(注:以下代码是基于ibatis3 beta4的扩展，ibatis3正式版如果实现改变，将会继续跟进修改)

iBatis3默认使用的分页是基于游标的分页，而这种分页在不同的数据库上性能差异不一致，最好的办法当然是使用类似hibernate的基于方言(Dialect)的物理分页功能。

iBatis3现在提供插件功能，通过插件我们可以编写自己的拦截器来拦截iBatis3的主要执行方法来完成相关功能的扩展。

能够拦截的的类如下:

Java代码 收藏代码

Executor

(update,query,flushStatements,commit,rollback,getTransaction,close,isClosed)

ParameterHandler

(getParameterObject,setParameters)

ResultSetHandler

(handleResultSets,handleOutputParameters)

StatementHandler

(prepare,parameterize,batch,update,query)

但此次我们感兴趣的是Executor.query()方法，查询方法最终都是执行该方法。

该方法完整是:

Java代码 收藏代码

Executor.query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

分页方言的基本实现:

以Mysql数据库示例,即有一个方法 query(sql,offset,limit);

Java代码 收藏代码

query("select * from user",5,10);

经过我们的拦截器拦截处理,变成

Java代码 收藏代码

query("select * from user limit 5,10",0,0);

1.拦截类实现:

Java代码 收藏代码

@Intercepts({@Signature(

type= Executor.class,

method = "query",

args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})

public class OffsetLimitInterceptor implements Interceptor{

static int MAPPED_STATEMENT_INDEX = 0;

static int PARAMETER_INDEX = 1;

static int ROWBOUNDS_INDEX = 2;

static int RESULT_HANDLER_INDEX = 3;

Dialect dialect;

public Object intercept(Invocation invocation) throws Throwable {

processIntercept(invocation.getArgs());

return invocation.proceed();

}

void processIntercept(final Object[] queryArgs) {

//queryArgs = query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler)

MappedStatement ms = (MappedStatement)queryArgs[MAPPED_STATEMENT_INDEX];

Object parameter = queryArgs[PARAMETER_INDEX];

final RowBounds rowBounds = (RowBounds)queryArgs[ROWBOUNDS_INDEX];

int offset = rowBounds.getOffset();

int limit = rowBounds.getLimit();

if(dialect.supportsLimit() && (offset != RowBounds.NO_ROW_OFFSET || limit != RowBounds.NO_ROW_LIMIT)) {

BoundSql boundSql = ms.getBoundSql(parameter);

String sql = boundSql.getSql().trim();

if (dialect.supportsLimitOffset()) {

sql = dialect.getLimitString(sql, offset, limit);

offset = RowBounds.NO_ROW_OFFSET;

} else {

sql = dialect.getLimitString(sql, 0, limit);

}

limit = RowBounds.NO_ROW_LIMIT;

queryArgs[ROWBOUNDS_INDEX] = new RowBounds(offset,limit);

BoundSql newBoundSql = new BoundSql(sql, boundSql.getParameterMappings(), boundSql.getParameterObject());

MappedStatement newMs = copyFromMappedStatement(ms, new BoundSqlSqlSource(newBoundSql));

queryArgs[MAPPED_STATEMENT_INDEX] = newMs;

}

}

private MappedStatement copyFromMappedStatement(MappedStatement ms,SqlSource newSqlSource) {

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());

builder.keyProperty(ms.getKeyProperty());

builder.timeout(ms.getTimeout());

builder.parameterMap(ms.getParameterMap());

builder.resultMaps(ms.getResultMaps());

builder.cache(ms.getCache());

MappedStatement newMs = builder.build();

return newMs;

}

public Object plugin(Object target) {

return Plugin.wrap(target, this);

}

public void setProperties(Properties properties) {

String dialectClass = new PropertiesHelper(properties).getRequiredString("dialectClass");

try {

dialect = (Dialect)Class.forName(dialectClass).newInstance();

} catch (Exception e) {

throw new RuntimeException("cannot create dialect instance by dialectClass:"+dialectClass,e);

}

System.out.println(OffsetLimitInterceptor.class.getSimpleName()+".dialect="+dialectClass);

}

public static class BoundSqlSqlSource implements SqlSource {

BoundSql boundSql;

public BoundSqlSqlSource(BoundSql boundSql) {

this.boundSql = boundSql;

}

public BoundSql getBoundSql(Object parameterObject) {

return boundSql;

}

}

}

2.ibatis3配置文件内容:

Xml代码 收藏代码

3.MysqlDialect实现

Java代码 收藏代码

public class MySQLDialect extends Dialect{

public boolean supportsLimitOffset(){

return true;

}

public boolean supportsLimit() {

return true;

}

public String getLimitString(String sql, int offset, int limit) {

return getLimitString(sql,offset,String.valueOf(offset),limit,String.valueOf(limit));

}

public String getLimitString(String sql, int offset,String offsetPlaceholder, int limit, String limitPlaceholder) {

if (offset > 0) {

return sql + " limit "+offsetPlaceholder+","+limitPlaceholder;

} else {

return sql + " limit "+limitPlaceholder;

}

}

}

其它数据库的Dialect实现请查看:

4.分页使用

直接调用SqlSession.selectList(statement, parameter, new RowBounds(offset,limit))即可使用物理分页

5.存在的问题

现不是使用占位符的方式(即不是“limit ?,?”而是使用“limit 8,20”)使用分页

完整代码可以查看即将发布的rapid-framework 3.0的ibatis3插件