public static enum Enumeration {
    STATUS_0(0), STATUS_1(1), STATUS_ERROR(-1);
    private final int code;
    Enumeration(int code) { this.code = code; }
    public int getCode() { return code; }
    public static Enumeration fromCode(int code) {
        switch(code) {
        case 0: return STATUS_0;
        case 1: return STATUS_1;
        default: return STATUS_ERROR;
        }
    }
}
public static class DirectTypeMappedEnumerationTestLibrary {
    public native Enumeration returnInt32Argument(Enumeration e);
    static {
        DefaultTypeMapper mapper = new DefaultTypeMapper();
        mapper.addTypeConverter(Enumeration.class, new TypeConverter() {
            public Object toNative(Object arg, ToNativeContext ctx) {
                return new Integer(((Enumeration)arg).getCode());
            }
            public Object fromNative(Object value, FromNativeContext context) {
                return Enumeration.fromCode(((Integer)value).intValue());
            }
            public Class nativeType() {
                return Integer.class;
            }
        });
        Map options = new HashMap();
        options.put(Library.OPTION_TYPE_MAPPER, mapper);

        Native.register(NativeLibrary.getInstance("testlib", options));
    }
}
public void testEnumerationConversion() {
    DirectTypeMappedEnumerationTestLibrary lib = new DirectTypeMappedEnumerationTestLibrary();
    Enumeration e = lib.returnInt32Argument(Enumeration.STATUS_1);
    assertEquals("Failed to convert enumeration", Enumeration.STATUS_1, e);
}

java -cp build/classes:build/test-classes:lib/junit.jar -Djna.library.path=build/native-darwin com.sun.jna.DirectTypeMapperTest
..E.....
Time: 0.06
There was 1 error:
1) testEnumerationConversion(com.sun.jna.DirectTypeMapperTest)java.lang.ExceptionInInitializerError
    at com.sun.jna.DirectTypeMapperTest.testEnumerationConversion(DirectTypeMapperTest.java:225)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at com.sun.jna.DirectTypeMapperTest.main(DirectTypeMapperTest.java:231)
Caused by: java.lang.IllegalArgumentException: Unsupported Structure field type class com.sun.jna.DirectTypeMapperTest$Enumeration
    at com.sun.jna.Structure$FFIType.get(Structure.java:1843)
    at com.sun.jna.Structure$FFIType.get(Structure.java:1798)
    at com.sun.jna.Native.register(Native.java:1447)
    at com.sun.jna.Native.register(Native.java:1175)
    at com.sun.jna.DirectTypeMapperTest$DirectTypeMappedEnumerationTestLibrary.<clinit>(DirectTypeMapperTest.java:221)
    ... 18 more

FAILURES!!!
Tests run: 7,  Failures: 0,  Errors: 1