Mybatis插件原理分析（三）分页插件

在Mybatis中插件最经常使用的是作为分页插件，接下来我们通过实现Interceptor来完成一个分页插件。

虽然Mybatis也提供了分页操作，通过在sqlSession的接口函数中设置RowBounds，给RowBounds设置初值（RowBounds源码）来实现逻辑分页，其实现原理就是通过sql查询所有的结果，并将结果放到List中，然后根据RowBouds的limit和offset数值来返回最后的数据，这种逻辑分页在数据量比较大的情况下对性能是有影响的。虽然我们可以自己写带有分页语句的sql来实现物理分页，如mysql下：select * from table limit 10 offset 1，来获得分页结果，但不同的数据库产品（mysql、oracle、sqlserver和oracle）对应的分页语句并不相同，如果要同时兼容所有的数据库产品，需要开发人员在每个分页sql中都编码几种数据库产品的分页语句，这样的开发效率实在是太低了，Mybatis的插件Interceptor给了我们一种根据数据库类型自动组装sql语句的方法。

PageInterceptor源码如下：

package com.tianjunwei.page;import java.lang.reflect.Constructor;import java.sql.Connection;import java.sql.DatabaseMetaData;import java.util.List;import java.util.Properties;import org.apache.ibatis.executor.Executor;import org.apache.ibatis.mapping.BoundSql;import org.apache.ibatis.mapping.MappedStatement;import org.apache.ibatis.mapping.ParameterMapping;import org.apache.ibatis.mapping.SqlSource;import org.apache.ibatis.plugin.Interceptor;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.session.ResultHandler;import org.apache.ibatis.session.RowBounds;import org.apache.ibatis.mapping.MappedStatement.Builder;import com.tianjunwei.page.dialect.Dialect;import com.tianjunwei.page.dialect.DialectFactory;/* * 分页插件我们只需要拦截Executor的query方法即可，在执行sql语句之前组装新的分页sql语句 */@Intercepts({@Signature(type= Executor.class,method = "query",args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})public class PageInterceptor implements Interceptor{    String dialectClass;    boolean asyncTotalCount = false;    String dataBaseType=null;    public static ThreadLocal<RowBounds> PageRowBounds = new ThreadLocal<RowBounds>();@SuppressWarnings({"rawtypes", "unchecked"})public Object intercept(final Invocation invocation) throws Throwable {//Executor的实现类        final Executor executor = (Executor) invocation.getTarget();        //Executor的query函数的参数        final Object[] queryArgs = invocation.getArgs();        final MappedStatement ms = (MappedStatement)queryArgs[0];        final Object parameter = queryArgs[1];        //rowBounds中有分页语句的limit和offset值        RowBounds rowBounds = (RowBounds)queryArgs[2];                if((PageRowBounds.get() != null)&&(PageRowBounds.get().getLimit() != RowBounds.NO_ROW_LIMIT || PageRowBounds.get().getOffset() != RowBounds.NO_ROW_OFFSET)){        rowBounds = PageRowBounds.get();        }                //如果不需要分页操作，直接返回，rowBounds为默认值时        if(rowBounds.getOffset() == RowBounds.NO_ROW_OFFSET                && rowBounds.getLimit() == RowBounds.NO_ROW_LIMIT){            return invocation.proceed();        }                //根据不同的数据库获取不到的分页方言来        if(dialectClass == null || "".equals(dialectClass)){                //判断数据源选择方言，暂时支持mysql、oracle、postgresql和sql server 2005 2008及2012        Connection connection = executor.getTransaction().getConnection();        DatabaseMetaData databaseMetaData = null;        if(connection != null){        databaseMetaData = connection.getMetaData();        }else {throw new Exception("connection is null");}                 String databaseProductName = databaseMetaData.getDatabaseProductName();        if( dataBaseType == null || "".equals(dataBaseType)){        dataBaseType = databaseProductName;        }        //通过xml方言的配置来获得方言类        if(databaseProductName != null && !