iBatis3基于方言(Dialect)
(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 <configuration> <plugins> <!-- 指定数据库分页方言Dialect, 其它方言:OracleDialect,SQLServerDialect,SybaseDialect,DB2Dialect,PostgreSQLDialect,MySQLDialect,DerbyDialect--> <plugin interceptor="cn.org.rapid_framework.ibatis3.plugin.OffsetLimitInterceptor"> <property name="dialectClass" value="cn.org.rapid_framework.jdbc.dialect.MySQLDialect"/> </plugin> </plugins>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost:3306/test" />
<property name="username" value="root" />
<property name="password" value="123456" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/company/project/model/mapper/BlogMapper.xml" />
</mappers> </configuration>
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