spotBugs and JDK-8194978: Javac produces dead code for try-with-resource (original) (raw)

Vicente Romero vicente.romero at oracle.com
Fri Nov 9 20:31:36 UTC 2018

Hi,

Sorry for the late reply. 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. 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. 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.

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: aload1 17: invokevirtual #6                  // Method java/nio/channels/FileChannel.size:()J 20: pop2 21: aload1 22: ifnull        52 25: aload1 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: aload0  1: iconst0  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: iconst0  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: astore1 16: aload1 17: invokevirtual #6                  // Method java/nio/channels/FileChannel.size:()J 20: pop2 21: aload1 22: ifnull        52 25: aload1 26: invokevirtual #7                  // Method java/nio/channels/FileChannel.close:()V 29: goto          52 32: astore2 33: aload1 34: ifnull        50 37: aload1 38: invokevirtual #7                  // Method java/nio/channels/FileChannel.close:()V 41: goto          50 44: astore3 45: aload2 46: aload3 47: invokevirtual #9                  // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V 50: aload2 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

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