A bug in Mockito will cause this test to fail at runtime with a ClassCastException


The problem

The JDK9 javac fixes a bug ([JDK-8058199] ( that was causing checkcast instructions to sometimes be skipped. Previously javac used the parameter types of a method symbol’s erased type as targets when translating the arguments. In JDK 9, javac has been fixed to use the inferred types as targets. The fix causes additional checkcasts to be generated if the inferred types do not have the same erasure.

The fix breaks Mockito answer strategies that pick types based on the erased method signature’s return type, and causes tests to fail with ClassCastExceptions when compiled with the JDK 9 javac.

This check is a work-around until the Mockito bug is fixed: [mockito#357] (

The affected answer strategies include:

The [RETURNS_DEFAULTS] ( strategy is usually unaffected, because it returns null as the default value of methods that return Object, and unbounded type parameters erase to Object.


class Foo {
  <T> T getFirst(Iterable<T> xs) {
    return xs.iterator().next();
class Test {
  @Mock Foo f;

  public void test() {
    Iterable<Boolean> it = Arrays.asList(false);

The JDK8 javac would have translated when(f.getFirst(it)) as:

INVOKEVIRTUAL Foo.getFirst (Ljava/lang/Iterable;)Ljava/lang/Object;
INVOKESTATIC org/mockito/Mockito.when (Ljava/lang/Object;)Lorg/mockito/stubbing/OngoingStubbing;

The JDK9 javac translates it as:

INVOKEVIRTUAL Foo.getFirst (Ljava/lang/Iterable;)Ljava/lang/Object;
CHECKCAST java/lang/Boolean
INVOKESTATIC org/mockito/Mockito.when (Ljava/lang/Object;)Lorg/mockito/stubbing/OngoingStubbing;

The erased return type of Foo.getFirst is Object, but the inferred return type of getFirst(Iterable<Boolean>) is Boolean. If the answer strategy returns Object the checkcast fails.


Suppress false positives by adding an @SuppressWarnings("MockitoCast") annotation to the enclosing element.