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BoundSql
		private 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MappedStatement
		private 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