​反射基础之Enum

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();
    }
    }
}

$ 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]

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)

Consider application which needs to dynamically modify the trace level in a server application which normally does not allow this change during runtime. Assume the instance of the server object is available. The SetTrace example shows how code can translate the String representation of an enum into an enum type and retrieve and set the value of a field storing an enum.
import java.lang.reflect.Field;
import static java.lang.System.out;

enum TraceLevel { OFF, LOW, MEDIUM, HIGH, DEBUG }

class MyServer {
    private TraceLevel level = TraceLevel.OFF;
}

public class SetTrace {
    public static void main(String... args) {
    TraceLevel newLevel = TraceLevel.valueOf(args[0]);

    try {
        MyServer svr = new MyServer();
        Class<?> c = svr.getClass();
        Field f = c.getDeclaredField("level");
        f.setAccessible(true);
        TraceLevel oldLevel = (TraceLevel)f.get(svr);
        out.format("Original trace level:  %s%n", oldLevel);

        if (oldLevel != newLevel) {
         f.set(svr, newLevel);
        out.format("    New  trace level:  %s%n", f.get(svr));
        }

        // production code should handle these exceptions more gracefully
    } catch (IllegalArgumentException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    } catch (NoSuchFieldException x) {
        x.printStackTrace();
    }
    }
}

$ java SetTrace OFF
Original trace level:  OFF
$ java SetTrace DEBUG
Original trace level:  OFF
    New  trace level:  DEBUG

IllegalArgumentException When Attempting to Instantiate an Enum Type
As has been mentioned, instantiation of enum types is forbidden. The EnumTrouble example attempts this.
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;

enum Charge {
    POSITIVE, NEGATIVE, NEUTRAL;
    Charge() {
    out.format("under construction%n");
    }
}

public class EnumTrouble {

    public static void main(String... args) {
    try {
        Class<?> c = Charge.class;

         Constructor[] ctors = c.getDeclaredConstructors();
         for (Constructor ctor : ctors) {
        out.format("Constructor: %s%n",  ctor.toGenericString());
         ctor.setAccessible(true);
         ctor.newInstance();
         }

        // production code should handle these exceptions more gracefully
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    } catch (InvocationTargetException x) {
        x.printStackTrace();
    }
    }
}

$ java EnumTrouble
Constructor: private Charge()
Exception in thread "main" java.lang.IllegalArgumentException: Cannot
  reflectively create enum objects
        at java.lang.reflect.Constructor.newInstance(Constructor.java:511)
        at EnumTrouble.main(EnumTrouble.java:22)

import java.lang.reflect.Field;

enum E0 { A, B }
enum E1 { A, B }

class ETest {
    private E0 fld = E0.A;
}

public class EnumTroubleToo {
    public static void main(String... args) {
    try {
        ETest test = new ETest();
        Field f = test.getClass().getDeclaredField("fld");
        f.setAccessible(true);
         f.set(test, E1.A);  // IllegalArgumentException

        // production code should handle these exceptions more gracefully
    } catch (NoSuchFieldException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    }
    }
}

$ java EnumTroubleToo
Exception in thread "main" java.lang.IllegalArgumentException: Can not set E0
  field ETest.fld to E1
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException
          (UnsafeFieldAccessorImpl.java:146)
        at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException
          (UnsafeFieldAccessorImpl.java:150)
        at sun.reflect.UnsafeObjectFieldAccessorImpl.set
          (UnsafeObjectFieldAccessorImpl.java:63)
        at java.lang.reflect.Field.set(Field.java:657)
        at EnumTroubleToo.main(EnumTroubleToo.java:16)