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本文主要介绍下spring jdbc的RowMapper

RowMapper

spring-jdbc-4.3.10.RELEASE-sources.jar!/org/springframework/jdbc/core/RowMapper.java

public interface RowMapper
    
      {    /**     * Implementations must implement this method to map each row of data     * in the ResultSet. This method should not call {@code next()} on     * the ResultSet; it is only supposed to map values of the current row.     * @param rs the ResultSet to map (pre-initialized for the current row)     * @param rowNum the number of the current row     * @return the result object for the current row     * @throws SQLException if a SQLException is encountered getting     * column values (that is, there's no need to catch SQLException)     */    T mapRow(ResultSet rs, int rowNum) throws SQLException;}复制代码
    

spring定义了这个RowMapper，来让应用去自定义数据库结果集与实体的映射，这样来把变化的部分隔离出去

ColumnMapRowMapper

spring-jdbc-4.3.7.RELEASE-sources.jar!/org/springframework/jdbc/core/ColumnMapRowMapper.java

public class ColumnMapRowMapper implements RowMapper

将结果集映射为map

SingleColumnRowMapper

spring-jdbc-4.3.10.RELEASE-sources.jar!/org/springframework/jdbc/core/SingleColumnRowMapper.java

public class SingleColumnRowMapper
    
      implements RowMapper
     
       {    private Class
       requiredType;    /**     * Create a new {@code SingleColumnRowMapper} for bean-style configuration.     * @see #setRequiredType     */    public SingleColumnRowMapper() {    }    /**     * Create a new {@code SingleColumnRowMapper}.     * 
      
Consider using the {@link #newInstance} factory method instead,     * which allows for specifying the required type once only.     * @param requiredType the type that each result object is expected to match     */    public SingleColumnRowMapper(Class
       
         requiredType) {        setRequiredType(requiredType);    }    /**     * Set the type that each result object is expected to match.     * 
       
      
If not specified, the column value will be exposed as     * returned by the JDBC driver.     */    public void setRequiredType(Class
       
         requiredType) {        this.requiredType = ClassUtils.resolvePrimitiveIfNecessary(requiredType);    }    /**     * Extract a value for the single column in the current row.     * 
       
      
Validates that there is only one column selected,     * then delegates to {@code getColumnValue()} and also     * {@code convertValueToRequiredType}, if necessary.     * @see java.sql.ResultSetMetaData#getColumnCount()     * @see #getColumnValue(java.sql.ResultSet, int, Class)     * @see #convertValueToRequiredType(Object, Class)     */    @Override    @SuppressWarnings("unchecked")    public T mapRow(ResultSet rs, int rowNum) throws SQLException {        // Validate column count.        ResultSetMetaData rsmd = rs.getMetaData();        int nrOfColumns = rsmd.getColumnCount();        if (nrOfColumns != 1) {            throw new IncorrectResultSetColumnCountException(1, nrOfColumns);        }        // Extract column value from JDBC ResultSet.        Object result = getColumnValue(rs, 1, this.requiredType);        if (result != null && this.requiredType != null && !this.requiredType.isInstance(result)) {            // Extracted value does not match already: try to convert it.            try {                return (T) convertValueToRequiredType(result, this.requiredType);            }            catch (IllegalArgumentException ex) {                throw new TypeMismatchDataAccessException(                        "Type mismatch affecting row number " + rowNum + " and column type '" +                        rsmd.getColumnTypeName(1) + "': " + ex.getMessage());            }        }        return (T) result;    }    /**     * Retrieve a JDBC object value for the specified column.     * 
      