("".equals(dataBaseType))){                dialectClass = DialectFactory.getDialectClass(dataBaseType,databaseProductName);                }else{        throw new Exception("the property of dialect is null");        }        setDialectClass(dialectClass);        }        final Dialect dialect;        try {        //初始化分页方言类            Class clazz = Class.forName(dialectClass);            Constructor constructor = clazz.getConstructor(new Class[]{MappedStatement.class, Object.class, RowBounds.class});            dialect = (Dialect)constructor.newInstance(new Object[]{ms, parameter, rowBounds});                } catch (Exception e) {            throw new ClassNotFoundException("Cannot create dialect instance: "+dialectClass,e);        }        final BoundSql boundSql = ms.getBoundSql(parameter);        //创建新的MappedStatement，此时的sql语句已经是符合数据库产品的分页语句        //dialect.getPageSQL()获得分页语句        //dialect.getParameterMappings(), dialect.getParameterObject(),添加了两个参数及其值，两个参数为_limit和_offset        queryArgs[0] = copyFromNewSql(ms,boundSql,dialect.getPageSQL(), dialect.getParameterMappings(), dialect.getParameterObject());        //sql语句的参数集合        queryArgs[1] = dialect.getParameterObject();        //设置为不分页，由新的sql语句进行物理分页        queryArgs[2] = new RowBounds(RowBounds.NO_ROW_OFFSET,RowBounds.NO_ROW_LIMIT);        return invocation.proceed();}//这个方法是用于mybatis接口编程过程中显示的指定分页参数public static void setPage(int pageNumber,int pageSize){RowBounds pageRowBounds = null;if(pageNumber > 0)pageRowBounds = new RowBounds((pageNumber-1)*pageSize, pageSize);else {pageRowBounds = new RowBounds(0, pageSize);}PageRowBounds.set(pageRowBounds);}//创建新的MappedStatement    private MappedStatement copyFromNewSql(MappedStatement ms, BoundSql boundSql,                                           String sql, List<ParameterMapping> parameterMappings, Object parameter){    //根据新的分页sql语句创建BoundSql        BoundSql newBoundSql = copyFromBoundSql(ms, boundSql, sql, parameterMappings, parameter);        //根据newBoundSql创建新的MappedStatement        return copyFromMappedStatement(ms, new BoundSqlSqlSource(newBoundSql));    }        //根据新的分页sql语句创建BoundSqlprivate BoundSql copyFromBoundSql(MappedStatement ms, BoundSql boundSql,String sql, List<ParameterMapping> parameterMappings,Object parameter) {BoundSql newBoundSql = new BoundSql(ms.getConfiguration(),sql, parameterMappings, parameter);for (ParameterMapping mapping : boundSql.getParameterMappings()) {    String prop = mapping.getProperty();    if (boundSql.hasAdditionalParameter(prop)) {        newBoundSql.setAdditionalParameter(prop, boundSql.getAdditionalParameter(prop));    }}return newBoundSql;}//根据newBoundSql创建新的MappedStatementprivate MappedStatement copyFromMappedStatement(MappedStatement ms,SqlSource newSqlSource) {Builder builder = new 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 && ms.getKeyProperties().length !=0){            StringBuffer keyProperties = new StringBuffer();            for(String keyProperty : ms.getKeyProperties()){                keyProperties.append(keyProperty).append(",");            }            keyProperties.delete(keyProperties.length()-1, keyProperties.length());builder.keyProperty(keyProperties.toString());}//setStatementTimeout()builder.timeout(ms.getTimeout());//setStatementResultMap()builder.parameterMap(ms.getParameterMap());//setStatementResultMap()        builder.resultMaps(ms.getResultMaps());builder.resultSetType(ms.getResultSetType());//setStatementCache()builder.cache(ms.getCache());builder.flushCacheRequired(ms.isFlushCacheRequired());builder.useCache(ms.isUseCache());return builder.build();}public Object plugin(Object target) {return Plugin.wrap(target, this);}/** * @Title: setProperties  * @Description: 方言插件配置时设置的参数 * @param properties 参数 * @return  void   * @2016年1月13日下午3:54:47 */public void setProperties(Properties properties) {dataBaseType = properties.getProperty("dialectType");}public static class BoundSqlSqlSource implements SqlSource {BoundSql boundSql;public BoundSqlSqlSource(BoundSql boundSql) {this.boundSql = boundSql;}public BoundSql getBoundSql(Object parameterObject) {return boundSql;}}    public void setDialectClass(String dialectClass) {        this.dialectClass = dialectClass;    }    public void setDataBaseType(String dataBaseType) {this.dataBaseType = dataBaseType;}    }

详细代码及工程请查看：https://github.com/IAMADOG/TTmybatis