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From: Patrick Roemer <sangamon@netcologne.de>
Newsgroups: comp.lang.java.programmer
Subject: Re: Does this make sense?
Date: Fri, 5 Apr 2019 17:43:32 +0200
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Xref: csiph.com comp.lang.java.programmer:38873

Responding to Eric Douglas:
>      public static String testMethod(final String db_connect_string, final String db_userid, final String db_password, final String dbDriver, final String queryText) throws ClassNotFoundException {
>           Class.forName(dbDriver);
>           Connection conn = null;
>           PreparedStatement ps = null;
>           ResultSet rs = null;
>           try {
>                conn = DriverManager.getConnection(db_connect_string, db_userid, db_password);
>                ps = conn.prepareStatement(queryText, ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
>                rs = ps.getResultSet();
>           } catch (final SQLException e) {
>                e.printStackTrace();
>                try {
>                     if (ps != null) {
>                          ps.close();
>                     }
>                     if (conn != null) {
>                          conn.close();
>                     }
>                     conn = DriverManager.getConnection("alternate connect string", db_userid, db_password);
>                     ps = conn.prepareStatement(queryText, ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
>                     rs = ps.getResultSet();
>                } catch (final SQLException e1) {
>                     e1.printStackTrace();
>                }
>           }
>           try {
>                while (rs.next()) {
>                     System.out.println(rs.getRow());
>                }
>           } catch (final SQLException e1) {
>                e1.printStackTrace();
>           } finally {
>                try {
>                     rs.close();
>                } catch (final SQLException e) {
>                     e.printStackTrace();
>                     throw new Error("SQL issue");
>                }
>           }
>           if (ps != null) {
>                try {
>                     ps.close();
>                } catch (final SQLException e) {
>                     e.printStackTrace();
>                     if (conn != null) {
>                          try {
>                               conn.close();
>                          } catch (final SQLException e1) {
>                               e1.printStackTrace();
>                               throw new Error("SQL issue");
>                          }
>                     }
>                     throw new Error("SQL issue");
>                }
>           }
>           if (conn != null) {
>                try {
>                     conn.close();
>                } catch (final SQLException e) {
>                     e.printStackTrace();
>                     throw new Error("SQL issue");
>                }
>           }
>           return null;
>      }

So you want to open a DB connection, run a prepared statement and obtain
a result set. If any of these steps fails along the way, you want to
retry these steps against a different DB URL. Once you have the result
set, you want to run code against it and *not* retry upon failure. And
of course you want to close all resources, no matter what code path has
been taken. Does that capture the intent?

This feels somewhat odd. I can imagine retrying against the mirror when
the primary DB isn't reachable. But if the statement/result fetching
fails on a live primary, why should I hope for better results against
the mirror?

Anyway, just going with it, I'd try to extract exception handling
responsibilities to reusable abstractions that can also be tested
standalone.

Assume you implement the following:

@FunctionalInterface
public interface CheckedFunction<I, O, E extends Exception> {
  O apply(I input) throws E;
}

@FunctionalInterface
public interface CheckedSupplier<O, E extends Exception> {
  O get() throws E;
}

public static <I, O, E extends Exception> O tryAlternatives(
    CheckedFunction<I, O, E> op,
    I firstInput,
    I... moreInputs
) throws E { ... }

public class CumulativeResource<T extends AutoCloseable>
    implements AutoCloseable {
  public final T resource;

  public interface CumulativeResourceBuilder<T extends AutoCloseable> {
    <S extends AutoCloseable> CumulativeResourceBuilder<S>
      cumulate(CheckedFunction<T, S, Exception> builder);
    CumulativeResource<T> build() throws Exception;
  }

  public static <T extends AutoCloseable> CumulativeResourceBuilder<T>
    from(CheckedSupplier<T, Exception> factory) { ... }
}

Then your actual code might become this:

CumulativeResource<ResultSet> fetchResult(String dbUrl)
    throws Exception {
  return
    CumulativeResource
      .from(() -> DriverManager.getConnection(dbUrl, user, password))
      .cumulate(c ->
        c.prepareStatement(
          query,
          ResultSet.TYPE_SCROLL_INSENSITIVE,
          ResultSet.CONCUR_READ_ONLY
        )
      )
      .cumulate(PreparedStatement::executeQuery)
      .build();
    }

void runQuery() throws Exception {
  try(CumulativeResource<ResultSet> rsRes =
      tryAlternatives(this::fetchResult, firstDbUrl, secondDbUrl)) {
    ResultSet rs = rsRes.resource;
    while (rs.next()) {
      System.out.println(rs.getRow());
    }
  }
}

Best regards,
Patrick