Preserving nullability contracts in runtime

[denis-zhdanov]

Thus PR does two things:

• configures the build to generate null-checks based on NonNull method parameters/return values annotations
• removes explicit null-checks

The above is achieved by the Traute javac plugin - it enhances compilation in a way to insert target checks into generated bytecode.

Let's consider RxMenuItem.actionViewEvents():

Source code

@CheckResult @NonNull
public static Observable<MenuItemActionViewEvent> actionViewEvents(@NonNull MenuItem menuItem) {
  return new MenuItemActionViewEventObservable(menuItem, Functions.PREDICATE_ALWAYS_TRUE);
}

Resulting bytecode looks like if it's compiled from the source below:

@CheckResult @NonNull
public static Observable<MenuItemActionViewEvent> actionViewEvents(@NonNull MenuItem menuItem) {
  if (munuItem == null) {
    throw new NullPointerException("menuItem == null");
  }
  Observable<MenuItemActionViewEvent> tmpTrauteVar1 = new MenuItemActionViewEventObservable(menuItem, Functions.PREDICATE_ALWAYS_TRUE);
  if (tmpTrauteVar1 == null) {
    throw new NullPointerException("Detected an attempt to return null from a method com.jakewharton.rxbinding2.view.RxMenuItem.actionViewEvents() marked by @android.support.annotation.NonNull");
  }
}

Bytecode

javap -c ./rxbinding/build/intermediates/classes/release/com/jakewharton/rxbinding2/view/RxMenuItem.class
...
  public static io.reactivex.Observable<com.jakewharton.rxbinding2.view.MenuItemActionViewEvent> actionViewEvents(android.view.MenuItem);
    Code:
       0: aload_0
       1: ifnonnull     14
       4: new           #1                  // class java/lang/NullPointerException
       7: dup
       8: ldc           #2                  // String menuItem == null
      10: invokespecial #3                  // Method java/lang/NullPointerException."<init>":(Ljava/lang/String;)V
      13: athrow
      14: new           #9                  // class com/jakewharton/rxbinding2/view/MenuItemActionViewEventObservable
      17: dup
      18: aload_0
      19: getstatic     #5                  // Field com/jakewharton/rxbinding2/internal/Functions.PREDICATE_ALWAYS_TRUE:Lio/reactivex/functions/Predicate;
      22: invokespecial #10                 // Method com/jakewharton/rxbinding2/view/MenuItemActionViewEventObservable."<init>":(Landroid/view/MenuItem;Lio/reactivex/functions/Predicate;)V
      25: astore_1
      26: aload_1
      27: ifnonnull     40
      30: new           #1                  // class java/lang/NullPointerException
      33: dup
      34: ldc           #11                 // String Detected an attempt to return null from a method com.jakewharton.rxbinding2.view.RxMenuItem.actionViewEvents() marked by @android.support.annotation.NonNull
      36: invokespecial #3                  // Method java/lang/NullPointerException."<init>":(Ljava/lang/String;)V
      39: athrow
      40: aload_1
      41: areturn

Please note that Traute is configured in the same way as the explicit checks:

• it throws NullPointerException from failed checks (default behavior)
• error message pattern is PARAMETER_NAME == null