使用mybatis时都是用的sqlmapper来做的数据库到java对象的映射，因此在针对一些特定数据库方言使用时无法在多个数据库上切换。

解决方案:

• mybatis篇

思路：

通过定义environment的id来指定使用不同的数据库映射文件，如下

<!--WizRtf2Html Charset=0 -->

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<environments default="mysql">
<environment id="mysql">
<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="pwd" />
    </dataSource>
</environment>
</environments>
    <mappers>
    <mapper resource="cn/dcr/mybatis/entity/UserMapper.xml" />
    </mappers>
</configuration>

environment的id是mysql映射文件是cn/dcr/mybatis/entity/UserMapper.xml

那么mybatis实际是读取的mysql/cn/dcr/mybatis/entity/UserMapper.xml

 

实现：

以org.apache.ibatis.builder.xml.XMLConfigBuilder类为蓝本创建一个新类org.apache.ibatis.builder.xml.XMLConfigBuilderEx

添加一个成员变量

private String dialect;

修改environmentsElement方法设置方言

private void environmentsElement(XNode context) throws Exception {

if (context != null) { if (environment == null) { environment = context.getStringAttribute("default"); } for (XNode child : context.getChildren()) { String id = child.getStringAttribute("id"); dialect = id.toLowerCase();//设置方言 if (isSpecifiedEnvironment(id)) { TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); Environment.Builder environmentBuilder = new Environment.Builder(id).transactionFactory(txFactory).dataSource(dsFactory.getDataSource()); configuration.setEnvironment(environmentBuilder.build()); } } } }

修改mapperElement方法

private void mapperElement(XNode parent) throws Exception {

if (parent != null) { for (XNode child : parent.getChildren()) { String resource = child.getStringAttribute("resource"); String url = child.getStringAttribute("url"); InputStream inputStream; if (resource != null && url == null) { if(dialect != null){ resource = dialect + "/" + resource;//从方言指定位置查找 } ErrorContext.instance().resource(resource); inputStream = Resources.getResourceAsStream(resource); XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); mapperParser.parse(); } else if (url != null && resource == null) { if(dialect != null){ url = dialect + "/" + url;//从方言指定位置查找 } ErrorContext.instance().resource(url); inputStream = Resources.getUrlAsStream(url); XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments()); mapperParser.parse(); } else { throw new BuilderException("A mapper element may only specify a url or resource, but not both."); } } } }

继承org.apache.ibatis.session.SqlSessionFactoryBuilder类创建一个新类org.apache.ibatis.session.SqlSessionFactoryBuilderEx用来加载org.apache.ibatis.builder.xml.XMLConfigBuilderEx

覆盖父类中的build方法

public SqlSessionFactory build(InputStream inputStream, String environment, Properties props) {

try { XMLConfigBuilderEx parser

=

new XMLConfigBuilderEx(inputStream, environment, props); Configuration config

= parser.parse();

return build(config); }

catch (Exception e) {

throw ExceptionFactory.wrapException(

"Error building SqlSession.", e); }

finally { ErrorContext.instance().reset();

try { inputStream.close(); }

catch (IOException e) {

// Intentionally ignore. Prefer previous error. } } }

调用org.apache.ibatis.builder.xml.XMLConfigBuilderEx来加载配置文件

 

• spring篇

思路：

自定义mybatis配置加载Bean在spring的配置文件中添加方言的配置让自定义Bean在加载mybatis的配置时可以使用不同的数据库映射文件，如下

<bean

id=

"sqlSessionFactory"

class=

"org.mybatis.spring.SqlSessionFactoryBeanEx">

<property

name=

"dataSource" ref=

"dataSource" /> <property

name=

"configLocation"

value=

"classpath:configuration.xml" /> <property

name=

"mapperLocations"

value=

"classpath*:${jdbc.dialect}/mappers/*.xml" /> </bean>

${jdbc.dialect}在配置文件中设置，如mysql，那么spring会把mysql/mappers下的所有映射文件加载进来

实现：

以org.mybatis.spring.SqlSessionFactoryBean为蓝本创建org.mybatis.spring.SqlSessionFactoryBeanEx类

将成员变量

private SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();

修改为

private SqlSessionFactoryBuilderEx sqlSessionFactoryBuilderEx = new SqlSessionFactoryBuilderEx();

并去掉setSqlSessionFactoryBuilder方法添加setSqlSessionFactoryBuilderEx方法

覆盖buildSqlSessionFactory方法

protected SqlSessionFactory buildSqlSessionFactory() throws IOException, IllegalAccessException, InstantiationException {

XMLConfigBuilderEx xmlConfigBuilderEx; Configuration configuration;

if (this.configLocation != null) { try { xmlConfigBuilderEx = new XMLConfigBuilderEx(this.configLocation.getInputStream(), null, this.configurationProperties); configuration = xmlConfigBuilderEx.parse(); } catch (Exception ex) { throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex); } finally { ErrorContext.instance().reset(); }

if (this.logger.isDebugEnabled()) { this.logger.debug("Parsed configuration file: '" + this.configLocation + "'"); } } else { if (this.logger.isDebugEnabled()) { this.logger.debug("Property 'configLocation' not specified, using default MyBatis Configuration"); } configuration = new Configuration(); }

if (this.transactionFactory == null) { this.transactionFactory = new SpringManagedTransactionFactory(this.dataSource); }

Environment environment = new Environment(this.environment, this.transactionFactory, this.dataSource);

configuration.setEnvironment(environment);

if (!ObjectUtils.isEmpty(this.mapperLocations)) { Map<String, XNode> sqlFragments = new HashMap<String, XNode>();

for (Resource mapperLocation : this.mapperLocations) { if (mapperLocation == null) { continue; }

// MyBatis holds a Map using "resource" name as a key. // If a mapper file is loaded, it searches for a mapper // interface type. // If the type is found then it tries to load the mapper file // again looking for this: // // String xmlResource = type.getName().replace('.', '/') + // ".xml"; // // So if a mapper interface exists, resource cannot be an // absolute path. // Otherwise MyBatis will throw an exception because // it will load both a mapper interface and the mapper xml file, // and throw an exception telling that a mapperStatement cannot // be loaded twice. String path; if (mapperLocation instanceof ClassPathResource) { path = ((ClassPathResource) mapperLocation).getPath(); } else { // this won't work if there is also a mapper interface in // classpath path = mapperLocation.toString(); }

try { XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(), configuration, path, sqlFragments); xmlMapperBuilder.parse(); } catch (Exception e) { throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e); } finally { ErrorContext.instance().reset(); }

if (this.logger.isDebugEnabled()) { this.logger.debug("Parsed mapper file: '" + mapperLocation + "'"); } } } else { if (this.logger.isDebugEnabled()) { this.logger.debug("Property 'mapperLocations' was not specified, only MyBatis mapper files specified in the config xml were loaded"); } }

return this.sqlSessionFactoryBuilderEx.build(configuration); }

转自：