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  1. JDK
  2. JDK-8194978

Javac produces dead code for try-with-resource

    Details

    • Type: Bug
    • Status: Resolved
    • Priority: P3
    • Resolution: Fixed
    • Affects Version/s: 7, 8, 9.0.4, 10
    • Fix Version/s: 11
    • Component/s: tools
    • Subcomponent:
    • Resolved In Build:
      b07
    • CPU:
      generic
    • OS:
      generic

      Description

      FULL PRODUCT VERSION :
      java version "1.8.0_152"
      Java(TM) SE Runtime Environment (build 1.8.0_152-b16)
      Java HotSpot(TM) 64-Bit Server VM (build 25.152-b16, mixed mode)


      ADDITIONAL OS VERSION INFORMATION :
      CYGWIN_NT-10.0 INFORMA-L7T6GPJ 2.9.0(0.318/5/3) 2017-09-12 10:18 x86_64 Cygwin

      A DESCRIPTION OF THE PROBLEM :
      A try-with-resource produces dead code. This results in inability to get to full-code-coverage (as reported by tools like jacoco).

      The root of the problem is the strange "aconst_null"/"astore" that happens which is subseqently put through a "ifnull" checks. Many have noted it, such as this thread:
      https://stackoverflow.com/questions/25615417/try-with-resources-introduce-unreachable-bytecode

      try-with-resource has been broken since introduction (I think) in Java7. Full coverage has never been possible. Using older try-finally techniques (ala Java6) allows for full coverage.


      STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
      compile code and view in javap - output shown in 'expected result'

      EXPECTED VERSUS ACTUAL BEHAVIOR :
      EXPECTED -
      There should be no aconst_null (line 24) or astore 5 (line 25).

      Having it causes the ifnull check (line 47) to be forced and lines 50-57 are unreachable, lines 60-69 unreachable (no exception possible from 50-57), and another forced outcome at line 100 making 103-110 unreachable, and 113-122 unreachable (no exception from 103-110 possible)
      ACTUAL -
       public void dummy() throws java.sql.SQLException;
          Code:
             0: ldc #88 // String 1
             2: astore_1
             3: ldc #89 // String
             5: astore_2
             6: ldc #89 // String
             8: astore_3
             9: aload_0
            10: invokevirtual #90 // Method baz:()V
            13: ldc #88 // String 1
            15: ldc #89 // String
            17: ldc #89 // String
            19: invokestatic #91 // Method java/sql/DriverManager.getConnection:(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/sql/Connection;
            22: astore 4
            24: aconst_null
            25: astore 5
            27: aload_0
            28: invokevirtual #92 // Method foo:()V
            31: aload 4
            33: ifnull 40
            36: aload_0
            37: invokevirtual #93 // Method bar:()V
            40: aload 4
            42: ifnull 135
            45: aload 5
            47: ifnull 72
            50: aload 4
            52: invokeinterface #94, 1 // InterfaceMethod java/sql/Connection.close:()V
            57: goto 135
            60: astore 6
            62: aload 5
            64: aload 6
            66: invokevirtual #96 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
            69: goto 135
            72: aload 4
            74: invokeinterface #94, 1 // InterfaceMethod java/sql/Connection.close:()V
            79: goto 135
            82: astore 6
            84: aload 6
            86: astore 5
            88: aload 6
            90: athrow
            91: astore 7
            93: aload 4
            95: ifnull 132
            98: aload 5
           100: ifnull 125
           103: aload 4
           105: invokeinterface #94, 1 // InterfaceMethod java/sql/Connection.close:()V
           110: goto 132
           113: astore 8
           115: aload 5
           117: aload 8
           119: invokevirtual #96 // Method java/lang/Throwable.addSuppressed:(Ljava/lang/Throwable;)V
           122: goto 132
           125: aload 4
           127: invokeinterface #94, 1 // InterfaceMethod java/sql/Connection.close:()V
           132: aload 7
           134: athrow
           135: aload_0
           136: invokevirtual #97 // Method bak:()V
           139: return
          Exception table:
             from to target type
                50 57 60 Class java/lang/Throwable
                27 40 82 Class java/lang/Throwable
                27 40 91 any
               103 110 113 Class java/lang/Throwable
                82 93 91 any


      ERROR MESSAGES/STACK TRACES THAT OCCUR :
      Code works OK, just produces dead code

      REPRODUCIBILITY :
      This bug can be reproduced always.

      ---------- BEGIN SOURCE ----------
      public void dummy() throws SQLException
      {
      final String dbUrl1 = "1";
      final String username = "";
      final String password = "";
      baz();
              try (Connection conn1 = DriverManager.getConnection(dbUrl1,username,password)) {
               foo();
               if (conn1 != null)
               bar();
              }
              
      bak();
      }
      public void foo() {/**/ }
      public void bar() {/**/ }
      public void baz() {/**/ }
      public void bak() {/**/ }

      ---------- END SOURCE ----------

      CUSTOMER SUBMITTED WORKAROUND :
      switch back to try-finally for autocloseables.

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              • Assignee:
                jlahoda Jan Lahoda
                Reporter:
                webbuggrp Webbug Group
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                Dates

                • Created:
                  Updated:
                  Resolved: