\htmlhr

% Arguments are: class name, formal parameter list
\def\reflectionAnno#1#2{\refqualclass{common/reflection/qual}{#1}\code{#2}}


\chapterAndLabel{Reflection resolution}{reflection-resolution}

A call to
\sunjavadoc{java.base/java/lang/reflect/Method.html\#invoke(java.lang.Object,java.lang.Object...)}{Method.invoke}
might reflectively invoke any method.  That method might place requirements
on its formal parameters, and it might return any value.  To reflect these
facts, the annotated JDK contains
conservative annotations for \<Method.invoke>.
These conservative library annotations often cause a checker to issue false
positive warnings when type-checking code that uses reflection.

If you supply the \<-AresolveReflection> command-line option, the Checker
Framework attempts to resolve reflection.  At each call to \<Method.invoke>
or \<Constructor.newInstance>, the Checker Framework first soundly estimates
which methods might be invoked at run time.  When type-checking the call, the
Checker Framework uses the library annotations for the possibly-invoked
methods, rather than the imprecise one for \<Method.invoke>.

If the estimate of invoked methods is small,
the checker issues fewer false positive warnings.
If the estimate of invoked methods is large, these types may be no better than the
conservative library annotations.

Reflection resolution is disabled by default, because it increases the time
to type-check a program.
% TODO: By how much, and how and when was that computed?
You should enable reflection resolution with the \<-AresolveReflection>
command-line option if, for some call site of \<Method.invoke> or
\<Constructor.newInstance> in your program:
\begin{enumerate}
\item
  the conservative library annotations on \<Method.invoke> or
  \<Constructor.newInstance> cause false positive warnings,
\item
  the set of possibly-invoked methods or constructors can be known at
  compile time,
  and
\item
  the reflectively invoked methods/constructors are on the class path at
  compile time.
\end{enumerate}

Reflection resolution does not change your source code or generated code.
In particular, it does not replace the \<Method.invoke> or
\<Constructor.newInstance> calls.

The command-line option \<-AresolveReflection=debug> outputs verbose
information about the reflection resolution process, which may be useful
for debugging.

Section~\ref{reflection-examples} gives an example of reflection resolution.
Then,
Section~\ref{methodval-and-classval-checkers} describes the MethodVal
and ClassVal Checkers, which reflection resolution uses internally.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\sectionAndLabel{Reflection resolution example}{reflection-examples}

Consider the following example, in which the Nullness Checker employs
reflection resolution to avoid issuing a false positive warning.

\begin{Verbatim}
public class LocationInfo {
    @NonNull Location getCurrentLocation() {  ...  }
}

public class Example {
    LocationInfo privateLocation = ... ;
    String getCurrentCity() throws Exception {
        Method getCurrentLocationObj = LocationInfo.class.getMethod("getCurrentLocation");
        Location currentLocation = (Location) getCurrentLocationObj.invoke(privateLocation);
        return currentLocation.nameOfCity();
    }
}
\end{Verbatim}

When reflection resolution is not enabled, the Nullness Checker uses conservative
annotations on the \<Method.invoke> method signature:

\quad \<{\bfseries @Nullable} Object invoke({\bfseries @NonNull} Object recv, {\bfseries @NonNull} Object ... args)>


This causes the Nullness Checker to issue the following warning even though
\<currentLocation> cannot be null.

\begin{Verbatim}
error: [dereference.of.nullable] dereference of possibly-null reference currentLocation
        return currentLocation.nameOfCity();
               ^
1 error
\end{Verbatim}

\begin{sloppypar}
When reflection resolution is enabled, the MethodVal Checker infers that the \<@MethodVal> annotation for \<getCurrentLocationObj>  is:
\end{sloppypar}

\quad \<@MethodVal(className="LocationInfo", methodName="getCurrentLocation", params=0)>

Based on this \<@MethodVal> annotation, the reflection resolver determines that
the reflective method call represents a call to \<getCurrentLocation> in class
\<LocationInfo>.
The reflection resolver uses this information to provide the
following precise procedure summary to the Nullness Checker, for this
call site only:

\quad \<{\bfseries @NonNull} Object invoke({\bfseries @NonNull} Object recv, {\bfseries @Nullable} Object ... args)>

Using this more precise signature, the Nullness Checker does not issue the false positive warning shown above.


