Ibatis原理分析

 ibatis是什么?ibatis是一个基于Java的"半自动化"的持久框架,"半自动化"是指ibatis并不会在运行期自动生成sql语句执行,而是具体的sql需要通过配置文件进行编写,并将所需的参数,以及返回的结果字段映射到指定的POJO.

      ibatis的核心类图

      SqlMapClient类是ibatis的门面,通过ibatis完成的持久化操作都是通过调用SqlMapClient类完成的,SqlMapClient将所有的操作都转给类SqlMapExecutorDelegate类,SqlMapExecutorDelegate类存放解析配置文件生成的类,比如数据源,parameterMap,resultMap, MappedStatement(对增删改查语句的封装)等,SqlExecutor是最终执行sql语句的地方.MappedStatement类包含对参数数组进行包装的ParameterMapping[]数组,对执行结果进行包装的ResultMapping[]数组,还有对各种不同的sql的包装类Sql

     比如对于一个插入语句,传进来的是一个对象,ibatis就会根据参数的数组包装类ParameterMapping[]将参数赋值.这个赋值是通过DataExchange类完成的,代码如下:

      public Object[] getProperties(Object object) {

    int i = 0;
  Object[] values = new Object[propertyNames.length];
  try {
    for (i = 0; i < propertyNames.length; i++) {
      try {
        values[i] = getters[i].invoke(object, NO_ARGUMENTS);
      } catch (Throwable t) {
        throw ClassInfo.unwrapThrowable(t);
      }
    }
  } catch (Throwable t) {
    throw new RuntimeException("Error getting property '" + getters[i].getName() + "' of '" + object + "'.  Cause: " + t, t);
  }
  return values;
}

其中object是传进来的要插入的对象,propertyNames就是ParameterMapping[]的参数数组,getters是object类的所有get方法的Method方法数组

       对于一个查询语句,对执行结果的处理,是通过对ResultMapping[]进行循环处理的,核心代码如下:

 

 

for (int i = 0; i < getResultMappings().length; i++) {
     ResultMapping mapping = (ResultMapping) getResultMappings()[i];
     errorContext.setMoreInfo(mapping.getErrorString());
     if (mapping.getStatementName() != null) {
       if (resultClass == null) {
         throw new SqlMapException("The result class was null when trying to get results for ResultMap named " + getId() + ".");
       } else if (Map.class.isAssignableFrom(resultClass)) {
         Class javaType = mapping.getJavaType();
         if (javaType == null) {
           javaType = Object.class;
         }
         columnValues[i] = getNestedSelectMappingValue(statementScope, rs, mapping, javaType);
       } else if (DomTypeMarker.class.isAssignableFrom(resultClass)) {
         Class javaType = mapping.getJavaType();
         if (javaType == null) {
           javaType = DomTypeMarker.class;
         }
         columnValues[i] = getNestedSelectMappingValue(statementScope, rs, mapping, javaType);
       } else {
         Probe p = ProbeFactory.getProbe(resultClass);
         Class type = p.getPropertyTypeForSetter(resultClass, mapping.getPropertyName());
         columnValues[i] = getNestedSelectMappingValue(statementScope, rs, mapping, type);
       }
       foundData = foundData || columnValues[i] != null;
     } else if (mapping.getNestedResultMapName() == null) {
       columnValues[i] = getPrimitiveResultMappingValue(rs, mapping);
       if (columnValues[i] == null) {
         columnValues[i] = doNullMapping(columnValues[i], mapping);
       } else {
         foundData = true;
       }
     }
   }

         其中,type为参数的java类型,   Probe p = ProbeFactory.getProbe(resultClass),Class type = p.getPropertyTypeForSetter(resultClass, mapping.getPropertyName())是通过反射获取java类型的

    getNestedSelectMappingValue(statementScope, rs, mapping, type)调用了prepareBeanParameterObject(StatementScope statementScope, ResultSet rs, ResultMapping mapping, Class parameterType)方法,方法如下:

 

      private Object prepareBeanParameterObject(StatementScope statementScope, ResultSet rs, ResultMapping mapping, Class parameterType)

  throws InstantiationException, IllegalAccessException, SQLException {
TypeHandlerFactory typeHandlerFactory = getDelegate().getTypeHandlerFactory();

Object parameterObject;
if (parameterType == null) {
  parameterObject = new HashMap();
} else {
  parameterObject = ResultObjectFactoryUtil.createObjectThroughFactory(parameterType);
}
String complexName = mapping.getColumnName();

if (complexName.indexOf('=') > -1
    || complexName.indexOf(',') > -1) {
  StringTokenizer parser = new StringTokenizer(complexName, "{}=, ", false);
  while (parser.hasMoreTokens()) {
    String propName = parser.nextToken();
    String colName = parser.nextToken();
    Class propType = PROBE.getPropertyTypeForSetter(parameterObject, propName);
    TypeHandler propTypeHandler = typeHandlerFactory.getTypeHandler(propType);
    Object propValue = propTypeHandler.getResult(rs, colName);
    PROBE.setObject(parameterObject, propName, propValue);
  }
} else {
  // single param
  TypeHandler propTypeHandler = typeHandlerFactory.getTypeHandler(parameterType);
  if (propTypeHandler == null) {
    propTypeHandler = typeHandlerFactory.getUnkownTypeHandler();
  }
  parameterObject = propTypeHandler.getResult(rs, complexName);
}

return parameterObject;

 typeHandlerFactory根据参数parameterType的类型决定采用哪个handler处理ResultSet的结果,以

StringTypeHandler为例,其getResult(ResultSet rs, String columnName)方法如下:

 

public Object getResult(ResultSet rs, String columnName)
    throws SQLException {
  Object s = rs.getString(columnName);
  if (rs.wasNull()) {
    return null;
  } else {
    return s;
  }
}