Q:

How can I define in clojure a java class during runtime and instantiate it

I have tried to use gen-and-load-class from clojure.core and then use custom class loader to call defineClass with generated bytecode, but when I call 
(foo.bar.MyClass.)

I'm getting
CompilerExceptionjava.lang.NoClassDefFoundError: Could not initialize class foo.bar.MyClass

UPDATE:
So I used deftype as suggested by @Elogent :
(defprotocol Struct
  (getX [this path] "Get value")
  (setX [this ^long value path] "Get value"))
(deftype Foo 
  [
  ^{:tag long :unsynchronized-mutable true} a 
  ^{:tag long :unsynchronized-mutable true} b 
  ^{:tag long :unsynchronized-mutable true} c]
  Struct
  (getX
    [this [head & tail]]
    (let [field (condp = head
          'a a
          'b b
          'c c)]
      (if (empty? tail)
        field
        (getX field tail))))
  (setX
    [this value [head & tail]]

    (if (empty? tail)
        (condp = head
          (set! a (long value))
          (set! b (long value))
          (set! c (long value)))
        (condp = head
          (setX a value tail)
          (setX b value tail)
          (setX c value tail)))))

After AOT when I do javap Foo.class I got:
public final class struct.core.Foo implements struct.core.Struct,clojure.lang.IType {
  public static final clojure.lang.Var const__0;
  public static final java.lang.Object const__1;
  public static final clojure.lang.Var const__2;
  public static final java.lang.Object const__3;
  public static final clojure.lang.Var const__4;
  public static final clojure.lang.AFn const__5;
  public static final clojure.lang.AFn const__6;
  public static final clojure.lang.AFn const__7;
  public static final clojure.lang.Var const__8;
  public static final clojure.lang.Var const__9;
  public static final clojure.lang.Var const__10;
  public static final clojure.lang.Var const__11;
  public static final clojure.lang.Var const__12;
  long a;
  long b;
  long c;
  public static {};
  public struct.core.Foo(long, long, long);
  public static clojure.lang.IPersistentVector getBasis();
  public java.lang.Object setX(java.lang.Object, java.lang.Object);
  public java.lang.Object getX(java.lang.Object);
}

This is exactly what I need. Thank you @Elogent!

A:

Unless you absolutely need all the flexibility of a Java class, I would advise that you use deftype instead. As explained here, deftype gives you access to lower-level constructs such as primitive types and mutability in an idiomatic way.

