LimitAndOffsetAdapter.java

/*
 *    Copyright 2016-2023 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       https://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package examples.paging;

import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.function.Function;

import org.mybatis.dynamic.sql.render.RenderingStrategies;
import org.mybatis.dynamic.sql.select.SelectModel;
import org.mybatis.dynamic.sql.select.render.SelectStatementProvider;

/**
 * This adapter modifies the generated SQL by adding a LIMIT and OFFSET clause at the end
 * of the generated SQL.  This can be used to create a paginated query.
 *
 * LIMIT and OFFSET has limited support in relational databases, so this cannot be considered
 * a general solution for all paginated queries (and that is why this adapter lives only in the
 * test source tree and is not packaged with the core library code).
 *
 * I believe it works in MySQL, HSQLDB, and Postgres.
 *
 * <b>Important Note: </b> this adapter is no longer required for limit and offset support as the
 * library now supports limit and offset natively. However, this remains a good example of altering the generated
 * SQL before it is executed.
 *
 * @author Jeff Butler
 */
public class LimitAndOffsetAdapter<R> {
    private final SelectModel selectModel;
    private final Function<SelectStatementProvider, R> mapperMethod;
    private final int limit;
    private final int offset;

    private LimitAndOffsetAdapter(SelectModel selectModel, Function<SelectStatementProvider, R> mapperMethod,
            int limit, int offset) {
        this.selectModel = Objects.requireNonNull(selectModel);
        this.mapperMethod = Objects.requireNonNull(mapperMethod);
        this.limit = limit;
        this.offset = offset;
    }

    public R execute() {
        return mapperMethod.apply(selectStatement());
    }

    private SelectStatementProvider selectStatement() {
        return new LimitAndOffsetDecorator(
                selectModel.render(RenderingStrategies.MYBATIS3));
    }

    public static <R> LimitAndOffsetAdapter<R> of(SelectModel selectModel,
            Function<SelectStatementProvider, R> mapperMethod, int limit, int offset) {
        return new LimitAndOffsetAdapter<>(selectModel, mapperMethod, limit, offset);
    }

    public class LimitAndOffsetDecorator implements SelectStatementProvider {
        private final Map<String, Object> parameters = new HashMap<>();
        private final String selectStatement;

        public LimitAndOffsetDecorator(SelectStatementProvider delegate) {
            parameters.putAll(delegate.getParameters());
            parameters.put("limit", limit);
            parameters.put("offset", offset);

            selectStatement = delegate.getSelectStatement() +
                    " LIMIT #{parameters.limit} OFFSET #{parameters.offset}";
        }

        @Override
        public Map<String, Object> getParameters() {
            return parameters;
        }

        @Override
        public String getSelectStatement() {
            return selectStatement;
        }
    }
}