\sectionAndLabel{MethodVal and ClassVal Checkers}{methodval-and-classval-checkers}

The implementation of reflection resolution internally uses the ClassVal
Checker (Section~\ref{classval-checker}) and the MethodVal Checker
(Section~\ref{methodval-checker}).  They are similar to the Constant
Value Checker (Section~\ref{constant-value-checker}) in that their
annotations estimate the run-time value of an expression.

In some cases, you may need to write annotations such as
\refqualclass{common/reflection/qual}{ClassVal},
\refqualclass{common/reflection/qual}{MethodVal},
\refqualclass{common/value/qual}{StringVal}, and
\refqualclass{common/value/qual}{ArrayLen} to aid in reflection
resolution.
Often, though, these annotations can be inferred
(Section~\ref{methodval-and-classval-inference}).


\subsectionAndLabel{ClassVal Checker}{classval-checker}

The ClassVal Checker defines the following annotations:

\begin{description}
\item[\reflectionAnno{ClassVal}{(String[] value)}]
If an expression has \<@ClassVal> type with a single argument,
then its exact run-time value is known at compile time.
For example, \<@ClassVal("java.util.HashMap")>
indicates that the \<Class> object represents the \<java.util.HashMap> class.

If multiple arguments are given, then the expression's run-time value is
known to be in that set.

The arguments are binary names
(\href{https://docs.oracle.com/javase/specs/jls/se11/html/jls-13.html#jls-13.1}{JLS \S 13.1}).

\item[\reflectionAnno{ClassBound}{(String[] value)}]
If an expression has \<@ClassBound> type, then its run-time value is known
to be upper-bounded by that type.
For example,
\<@ClassBound("java.util.HashMap")> indicates that the \<Class> object
represents \<java.util.HashMap> or a subclass of it.

If multiple arguments are given, then the run-time value is equal to or a
subclass of some class in that set.

The arguments are binary names
(\href{https://docs.oracle.com/javase/specs/jls/se11/html/jls-13.html#jls-13.1}{JLS \S 13.1}).

\item[\reflectionAnno{UnknownClass}{}] Indicates that there is no
  compile-time information about the run-time value of the class --- or
  that the Java type is not \<Class>.
  This is the default qualifier, and it may not be written in source code.

\item[\reflectionAnno{ClassValBottom}{}] Type given to the \<null> literal.
  It may not be written in source code.
\end{description}
\begin{figure}
\includeimage{classval}{6cm}
\caption{Partial type hierarchy for the ClassVal type system. The type qualifiers in gray (\<@UnknownClass>
and \<@ClassValBottom>) should never be written in source code; they are used internally by the type system.}
\label{fig-classval-hierarchy}
\end{figure}

\subsubsectionAndLabel{Subtyping rules}{classval-subtyping-rules}
Figure~\ref{fig-classval-hierarchy} shows part of the type hierarchy of  the
ClassVal type system.
\<@ClassVal(A)> is a subtype of \<@ClassVal(B)> if A is a subset of B.
\<@ClassBound(A)> is a subtype of \<@ClassBound(B)> if A is a subset of B.
\<@ClassVal(A)> is a subtype of \<@ClassBound(B)> if A is a subset of B.


\subsectionAndLabel{MethodVal Checker}{methodval-checker}

The MethodVal Checker defines the following annotations:

\begin{description}
\item[\reflectionAnno{MethodVal}{(String[] className, String[] methodName, int[] params)}]
Indicates that an expression of type \<Method> or \<Constructor> has a
run-time value in a given set.  If the set has size $n$, then each of
\<@MethodVal>'s arguments is an array of size $n$, and the $i$th method in the set is
represented by \{ className[i], methodName[i], params[i] \}.
For a constructor, the method name is ``\code{<init>}''.
% to avoid an additional annotation for constructors.

