When Does the Finally Block Run Relative to the Return

问题

I stumbled across an interesting error yesterday and have since fixed it, but it still was bothering me this morning, so I would like to see if anyone can shed some light on the issue.

The code in question:

final ResultSet rs = prepStatement.executeQuery();
try
{
   if (!rs.next())
   {
      throw new IllegalStateException("Expected non-empty result");
   }
   return rs.getInt(0 + 1);
}
finally
{
   rs.close();
}

Now for the part that doesn't make since. Every once in a while, the return statement will throw an exception indicating that getInt(int) has been called on a closed ResultSet. I verified that the prepared statement is not being closed anywhere in the code, and if the database was closing, I would see other errors as well. This leads me to believe that somehow, occasionally, the finally block is being executed before the return statement. The only thing I can think of is that the hotspot compiler doesn't always get this right. I'm using the Oracle JVM listed below.

java version "1.7.0_45"
Java(TM) SE Runtime Environment (build 1.7.0_45-b18)
Java HotSpot(TM) 64-Bit Server VM (build 24.45-b08, mixed mode)

I feel like I should mention that I have seen the host of other questions about this ordering, but they all seem to indicate that it is set in stone, where I seem to be witnessing something different.

Try-catch-finally-return clarification
https://stackoverflow.com/questions/20164755/the-order-of-invoking-finally-block
Does finally always execute in Java?

回答1:

I wrote and compiled the following class

public class Examples {
    public int answer(PreparedStatement prepStatement) throws SQLException {
        final ResultSet rs = prepStatement.executeQuery();
        try {
            if (!rs.next()) {
                throw new IllegalStateException("Expected non-empty result");
            }
            return rs.getInt(1);
        } finally {
            rs.close();
        }
    }
}

with the following commands

[s_delima@ml-l-sotiriosd Downloads]$ java -version
java version "1.7.0_45"
Java(TM) SE Runtime Environment (build 1.7.0_45-b18)
Java HotSpot(TM) 64-Bit Server VM (build 24.45-b08, mixed mode)
[s_delima@ml-l-sotiriosd Downloads]$ javac Examples.java 
[s_delima@ml-l-sotiriosd Downloads]$ /usr/java/latest/bin/javap -c Examples
Compiled from "Examples.java"
public class Examples {
  public Examples();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public int answer(java.sql.PreparedStatement) throws java.sql.SQLException;
    Code:
       0: aload_1       
       1: invokeinterface #2,  1            // InterfaceMethod java/sql/PreparedStatement.executeQuery:()Ljava/sql/ResultSet;
       6: astore_2      
       7: aload_2       
       8: invokeinterface #3,  1            // InterfaceMethod java/sql/ResultSet.next:()Z
      13: ifne          26
      16: new           #4                  // class java/lang/IllegalStateException
      19: dup           
      20: ldc           #5                  // String Expected non-empty result
      22: invokespecial #6                  // Method java/lang/IllegalStateException."<init>":(Ljava/lang/String;)V
      25: athrow        
      26: aload_2       
      27: iconst_1      
      28: invokeinterface #7,  2            // InterfaceMethod java/sql/ResultSet.getInt:(I)I
      33: istore_3      
      34: aload_2       
      35: invokeinterface #8,  1            // InterfaceMethod java/sql/ResultSet.close:()V
      40: iload_3       
      41: ireturn       
      42: astore        4
      44: aload_2       
      45: invokeinterface #8,  1            // InterfaceMethod java/sql/ResultSet.close:()V
      50: aload         4
      52: athrow        
    Exception table:
       from    to  target type
           7    34    42   any
          42    44    42   any
}

If you follow along with byte code instructions, you will see that at 28, the rs.getInt(1) is invoked and its value is stored at 33. The rs.close() is invoked at 35. The stored value is retrieved at 40 and returned at 41.

What you are experiencing must come from some other point in your code.

来源：https://stackoverflow.com/questions/20380869/when-does-the-finally-block-run-relative-to-the-return

标签

java

try-finally