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Spring-JDBC extension library. Provides IterableJdbcTemplate and IterableNamedParameterJdbcTemplate that can return query result in lazy mode as iterators
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package com.alexkasko.springjdbc.iterable;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.jdbc.InvalidResultSetAccessException;
import org.springframework.jdbc.core.*;
import org.springframework.jdbc.datasource.DataSourceUtils;
import org.springframework.jdbc.support.JdbcUtils;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.NoSuchElementException;

/**
 * Closable iterator implementation, returns mapped roes from provided result set.
 * Will be closed automatically on exhaustion.
 * Iterator implementation borrowed from guava's {@code AbstractIterator}.
 * NOT thread-safe.
 *
 * @author alexkasko
 * Date: 11/7/12
 */
class PreparedStatementCloseableIterator implements CloseableIterator {
    private static final Log logger = LogFactory.getLog(PreparedStatementCloseableIterator.class);

    private final DataSource ds;
    private final Connection conn;
    private final PreparedStatementCreator psc;
    private final PreparedStatementSetter pss;
    private final PreparedStatement ps;
    private final ResultSet wrappedRs;
    private final ResultSet rsToUse;
    private final RowMapper mapper;

    private enum State {READY, NOT_READY, DONE, FAILED}

    private State state = State.NOT_READY;
    private T next;
    private boolean closed = false;
    private int rowNum = 0;

    /**
     * Constructor
     *
     * @param ds provided here for proper JDBC resources releasing
     * @param conn provided here for proper JDBC resources releasing
     * @param psc provided here for proper JDBC resources releasing
     * @param ps provided here for proper JDBC resources releasing
     * @param wrappedRs provided here for proper JDBC resources releasing
     * @param rsToUse result set to iterate over
     * @param mapper row mapper to use
     */
    PreparedStatementCloseableIterator(DataSource ds, Connection conn, PreparedStatementCreator psc,
                                       PreparedStatementSetter pss, PreparedStatement ps,
                                       ResultSet wrappedRs, ResultSet rsToUse, RowMapper mapper) {
        this.ds = ds;
        this.conn = conn;
        this.psc = psc;
        this.pss = pss;
        this.ps = ps;
        this.wrappedRs = wrappedRs;
        this.rsToUse = rsToUse;
        this.mapper = mapper;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public final T next() {
        if(!hasNext()) throw new NoSuchElementException();
        state = State.NOT_READY;
        return next;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public final boolean hasNext() {
        if(state == State.FAILED) throw new IllegalStateException();
        switch(state) {
            case DONE:
                return false;
            case READY:
                return true;
            default:
        }
        return tryToComputeNext();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void remove() {
        throw new UnsupportedOperationException("remove");
    }

    /**
     * Releases JDBC resources exactly the same way {@code JdbcTemplate} does.
     * See finally clause {@code JdbcTemplate#query(org.springframework.jdbc.core.PreparedStatementCreator, org.springframework.jdbc.core.PreparedStatementSetter, org.springframework.jdbc.core.ResultSetExtractor)}
     * See finally clause {@code JdbcTemplate#execute(org.springframework.jdbc.core.PreparedStatementCreator, org.springframework.jdbc.core.PreparedStatementCallback)}
     * See finally clause {@code JdbcTemplate#query(org.springframework.jdbc.core.PreparedStatementCreator, org.springframework.jdbc.core.PreparedStatementSetter, org.springframework.jdbc.core.ResultSetExtractor)}
     */
    @Override
    public void close() {
        if(closed) return;
        JdbcUtils.closeResultSet(wrappedRs);
        if(pss instanceof ParameterDisposer) {
            ((ParameterDisposer) pss).cleanupParameters();
        }
        if(psc instanceof ParameterDisposer) {
            ((ParameterDisposer) psc).cleanupParameters();
        }
        JdbcUtils.closeStatement(ps);
        DataSourceUtils.releaseConnection(conn, ds);
        closed = true;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean isClosed() {
        return closed;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        if(!closed) logger.warn("GC level message: iterator wasn't closed: [" + this + "]");
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public String toString() {
        final StringBuilder sb = new StringBuilder();
        sb.append("ResultSetCloseableIterator");
        sb.append("{ds=").append(ds);
        sb.append(", conn=").append(conn);
        sb.append(", psc=").append(psc);
        sb.append(", ps=").append(ps);
        sb.append(", wrappedRs=").append(wrappedRs);
        sb.append(", rsToUse=").append(rsToUse);
        sb.append(", mapper=").append(mapper);
        sb.append(", closed=").append(closed);
        sb.append(", rowNum=").append(rowNum);
        sb.append('}');
        return sb.toString();
    }

    /**
     *  Wrapper method for exceptions catching
     *
     * @return next iter value or {@code endOfData()}
     * @throws InvalidResultSetAccessException on result set access problem
     */
    private T computeNext() {
        try {
            return computeNextInternal();
        } catch(SQLException e) {
            throw new InvalidResultSetAccessException(e);
        }
    }

    /**
     * Iterator logic is here. Closes iterator on exhausted result set
     *
     * @return next iter value or {@code endOfData()}
     * @throws SQLException on result set access problem
     */
    private T computeNextInternal() throws SQLException {
        if(closed) return endOfData();
        if(rsToUse.next()) return mapper.mapRow(rsToUse, rowNum++);
        close();
        return endOfData();
    }

    /**
     * borrowed from guava's {@code AbstractIterator}
     */
    private T endOfData() {
        state = State.DONE;
        return null;
    }

    /**
     * borrowed from guava's {@code AbstractIterator}
     */
    private boolean tryToComputeNext() {
        state = State.FAILED; // temporary pessimism
        next = computeNext();
        if(state != State.DONE) {
            state = State.READY;
            return true;
        }
        return false;
    }
}