简要描述mybatis插件实现分页

第一步：在mybatis配置文件中配置拦截器

<plugins>
    <plugin interceptor="x.x.x.x.x.x.xxxInterceptor" />
</plugins>

第二步：实现拦截器

package com.mg.background.common.persistence.interceptor;

import java.util.Properties;

import org.apache.commons.lang3.StringUtils;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.SqlSource;
import org.apache.ibatis.plugin.Intercepts;
import org.apache.ibatis.plugin.Invocation;
import org.apache.ibatis.plugin.Plugin;
import org.apache.ibatis.plugin.Signature;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;

import com.mg.background.common.persistence.Pages;
import com.mg.background.common.persistence.utils.Reflections;

//拦截器注解，拦截器必须实现Interceptor接口
//AbstractInterceptor实现了Interceptor接口
//@Intercepts的参数中为@Signature的数组，表示要拦截的类和类中要拦截的方法
//args为这个方法需要的参数
//MappedStatement.class 为一个节点对象 即<select/><insert/>之类
//RowBounds.class分页对象 他的offset和limit属性表示要展示数据的范围
//ResultHandler.class结果集映射
@Intercepts({ @Signature(type = Executor.class, method = "query", args = { MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class }) })
public class PaginationInterceptor extends AbstractInterceptor {
    private static final long serialVersionUID = 4989671349466153547L;

    @Override
    public Object intercept(Invocation invocation) throws Throwable {
    final MappedStatement mappedStatement = (MappedStatement) invocation.getArgs()[0];
        Object parameter = invocation.getArgs()[1];
        //boundSql为一条sql语句动态运行对象
        //mappedStatement.getBoundSql(parameter);动态计算sql运行的条件
        //即<if/>条件的计算，然后生成要运行的sql
        BoundSql boundSql = mappedStatement.getBoundSql(parameter);
        //得到入参类型
        Object parameterObject = boundSql.getParameterObject();
        //获取分页参数对象
        Pages<Object> pages = null;
        if (parameterObject != null) {
            //利用反射机制访问对象属性
            pages = convertParameter(parameterObject, pages);
        }
        //如果设置了分页对象，则进行分页
        if (pages != null && pages.getPageSize() != -1) {
            if (StringUtils.isBlank(boundSql.getSql())) {
                return null;
            }
            String originalSql = boundSql.getSql().trim();
            //得到总记录数
            pages.setTotal(SQLHelper.getCount(originalSql, null, mappedStatement, parameterObject, boundSql, log));
            //分页查询 本地化对象 修改数据库注意修改实现
            String pageSql = SQLHelper.generatePageSql(originalSql, pages, DIALECT);
            //                if (log.isDebugEnabled()) {
            //                    log.debug("PAGE SQL:" + StringUtils.replace(pageSql, "\n", ""));
            //                }
            invocation.getArgs()[2] = new RowBounds(RowBounds.NO_ROW_OFFSET, RowBounds.NO_ROW_LIMIT);
            BoundSql newBoundSql = new BoundSql(mappedStatement.getConfiguration(), pageSql, boundSql.getParameterMappings(), boundSql.getParameterObject());
            //解决MyBatis 分页foreach 参数失效 start
            if (Reflections.getFieldValue(boundSql, "metaParameters") != null) {
                MetaObject mo = (MetaObject) Reflections.getFieldValue(boundSql, "metaParameters");
                Reflections.setFieldValue(newBoundSql, "metaParameters", mo);
            }
            //解决MyBatis 分页foreach 参数失效 end
            MappedStatement newMs = copyFromMappedStatement(mappedStatement, new BoundSqlSqlSource(newBoundSql));
            invocation.getArgs()[0] = newMs;
        }
        //        }
        return invocation.proceed();
    }

    //绑定插件
    @Override
    public Object plugin(Object target) {
        //如果target有接口返回代理类，没有返回类本身
        //如果代理类wrap方法会绑定一个拦截器，调用plugin的invoke方法
        //将拦截的方法的参数初始化成一个Invocation对象 然后调用intercept方法
        return Plugin.wrap(target, this);
    }

    //初始化拦截器时将配置文件中拦截器下配置的properties设置进来
    @Override
    public void setProperties(Properties properties) {
        super.initProperties(properties);
    }

    private MappedStatement copyFromMappedStatement(MappedStatement ms, SqlSource newSqlSource) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), ms.getId(), newSqlSource, ms.getSqlCommandType());
        builder.resource(ms.getResource());
        builder.fetchSize(ms.getFetchSize());
        builder.statementType(ms.getStatementType());
        builder.keyGenerator(ms.getKeyGenerator());
        if (ms.getKeyProperties() != null) {
            for (String keyProperty : ms.getKeyProperties()) {
                builder.keyProperty(keyProperty);
            }
        }
        builder.timeout(ms.getTimeout());
        builder.parameterMap(ms.getParameterMap());
        builder.resultMaps(ms.getResultMaps());
        builder.cache(ms.getCache());
        return builder.build();
    }

    public static class BoundSqlSqlSource implements SqlSource {
        BoundSql boundSql;

        public BoundSqlSqlSource(BoundSql boundSql) {
            this.boundSql = boundSql;
        }

        public BoundSql getBoundSql(Object parameterObject) {
            return boundSql;
        }
    }
}