spring jdbc是如何實現駝峯和下劃線的自動映射的

今天在看寫代碼的時候忽然發現，在類和數據庫字段之間沒有作過任何的映射關係，可是數據會自動的映射好 
如下是spring代碼的具體實現直接貼代碼 
講述映射實現的過程 
關注下BeanPropertyRowMapper這個類

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());
            }
        }
    }

這段代碼是實現映射的地方

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

這個方法是把java類中的駝峯轉化成數據庫的下劃線的方法

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

這段代碼是檢測當前字段是否須要處理駝峯命名，若是是須要轉化的則須要從新賦值，key是數據庫的對應字段名

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;
    }

這個方法中的PD是經過數據庫的column獲取的，正好能夠跟上面講述的轉化賦值對應，而後這個從mappedFields中獲取到的PD中的name就是該數據庫字段對應的javaBean中的值

至此spring的默認規則的映射實現已經講述完了

附連接 
使用Spring的NamedParameterJdbcTemplate完成DAO操做 
（這個是NamedParameterJdbcTemplate的一個操做講解） 
http://zmx.iteye.com/blog/373736