Java SneakyThrow of exceptions, type erasure

Question

Can someone explain this code?

public class SneakyThrow {


  public static void sneakyThrow(Throwable ex) {
    SneakyThrow.<RuntimeException>sneakyThrowInner(ex);
  }

  private static <T extends Throwable> T sneakyThrowInner(Throwable ex) throws T {
    throw (T) ex;
  }



  public static void main(String[] args) {
    SneakyThrow.sneakyThrow(new Exception());
  }


}

It may seems strange, but this doesn't produce a cast exception, and permits to throw a checked exception without having to declare it in the signature, or to wrap it in an unchecked exception.

Notice that neither sneakyThrow(...) or the main are declaring any checked exception, but the output is:

Exception in thread "main" java.lang.Exception
    at com.xxx.SneakyThrow.main(SneakyThrow.java:20)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

This hack is used in Lombok, with the annotation @SneakyThrow, which permits to throw checked exceptions without declaring them.

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I know it has something to do with type erasure, but i'm not sure to understand every part of the hack.

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Edit: I know that we can insert an Integer in a List<String> and that checked/unchecked exceptions distinction is a compile time feature.

When casting from a non-generic type like List to a generic type like List<XXX> the compiler produces a warning. But it's less common to cast to a generic type directly like (T) ex in the above code.

If you want, the part that seems strange for me is that I understand that inside the JVM a List<Dog> and List<Cat> looks the same, but the above code seems to mean that finally we can also assign a value of type Cat to a variable of type Dog or something like that.

Answer 1

If you compile it with -Xlint you'll get a warning:

c:\Users\Jon\Test>javac -Xlint SneakyThrow.java
SneakyThrow.java:9: warning: [unchecked] unchecked cast
    throw (T) ex;
              ^
  required: T
  found:    Throwable
  where T is a type-variable:
    T extends Throwable declared in method <T>sneakyThrowInner(Throwable)
1 warning

That's basically saying "This cast isn't really checked at execution time" (due to type erasure) - so the compiler reluctantly assumes you're doing the right thing, knowing that it won't actually be checked.

Now it's only the compiler which cares about checked and unchecked exceptions - it's not part of the JVM at all. So once you've got past the compiler, you're home free.

I'd strongly advise you to avoid doing this though.

In many cases there's a "real" check when you're using generics because something uses the desired type - but that's not always the case. For example:

List<String> strings = new ArrayList<String>();
List raw = strings;
raw.add(new Object()); // Haha! I've put a non-String in a List<String>!
Object x = strings.get(0); // This doesn't need a cast, so no exception...