对返回值的处理

有些sql查询会返回一些复杂类型，这些复杂类型没有办法简单的通过xml或者注解配置来实现，这种时候我们需要实现mybatis 的ResultHandler接口，来做自定义的对象属性映射。

ResultHandler例子：

Mybatis在调用ResultHandler的handleResult(ResultContext)方法之后生成bean，首先我们定义一个复杂对象。

public interface GrandFatherMapper {
    public static class GrandFatherWithGrandChildren {
        public GrandFather grandFather;
        public Child child;
    }
    public void selectComplex(ResultHandler handler);
}

com.xxx.xxx.BaseResultMap是另一个实体的mapper的resultMap。
这个查询，返回的每一条记录都是
{key=..., value=...}

这个结果集并不符合要求。不过通过ResultHandler来处理每一条记录就可以达到要求了。
看下Mybatis源码里面有关继承ResultHandler的DefaultMapResultHandler类。

public class DefaultMapResultHandler<K, V> implements ResultHandler {  
  
  private final Map<K, V> mappedResults;  
  private final String mapKey;  
  
  @SuppressWarnings("unchecked")  
  public DefaultMapResultHandler(String mapKey, ObjectFactory objectFactory) {  
    this.mappedResults = objectFactory.create(Map.class);  
    this.mapKey = mapKey;  
  }  
  
  public void handleResult(ResultContext context) {  
    // TODO is that assignment always true?  
    final V value = (V) context.getResultObject();  
    final MetaObject mo = MetaObject.forObject(value);  
    // TODO is that assignment always true?  
    final K key = (K) mo.getValue(mapKey);  
    mappedResults.put(key, value);  
  }  
  
  public Map<K, V> getMappedResults() {  
    return mappedResults;  
  }  
}  

这个DefaultMapResultHandler实现handleResult接口，处理每条数据。

可以模仿这个类写个类来装配自己所需要的Map。

private class MapResultHandler implements ResultHandler {  
  
        @SuppressWarnings("rawtypes")  
        private final Map mappedResults = new HashMap();  
          
        @SuppressWarnings({ "unchecked", "rawtypes" })  
        @Override  
        public void handleResult(ResultContext context) {  
            Map map = (Map) context.getResultObject();  
            mappedResults.put(map.get("key"), map.get("value"));  
        }  
          
        public Map getMappedResults() {  
            return mappedResults;  
        }  
          
    }  

this.getSqlSession().select(getWholeSqlId("getUaMapByTimestamp"),handler);  

这样就可以得到需要的结果集了。

使用自定义的数据库源

Mybatis支持使用自定义的数据源，这里我们使用HikariDataSourceFactory举例
首先需要在mybatis-config.xml指定数据源，

<?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>
    <typeAliases>
        <typeAlias type='com.xxx.xxx.xxx.HikariDataSourceFactory' alias='HikariDataSourceFactory'/>
    </typeAliases>
...
    <environments default='development'>
        <environment id='development'>
            <transactionManager type='JDBC'/>
            <dataSource type='HikariDataSourceFactory'>
                <property name="mysql.driverClassName" value="${mysql.driverClassName}"/>
                <property name="mysql.jdbcUrl" value="${mysql.jdbcUrl}"/>
                <property name="mysql.username" value="${mysql.username}"/>
                <property name="mysql.password" value="${mysql.password}"/>
                <property name="mysql.maximumPoolSize" value="${mysql.maximumPoolSize}"/>
                <property name="mysql.autoCommit" value="${mysql.autoCommit}"/>
                <property name="mysql.cachePrepStmts" value="${mysql.cachePrepStmts}"/>
                <property name="mysql.prepStmtCacheSize" value="${mysql.prepStmtCacheSize}"/>
                <property name="mysql.prepStmtCacheSqlLimit" value="${mysql.prepStmtCacheSqlLimit}"/>
            </dataSource>
        </environment>
    </environments>
...
</configuration>

然后我们实现DataSourceFactory方法，mybatis会在加载xml配置文件时，使用我们设置好的数据源

public class HikariDataSourceFactory implements DataSourceFactory {

    private Properties properties;
    private static final Logger LOGGER = LoggerFactory.getLogger(HikariDataSourceFactory.class);