Consider the following example:

\begin{Verbatim}
@MethodVal(className={"java.util.HashMap", "java.util.HashMap"},
           methodName={"containsKey", "containsValue"},
           params={1, 1})
\end{Verbatim}

\noindent
This \<@MethodVal> annotation indicates that the \<Method>
is either \<HashMap.containsKey> with 1 formal parameter or
\<HashMap.containsValue> with 1 formal parameter.

The \<@MethodVal> type qualifier indicates the number of
parameters that the method takes, but not their type.  This means that the
Checker Framework's reflection resolution cannot distinguish among
overloaded methods.

\item[\reflectionAnno{UnknownMethod}{}] Indicates that there is no
  compile-time information about the run-time value of the method --- or
  that the Java type is not \<Method> or \<Constructor>.
  This is the default qualifier, and it may not be written in source code.

\item[\reflectionAnno{MethodValBottom}{}] Type given to the \<null> literal.
  It may not be written in source code.
\end{description}

\begin{figure}
\includeimage{methodval}{3.5cm}
\caption{Partial type hierarchy for the MethodVal type system. The type qualifiers in gray (\<@UnknownMethod>
and \<@MethodValBottom>) should never be written in source code; they are used internally by the type system.}
\label{fig-methodval-hierarchy}
\end{figure}

\subsubsectionAndLabel{Subtyping rules}{methodval-subtyping-rules}
Figure~\ref{fig-methodval-hierarchy} shows part of the type hierarchy of  the
MethodVal type system.  \<@MethodVal(classname=CA, methodname=MA, params=PA)> is a subtype of
\<@MethodVal(classname=CB, methodname=MB, params=PB)> if

\[\forall \textrm{indexes} i \exists \textrm{an index} j:  CA[i] = CB[j], MA[i] = MA[j], and PA[i] = PB[j]\]

\noindent
where CA, MA, and PA are lists of equal size and CB, MB, and PB are lists of equal size.



\subsectionAndLabel{MethodVal and ClassVal inference}{methodval-and-classval-inference}

The developer rarely has to write \<@ClassVal> or \<@MethodVal>
annotations, because the Checker Framework infers them according to
Figure~\ref{fig:reflection-inference}.  Most readers can skip this
section, which explains the inference rules.

\input{reflection-inference-rules}

The ClassVal Checker infers the exact class name (\<@ClassVal>) for a
\<Class> literal (\<C.class>), and for a static method call (e.g.,
\<Class.forName(arg)>, \<ClassLoader.loadClass(arg)>, ...) if the argument is a
statically computable expression.  In contrast, it infers an upper bound
(\<@ClassBound>) for instance method calls (e.g., \<obj.getClass()>).

The MethodVal Checker infers \<@MethodVal> annotations for \<Method> and
\<Constructor> types that have been created using a method call to Java's Reflection
API\@:
\begin{itemize}
    \item \code{Class.getMethod(String name, Class<?>...~paramTypes)}
    \item \code{Class.getConstructor(Class<?>...~paramTypes)}
\end{itemize}

Note that an exact class name is necessary to precisely resolve
reflectively-invoked constructors since a constructor in a subclass does not
override a constructor in its superclass. This means that the MethodVal Checker
does not infer a \<@MethodVal> annotation for \<Class.getConstructor> if the
type of that class is \<@ClassBound>. In contrast, either an exact class name or a bound
is adequate to resolve reflectively-invoked methods because of the subtyping
rules for overridden methods.


%%  LocalWords:  java AresolveReflection newInstance MethodVal ClassVal
%%  LocalWords:  StringVal ArrayLen jls ClassBound UnknownClass params
%%  LocalWords:  ClassValBottom className methodName init containsKey arg
%%  LocalWords:  containsValue UnknownMethod MethodValBottom classname
%%  LocalWords:  methodname forName ClassLoader loadClass getClass recv
%%  LocalWords:  getMethod paramTypes getConstructor args currentLocation
%%  LocalWords:  getCurrentLocationObj LocationInfo getCurrentLocation
%%  LocalWords:  methodval classval
