spotBugs and JDK-8194978: Javac produces dead code for try-with-resource
Enrico Olivelli
eolivelli at gmail.com
Fri Jan 25 20:44:11 UTC 2019
Up
Regards
Enrico
Il 10 gen 2019 8:13 PM, "Enrico Olivelli" <eolivelli at gmail.com> ha scritto:
> Kindly pinging on this..
>
> Cheers
> Enrico
>
> Il lun 7 gen 2019, 14:28 Enrico Olivelli <eolivelli at gmail.com> ha scritto:
>
>> Two questions/ideas:
>> 1) Wouldn't it be possible for javac to annotate such code blocks and
>> write into the class file that that code is "synthetic" ? I really
>> don't know where to store such flags, but maybe someone could
>> elaborate more this idea
>> 2) Why isn't javac adding the "$closeResource" method ?
>>
>> Enrico
>>
>> Il giorno ven 9 nov 2018 alle ore 23:01 Alex Buckley
>> <alex.buckley at oracle.com> ha scritto:
>> >
>> > On 11/9/2018 12:31 PM, Vicente Romero wrote:
>> > > My evaluation on this issue is that javac is generating code
>> > > according to the spec, see JLS11 14.20.3.1 [1]. There is an explicit
>> > > check for null on the resource before invoking the close() method.
>> > > This is the null check javac is generating.
>> >
>> > I confirm that the explicit null check on the resource variable is
>> > required, per the translation in
>> > https://docs.oracle.com/javase/specs/jls/se11/html/
>> jls-14.html#jls-14.20.3.1-140
>> > -- it has been specified that way since JLS7.
>> >
>> > > Fix for JDK-8194978 went a bit further trying to eliminate dead code
>> > > for some cases for which javac could statically determine that the
>> > > resource would be different from null. In particular if the resource
>> > > was initialized with a new class expression.
>> >
>> > I confirm it's acceptable to not perform the null check in generated
>> > code if you can prove the resource variable is non-null. Frankly,
>> > though, there is so much that can occur in a "new class expression" --
>> > `new Outer(a(b(c)))<>.new <String>Inner(d(e(f)))` -- that I would be
>> > wary of trying to prove anything.
>> >
>> > > It could be argued that we could try to analyze the body and if we
>> > > find out that a NPE must be thrown in the body of the try, then the
>> > > null check on the resource would be deemed unnecessary. But then we
>> > > can get to an implementation that will be out of sync with the spec
>> > > plus it probably won't cover all cases.
>> >
>> > Right, let's not be too clever.
>> >
>> > Alex
>> >
>> > > Thanks,
>> > > Vicente
>> > >
>> > > [1]
>> > > https://docs.oracle.com/javase/specs/jls/se11/html/
>> jls-14.html#jls-14.20.3.1
>> > >
>> > > On 11/9/18 10:22 AM, Ismael Juma wrote:
>> > >> Any comments on this? Many people are disabling spotBugs when
>> > >> compiling with Java 11 due to this issue:
>> > >>
>> > >> https://github.com/spotbugs/spotbugs/issues/756
>> > >>
>> > >> Ismael
>> > >>
>> > >> On Thu, Sep 13, 2018 at 10:05 PM Ismael Juma <mlists at juma.me.uk
>> > >> <mailto:mlists at juma.me.uk>> wrote:
>> > >>
>> > >> Hi all,
>> > >>
>> > >> JDK-8194978 introduced some changes to the bytecode generated by
>> > >> javac for the try with resource construct. In the following code,
>> > >> it seems to generate a null check on a reference after invoking a
>> > >> method on it:
>> > >>
>> > >> public static void readFileAsString(String path) throws
>> > >> IOException {
>> > >> try (FileChannel fc = FileChannel.open(Paths.get(path)))
>> {
>> > >> fc.size();
>> > >> }
>> > >>
>> > >> }
>> > >>
>> > >> In line 16 to 22 of the bytecode, it looks like we check for null
>> > >> after calling a method on the fc reference:
>> > >>
>> > >> 16: aload_1
>> > >> 17: invokevirtual #6 // Method
>> > >> java/nio/channels/FileChannel.size:()J
>> > >> 20: pop2
>> > >> 21: aload_1
>> > >> 22: ifnull 52
>> > >> 25: aload_1
>> > >> 26: invokevirtual #7 // Method
>> > >> java/nio/channels/FileChannel.close:()V
>> > >>
>> > >> Is this intentional? I ask because this pattern triggers a
>> > >> spotBugs warning since it often implies a bug in user's code:
>> > >>
>> > >> RCN | Nullcheck of fc at line 10 of value previously dereferenced
>> > >> in TryTest.readFileAsString(String, Charset)
>> > >>
>> > >> Note that this works fine in Java versions older than Java 11.
>> > >> Since this spotBugs warning is generally useful, it would be
>> handy
>> > >> if javac did not trigger it. Alternatively, if there's a good way
>> > >> to detect the code that was generated by javac, spotBugs could be
>> > >> updated to ignore it. For reference, this was discussed in the
>> > >> spotBugs issue tracker:
>> > >>
>> > >> https://github.com/spotbugs/spotbugs/issues/756
>> > >>
>> > >> And method bytecode in full:
>> > >>
>> > >> public static void readFileAsString(java.lang.String) throws
>> > >> java.io.IOException;
>> > >> Code:
>> > >> 0: aload_0
>> > >> 1: iconst_0
>> > >> 2: anewarray #2 // class
>> java/lang/String
>> > >> 5: invokestatic #3 // Method
>> > >> java/nio/file/Paths.get:(Ljava/lang/String;[Ljava/lang/
>> String;)Ljava/nio/file/Path;
>> > >> 8: iconst_0
>> > >> 9: anewarray #4 // class
>> > >> java/nio/file/OpenOption
>> > >> 12: invokestatic #5 // Method
>> > >> java/nio/channels/FileChannel.open:(Ljava/nio/
>> file/Path;[Ljava/nio/file/OpenOption;)Ljava/nio/channels/FileChannel;
>> > >> 15: astore_1
>> > >> 16: aload_1
>> > >> 17: invokevirtual #6 // Method
>> > >> java/nio/channels/FileChannel.size:()J
>> > >> 20: pop2
>> > >> 21: aload_1
>> > >> 22: ifnull 52
>> > >> 25: aload_1
>> > >> 26: invokevirtual #7 // Method
>> > >> java/nio/channels/FileChannel.close:()V
>> > >> 29: goto 52
>> > >> 32: astore_2
>> > >> 33: aload_1
>> > >> 34: ifnull 50
>> > >> 37: aload_1
>> > >> 38: invokevirtual #7 // Method
>> > >> java/nio/channels/FileChannel.close:()V
>> > >> 41: goto 50
>> > >> 44: astore_3
>> > >> 45: aload_2
>> > >> 46: aload_3
>> > >> 47: invokevirtual #9 // Method
>> > >> java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
>> > >> 50: aload_2
>> > >> 51: athrow
>> > >> 52: return
>> > >> Exception table:
>> > >> from to target type
>> > >> 16 21 32 Class java/lang/Throwable
>> > >> 37 41 44 Class java/lang/Throwable
>> > >>
>> > >> Thanks,
>> > >> Ismael
>> > >>
>> > >
>>
> --
>
>
> -- Enrico Olivelli
>
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