    @Override
    public void setProperties(Properties props) {
        this.properties = props;
    }

    @Override
    public DataSource getDataSource() {
        try {
            HikariConfig hikariConfig = new HikariConfig();
            hikariConfig.setJdbcUrl(properties.getProperty(MySQLDAOFactory.CONF_JDBC_URL, ""));
            hikariConfig.setUsername(properties.getProperty(MySQLDAOFactory.CONF_USERNAME, ""));
            hikariConfig.setPassword(properties.getProperty(MySQLDAOFactory.CONF_PASSWORD, ""));
            hikariConfig.setMaximumPoolSize(Integer.parseInt(properties.getProperty(MySQLDAOFactory.CONF_MAX_POOLSIZE, "10")));
            hikariConfig.setAutoCommit(Boolean.valueOf(properties.getProperty(MySQLDAOFactory.CONF_AUTO_COMMIT, "false")
            ));
            hikariConfig.addDataSourceProperty("cachePrepStmts",
                    properties.getProperty(MySQLDAOFactory.CONF_CACHE_PREPARESTATEMENT, "true"));
            hikariConfig.addDataSourceProperty("prepStmtCacheSize",
                    properties.getProperty(MySQLDAOFactory.CONF_PREPARESTATEMENT_CACHE_SIZE, "300"));
            hikariConfig.addDataSourceProperty("prepStmtCacheSqlLimit",
                    properties.getProperty(MySQLDAOFactory.CONF_PREPARESTATEMENT_CACHE_LIMIT, "2048"));
            return new HikariDataSource(hikariConfig);
        } catch (Exception e) {
            LOGGER.error("create HikariConfig data source error {} ", e);
        }
        return null;
    }
}

接下来在初始化函数中调用。

    public void init(ServiceConfig config) {
        try {
            Properties properties = new Properties();
            properties.setProperty(MySQLDAOFactory.CONF_DRIVER_CLASSNAME, config.getConfig(MySQLDAOFactory.CONF_JDBC_URL, "com.mysql.cj.jdbc.Driver"));
            properties.setProperty(MySQLDAOFactory.CONF_JDBC_URL, config.getConfig(MySQLDAOFactory.CONF_JDBC_URL, ""));
            properties.setProperty(MySQLDAOFactory.CONF_USERNAME, config.getConfig(MySQLDAOFactory.CONF_USERNAME, ""));
            properties.setProperty(MySQLDAOFactory.CONF_PASSWORD, config.getConfig(MySQLDAOFactory.CONF_PASSWORD, ""));
            properties.setProperty(MySQLDAOFactory.CONF_MAX_POOLSIZE, config.getConfig(MySQLDAOFactory.CONF_MAX_POOLSIZE, "10"));
            properties.setProperty(MySQLDAOFactory.CONF_AUTO_COMMIT, config.getConfig(MySQLDAOFactory.CONF_AUTO_COMMIT, "false"));
            properties.setProperty(MySQLDAOFactory.CONF_CACHE_PREPARESTATEMENT, config.getConfig(MySQLDAOFactory.CONF_CACHE_PREPARESTATEMENT, "true"));
            properties.setProperty(MySQLDAOFactory.CONF_PREPARESTATEMENT_CACHE_SIZE, config.getConfig(MySQLDAOFactory.CONF_PREPARESTATEMENT_CACHE_SIZE, "300"));
            properties.setProperty(MySQLDAOFactory.CONF_PREPARESTATEMENT_CACHE_LIMIT, config.getConfig(MySQLDAOFactory.CONF_PREPARESTATEMENT_CACHE_LIMIT, "2048"));

            try (Reader reader = Resources.getResourceAsReader("mybatis-config.xml")) {
                factory = new SqlSessionFactoryBuilder().build(reader, properties);
                LOGGER.info("mybatis init success.");
            } catch (IOException e) {
                throw new RuntimeException(e.getMessage());
            }
        } catch (Exception e) {
            LOGGER.error("MyBatisSqlSession init failed ", e);
        }
    }