spring jdbc是如何实现驼峰和下划线的自动映射的

今天在看写代码的时候突然发现，在类和数据库字段之间没有做过任何的映射关系，但是数据会自动的映射好
以下是spring代码的具体实现直接贴代码
讲述映射实现的过程
关注下BeanPropertyRowMapper这个类

protected void initialize(Class<T> mappedClass) {
this.mappedClass = mappedClass;
this.mappedFields = new HashMap<String, PropertyDescriptor>();
this.mappedProperties = new HashSet<String>();
PropertyDescriptor[] pds = BeanUtils.getPropertyDescriptors(mappedClass);
for (PropertyDescriptor pd : pds) {
if (pd.getWriteMethod() != null) {
this.mappedFields.put(pd.getName().toLowerCase(), pd);
String underscoredName = underscoreName(pd.getName());
if (!pd.getName().toLowerCase().equals(underscoredName)) {
this.mappedFields.put(underscoredName, pd);
}
this.mappedProperties.add(pd.getName());
}
}
}

这段代码是实现映射的地方

String underscoredName = underscoreName(pd.getName());

这个方法是把java类中的驼峰转化成数据库的下划线的方法

if (!pd.getName().toLowerCase().equals(underscoredName)) {
this.mappedFields.put(underscoredName, pd);
}

这段代码是检测当前字段是否需要处理驼峰命名，如果是需要转化的则需要重新赋值，key是数据库的对应字段名

public T mapRow(ResultSet rs, int rowNumber) throws SQLException {
Assert.state(this.mappedClass != null, "Mapped class was not specified");
T mappedObject = BeanUtils.instantiate(this.mappedClass);
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(mappedObject);
initBeanWrapper(bw);

ResultSetMetaData rsmd = rs.getMetaData();
int columnCount = rsmd.getColumnCount();
Set<String> populatedProperties = (isCheckFullyPopulated() ? new HashSet<String>() : null);

for (int index = 1; index <= columnCount; index++) {
String column = JdbcUtils.lookupColumnName(rsmd, index);
PropertyDescriptor pd = this.mappedFields.get(column.replaceAll(" ", "").toLowerCase());
if (pd != null) {
try {
Object value = getColumnValue(rs, index, pd);
if (logger.isDebugEnabled() && rowNumber == 0) {
logger.debug("Mapping column '" + column + "' to property '" +
pd.getName() + "' of type " + pd.getPropertyType());
}
try {
bw.setPropertyValue(pd.getName(), value);
}
catch (TypeMismatchException e) {
if (value == null && primitivesDefaultedForNullValue) {
logger.debug("Intercepted TypeMismatchException for row " + rowNumber +
" and column '" + column + "' with value " + value +
" when setting property '" + pd.getName() + "' of type " + pd.getPropertyType() +
" on object: " + mappedObject);
}
else {
throw e;
}
}
if (populatedProperties != null) {
populatedProperties.add(pd.getName());
}
}
catch (NotWritablePropertyException ex) {
throw new DataRetrievalFailureException(
"Unable to map column " + column + " to property " + pd.getName(), ex);
}
}
}

if (populatedProperties != null && !populatedProperties.equals(this.mappedProperties)) {
throw new InvalidDataAccessApiUsageException("Given ResultSet does not contain all fields " +
"necessary to populate object of class [" + this.mappedClass + "]: " + this.mappedProperties);
}

return mappedObject;
}

这个方法中的PD是通过数据库的column获取的，正好可以跟上面讲述的转化赋值对应，然后这个从mappedFields中获取到的PD中的name就是该数据库字段对应的javaBean中的值

至此spring的默认规则的映射实现已经讲述完了

附链接
使用Spring的NamedParameterJdbcTemplate完成DAO操作
（这个是NamedParameterJdbcTemplate的一个操作讲解）
http://zmx.iteye.com/blog/373736