In case of OpendJDK (and JDKs built atop it), the LambdaMetaFactory generates a mostly ordinary class file (just accessing private member(s) of another class) and uses a special method in sun.misc.Unsafe, to create an anonymous class.
Creating a similar class file accessing a field, is straight-forward and creating an anonymous class with it, does work, as can be demonstrated with the following quick&dirty program:
public class Generator {
public static void main(String[] args) throws Throwable {
ToIntFunction<Thread> ft=generateIntFieldAccessor(Thread.class, "threadStatus");
System.out.println(ft.applyAsInt(Thread.currentThread()));
}
private static <X> ToIntFunction<X> generateIntFieldAccessor(
Class<? super X> c, String name) throws Throwable {
byte[] code = Generator.generateIntReaderCode(c.getDeclaredField(name));
Class<?> unsafe = Class.forName("sun.misc.Unsafe");
Field u = unsafe.getDeclaredField("theUnsafe");
u.setAccessible(true);
Object theUnsafe = u.get(null);
Class<ToIntFunction<X>> gen = (Class<ToIntFunction<X>>)
MethodHandles.publicLookup().bind(theUnsafe, "defineAnonymousClass",
MethodType.methodType(
Class.class, Class.class, byte[].class, Object[].class))
.invokeExact(c, code, (Object[])null);
return gen.getConstructor().newInstance();
}
private static final String HEAD = "Êþº¾\0\0\0004\0\24\7\0\21\7\0\t\7\0\n\7\0\22"
+ "\n\0\2\0\6\f\0\13\0\f\t\0\4\0\b\f\0\23\0\20\1\0\20java/lang/Object\1\0\40"
+ "java/util/function/ToIntFunction\1\0\6<init>\1\0\3()V\1\0\4Code\1\0\n"
+ "applyAsInt\1\0\25(Ljava/lang/Object;)I\1\0\1I";
private static final String TAIL = "\0001\0\1\0\2\0\1\0\3\0\0\0\2\0\1\0\13\0\f\0"
+ "\1\0\r\0\0\0\21\0\1\0\1\0\0\0\5*·\0\5±\0\0\0\0\0\21\0\16\0\17\0\1\0\r\0\0"
+ "\0\24\0\1\0\2\0\0\0\b+À\0\4´\0\7¬\0\0\0\0\0\0";
public static byte[] generateIntReaderCode(Field f) {
return new ByteArrayOutputStream(HEAD.length() + TAIL.length() + 100) {
@SuppressWarnings("deprecation") byte[] get() {
HEAD.getBytes(0, count = HEAD.length(), buf, 0);
try(DataOutputStream dos = new DataOutputStream(this)) {
String decl = f.getDeclaringClass().getName().replace('.', '/');
dos.writeByte(1); dos.writeUTF(decl+"$"+f.getName()+"$access");
dos.writeByte(1); dos.writeUTF(decl);
dos.writeByte(1); dos.writeUTF(f.getName());
} catch (IOException ex) {
throw new UncheckedIOException(ex);
}
int dynSize = count;
byte[] result = Arrays.copyOf(buf, dynSize + TAIL.length());
TAIL.getBytes(0, TAIL.length(), result, dynSize);
return result;
}
}.get();
}
}
Demo on Ideone
Of course, for production code you should better resort to one of the commonly used code generation libraries, to have maintainable factory code. E.g., OpenJDK’s LambdaMetaFactory uses the ASM library under the hood.
If your attempt to implement a similar solution failed, you have to post what you’ve tried, so we can help identifying the problem. But perhaps, knowing that it is possible in general, does already help you.
