throws java.lang.InterruptedException
Method: public long java.util.concurrent.TimeUnit.toDays(long)
Method: public long java.util.concurrent.TimeUnit.toHours(long)
Method: public long java.util.concurrent.TimeUnit.toMicros(long)
Method: public long java.util.concurrent.TimeUnit.toMillis(long)
Method: public long java.util.concurrent.TimeUnit.toMinutes(long)
Method: public long java.util.concurrent.TimeUnit.toSeconds(long)
Method: static long java.util.concurrent.TimeUnit.x(long,long,long)
