Reducing "exception is never thrown in body of corresponding try statement" Error to a Warning

[Brian Goetz]

I will admit that this strictness is sometimes annoying.  

Your goal here, which is a good one, is to allow narrowing the set of checked exceptions to be a source-compatible change.  I seriously doubt much consideration was given in 1995 for the source-compatibility of _removing_ exceptions from a descriptor, even though this should be a compatible change (as it narrows the range of accepted effects.)  You correctly argue that this would be a compatible change for clients that hadn’t gotten the memo about the narrowed exception set.  

Unfortunately, there’s another group we have to worry about — subclasses.  Because, if you have a hierarchy:

    class Foo {
        void m() throws Exception { };
    }

    class Bar extends Foo {
        @Override
        void m() throws Exception { };
    }

where Foo and Bar are in separate maintenance domains, then reducing the set of exceptions in Foo:

    class Foo {
        void m() throws IOException { };
    }

now renders Bar uncompilable, because it throws a wider set of exceptions than the method it overrides (violating LSP) and therefore reducing the set of exceptions for overridable methods is not a source-compatible change for this group.  

(General observation: It’s really easy to forget the “subclasses” part of compatibility — it’s easy to focus only on the clients.  This has bitten me more than once.)  

Given that this change still would not allows us to say reducing the set of thrown exceptions is a source-compatible change, I’d think that the benefit is reduced somewhat?


> On Oct 20, 2019, at 6:36 PM, Mike Duigou <openjdk at duigou.org> wrote:
> 
> Hello all;
> 
> As part of my Oracle CodeOne talk "Exceptions 2020" I made two proposals for improving Java exceptions. One of the proposals was to remove the compile time error produced when and exception is caught that is not thrown by the enclosed statements. Specifically I am proposing the removal of the following section from JLS 11.2.3
> 
>    "It is a compile-time error if a catch clause can catch checked exception class E1
>     and it is not the case that the try block corresponding to the catch clause can
>     throw a checked exception class that is a subclass or superclass of E1, unless E1
>     is Exception or a superclass of Exception."
> 
> The advantage of removal of this rule is that removal allows API authors to better document the actual exceptions thrown by constructors and methods. Vestigial or inaccurate throws specifications can be eliminated without "breaking" calling code. This makes it easier to evolve the API by narrowing (including to empty), the exception specification of method without impacting existing code which uses the API.
> 
> To allow application authors to clean up dead code an equivalent warning would still be provided and probably should be encouraged by the JLS specification. The warning would alert them that the catch clause will not be executed.
> 
> As an API author I have been frustrated many times by being unable to reduce or remove a method's exception specification without causing compatibility issues for my users and I have also been frustrated when libraries did choose to alter their method exception specifications and "broke" my code which caught the no longer thrown exceptions. Does this seem like a feature/change worth pursuing?
> 
> Cheers,
> 
> Mike
> 
> The changes to javac needed to accomplish this change are quite simple:
> 
> diff --git a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java
> index 0c1fead9a4..f7793d0605 100644
> --- a/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java
> +++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Flow.java
> @@ -1271,7 +1277,7 @@ public class Flow {
>             } else if (!chk.isUnchecked(pos, exc) &&
>                     !isExceptionOrThrowable(exc) &&
>                     !chk.intersects(exc, thrownInTry)) {
> -                log.error(pos, Errors.ExceptNeverThrownInTry(exc));
> +                log.warning(pos, Warnings.ExceptNeverThrownInTry(exc));
>             } else {
>                 List<Type> catchableThrownTypes = chk.intersect(List.of(exc), thrownInTry);
>                 // 'catchableThrownTypes' cannnot possibly be empty - if 'exc' was an
> diff --git a/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties b/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties
> index 1f29df0704..e29b5583e4 100644
> --- a/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties
> +++ b/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties
> @@ -533,10 +533,6 @@ compiler.err.error.reading.file=\
> compiler.err.except.already.caught=\
>     exception {0} has already been caught
> 
> -# 0: type
> -compiler.err.except.never.thrown.in.try=\
> -    exception {0} is never thrown in body of corresponding try statement
> -
> # 0: symbol
> compiler.err.final.parameter.may.not.be.assigned=\
>     final parameter {0} may not be assigned
> @@ -2019,6 +2015,14 @@ compiler.warn.unchecked.generic.array.creation=\
> compiler.warn.unchecked.varargs.non.reifiable.type=\
>     Possible heap pollution from parameterized vararg type {0}
> 
> +# 0: type
> +compiler.warn.except.never.thrown.in.try=\
> +    exception {0} is never thrown in body of corresponding try statement
> +
> # 0: symbol
> compiler.warn.varargs.unsafe.use.varargs.param=\
>     Varargs method could cause heap pollution from non-reifiable varargs parameter {0}