[jira] [Created] (THRIFT-2556) TUnion copy constructor throws in some unexpected cases

[JIRA]

Jeremiah Menétrey created THRIFT-2556:
-----------------------------------------

             Summary: TUnion copy constructor throws in some unexpected cases
                 Key: THRIFT-2556
                 URL: https://issues.apache.org/jira/browse/THRIFT-2556
             Project: Thrift
          Issue Type: Bug
          Components: Java - Library
    Affects Versions: 0.7
         Environment: Java 1.7
            Reporter: Jeremiah Menétrey


The copy constructor of {{TUnion}}
{code}protected TUnion(TUnion<T, F> other) { ... {code}
throws when the instantiated class and the copied one are not exactly the same.

This prevents using the copy constructor pattern in some cases, in particular 
when used for wrapping thrift struct objects. E.g.:

Let's say the following {{union}} class is generated:
{code}public class MyUnion extends TUnion<MyUnion, MyUnion._Fields> { ... {code}

Then the following wrapper class (that redefines some behavior) will compile 
but throw upon instantiation:
{code}
public class MyWrapper extends MyUnion {
  public MyWrapper(MyUnion wrapper) {
    super(wrapper); // Throws a ClassCastException
  }

  @Override
  public String toString() {
    // Generates a better String representation for the use-case
    return "foobar";
  }
}
{code}

Also, the idiomatic constructor of the copy constructor pattern:
{code}public MyWrapper(MyWrapper other) { ...{code}
won't work if the provided instance is a subclass of {{MyWrapper}}.

I don't think that the goal was to prevent subclassing, as generated {{TUnion}} 
subclasses are not {{final}}.

Maybe using {{Class.isInstance(Object)}} or {{Class.isAssignableFrom(Class)}} 
in the copy constructor of {{TUnion}} instead of checking for strict equality 
would be a solution, but I am not sure of all the implications of doing so for 
the rest of the framework.



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