One mystery string in Java

Question

Say I have jar file jarA and jarB.

class A is in jarA:

class A {
  public static final String SQL = "select `col1`, `col2`, ... `col50` from `table`";
}

class B is in jarB:

class B {
    public void loadData() {
        String sql = "some other sql";
        ...
        A a = null;
        sql = a.SQL + " where something = something"; //This actually works, I never thought about it before I saw this code. 
        executeSQL(sql); //jdbc stuff
    }
}

When I run (and later debug) jarB with jarA in the class path, weird thing happened:

1. the sql executed was missing col25, it was like select col1, col2, ..., col24, col26, ..., col50 from ...

2. when I debug it, paused somewhere before the executeSQL(sql) thing, I ran some expressions below:

   A.SQL returns the correct string(with col25)

   a.SQL also returns correctly

   sql is almost correct except that col25 is not there

I added -verbose:class at startup, I can see that class A was indeed loaded from jarA. Then I unzipped jarA, found A.class, reverse engineered it, I can see the string SQL is correct.

I know there's definitely something wrong. But any suggestions where I should look into, or any wild guesses?

Answer 1

SQL is a compile-time constant. That means that, when compiling the class B, the A.SQL string value will be inlined in B's byte-code.

So, my guess is that col25 was missing in A.SQL, B was compiled against this incorrect class, then A.SQL was fixed and recompiled, but B was not recompiled. So its bytecode still contains the incorrect sql string.

Recompile B, or, if SQL is not actually a constant (i.e. a value that will never, ever, ever change), then don't expose it as a public constant. Instead, define a method returning the SQL string, and use that method from the B class.