Setting outer variable from anonymous inner class

Question

Is there any way to access caller-scoped variables from an anonymous inner class in Java?

Here\'s the sample code to understand what I need:

public L         


        

Answer 1

This situation arises a lot in Java, and the cleanest way to handle it is with a simple value container class. It's the same type thing as the array approach, but it's cleaner IMO.

public class ValContainer<T> {
    private T val;

    public ValContainer() {
    }

    public ValContainer(T v) {
        this.val = v;
    }

    public T getVal() {
        return val;
    }

    public void setVal(T val) {
        this.val = val;
    }
}

Answer 2

The standard solution to this is to return a value. See, for instance, ye olde java.security.AccessController.doPrivileged.

So the code would look something like this:

public Long getNumber(
    final String type, final String refNumber, final Long year
) throws ServiceException {
    try {
        Session session = PersistenceHelper.getSession();
        return session.doWork(new Work<Long>() {
            public Long execute(Connection conn) throws SQLException {
                CallableStatement st = conn.prepareCall("{ CALL PACKAGE.procedure(?, ?, ?, ?) }");
                try {
                    st.setString(1, type);
                    st.setString(2, refNumber);
                    st.setLong(3, year);
                    st.registerOutParameter(4, OracleTypes.NUMBER);
                    st.execute();
                    return st.getLong(4);
                } finally {
                    st.close();
                }
            }
        });
    } catch (Exception e) {
        throw ServiceException(e);
    }
}

(Also fixed the potential resource leak, and returning null for any error.)

Update: So apparently Work is from a third-party library and can't be altered. So I suggest not using it, at least isolate your application from so that you are not using it directly. Something like:

public interface WithConnection<T> {
    T execute(Connection connnection) throws SQLException;
}
public class SessionWrapper {
    private final Session session;
    public SessionWrapper(Session session) {
        session = nonnull(session);
    }
    public <T> T withConnection(final WithConnection<T> task) throws Service Exception {
        nonnull(task);
        return new Work() {
            T result;
            {
                session.doWork(this);
            }
            public void execute(Connection connection) throws SQLException {
                result = task.execute(connection);
            }
        }.result;
    }
}

Answer 3

If the containing class is MyClass -->

MyClass.this.variable = value;

Do not remember if this would work with a private variable (I think it would work).

Only works for attributes of the class (class variable). Does not work for method local variables. In JSE 7 probably there will be closures to do that kind of thing.