spotBugs and JDK-8194978: Javac produces dead code for try-with-resource

[Ismael Juma]

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
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