Unloading LambdaForm bytecode

[Martin Traverso]

Do the generated bytecodes for LambdaForms ever get unloaded? In Presto (a
distributed SQL engine) we generate bytecode on the fly for each query and
use invokedynamic in a number of places as a way to link constants. We
recently ran into a permgen leak (we run 7u45). Looking at the output of
-verbose:class, I noticed a bunch of lines like these:

    [Loaded java.lang.invoke.LambdaForm$MH227/995942612 from
java.lang.invoke.LambdaForm]
    [Loaded java.lang.invoke.LambdaForm$MH228/489675590 from
java.lang.invoke.LambdaForm]
    [Loaded java.lang.invoke.LambdaForm$MH229/2095286826 from
java.lang.invoke.LambdaForm]
    [Loaded java.lang.invoke.LambdaForm$MH230/538456879 from
java.lang.invoke.LambdaForm]
    [Loaded java.lang.invoke.LambdaForm$MH231/1550961803 from
java.lang.invoke.LambdaForm]
    ...

We load our generated bytecode in standalone classloaders and it eventually
gets unloaded. I never see an unload message for the LambdaForm classes,
though.

This is what’s inside one of those classes. They all look similar:

    > javap -c 'LambdaForm$MH499.class'

    Compiled from "LambdaForm$MH499"
    final class java.lang.invoke.LambdaForm$MH499 extends
java.lang.invoke.LambdaForm {
      static java.lang.Object identity(java.lang.Object);
        Code:
           0: aload_0
           1: checkcast     #12                 // class
java/lang/invoke/BoundMethodHandle$Species_L
           4: getfield      #16                 // Field
java/lang/invoke/BoundMethodHandle$Species_L.argL0:Ljava/lang/Object;
           7: astore_1
           8: ldc           #18                 // String
CONSTANT_PLACEHOLDER_0 <<MethodHandle(Object)Object>>
          10: checkcast     #20                 // class
java/lang/invoke/MethodHandle
          13: aload_1
          14: invokevirtual #23                 // Method
java/lang/invoke/MethodHandle.invokeBasic:(Ljava/lang/Object;)Ljava/lang/Object;
          17: areturn

      static void dummy();
        Code:
           0: ldc           #27                 // String
identity=Lambda(a0:L)=>{\n    t1:L=Species_L.argL0(a0:L);\n
 t2:L=ValueConversions.identity(t1:L);t2:L}
           2: pop
           3: return
    }
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