The default implementation calls     * {@link JdbcUtils#getResultSetValue(java.sql.ResultSet, int, Class)}.     * If no required type has been specified, this method delegates to     * {@code getColumnValue(rs, index)}, which basically calls     * {@code ResultSet.getObject(index)} but applies some additional     * default conversion to appropriate value types.     * @param rs is the ResultSet holding the data     * @param index is the column index     * @param requiredType the type that each result object is expected to match     * (or {@code null} if none specified)     * @return the Object value     * @throws SQLException in case of extraction failure     * @see org.springframework.jdbc.support.JdbcUtils#getResultSetValue(java.sql.ResultSet, int, Class)     * @see #getColumnValue(java.sql.ResultSet, int)     */    protected Object getColumnValue(ResultSet rs, int index, Class
        requiredType) throws SQLException {        if (requiredType != null) {            return JdbcUtils.getResultSetValue(rs, index, requiredType);        }        else {            // No required type specified -> perform default extraction.            return getColumnValue(rs, index);        }    }    /**     * Retrieve a JDBC object value for the specified column, using the most     * appropriate value type. Called if no required type has been specified.     * 
      
The default implementation delegates to {@code JdbcUtils.getResultSetValue()},     * which uses the {@code ResultSet.getObject(index)} method. Additionally,     * it includes a "hack" to get around Oracle returning a non-standard object for     * their TIMESTAMP datatype. See the {@code JdbcUtils#getResultSetValue()}     * javadoc for details.     * @param rs is the ResultSet holding the data     * @param index is the column index     * @return the Object value     * @throws SQLException in case of extraction failure     * @see org.springframework.jdbc.support.JdbcUtils#getResultSetValue(java.sql.ResultSet, int)     */    protected Object getColumnValue(ResultSet rs, int index) throws SQLException {        return JdbcUtils.getResultSetValue(rs, index);    }    /**     * Convert the given column value to the specified required type.     * Only called if the extracted column value does not match already.     * 
      
If the required type is String, the value will simply get stringified     * via {@code toString()}. In case of a Number, the value will be     * converted into a Number, either through number conversion or through     * String parsing (depending on the value type).     * @param value the column value as extracted from {@code getColumnValue()}     * (never {@code null})     * @param requiredType the type that each result object is expected to match     * (never {@code null})     * @return the converted value     * @see #getColumnValue(java.sql.ResultSet, int, Class)     */    @SuppressWarnings("unchecked")    protected Object convertValueToRequiredType(Object value, Class
        requiredType) {        if (String.class == requiredType) {            return value.toString();        }        else if (Number.class.isAssignableFrom(requiredType)) {            if (value instanceof Number) {                // Convert original Number to target Number class.                return NumberUtils.convertNumberToTargetClass(((Number) value), (Class
       
        ) requiredType);            }            else {                // Convert stringified value to target Number class.                return NumberUtils.parseNumber(value.toString(),(Class
        
         ) requiredType);            }        }        else {            throw new IllegalArgumentException(                    "Value [" + value + "] is of type [" + value.getClass().getName() +                    "] and cannot be converted to required type [" + requiredType.getName() + "]");        }    }    /**     * Static factory method to create a new {@code SingleColumnRowMapper}     * (with the required type specified only once).     * @param requiredType the type that each result object is expected to match     * @since 4.1     */    public static 
         
           SingleColumnRowMapper
          
            newInstance(Class
           
             requiredType) { return new SingleColumnRowMapper
            
             (requiredType); }}复制代码
            
           
          
         
        
       
     
    

映射单个字段，比如count(*)这种

BeanPropertyRowMapper

spring-jdbc-4.3.7.RELEASE-sources.jar!/org/springframework/jdbc/core/BeanPropertyRowMapper.java

public class BeanPropertyRowMapper
    
      implements RowMapper
     
       {//...    protected void initialize(Class
      
        mappedClass) {        this.mappedClass = mappedClass;        this.mappedFields = new HashMap
       
        ();        this.mappedProperties = new HashSet
        
         ();        PropertyDescriptor[] pds = BeanUtils.getPropertyDescriptors(mappedClass);        for (PropertyDescriptor pd : pds) {            if (pd.getWriteMethod() != null) {                this.mappedFields.put(lowerCaseName(pd.getName()), pd);                String underscoredName = underscoreName(pd.getName());                if (!lowerCaseName(pd.getName()).equals(underscoredName)) {                    this.mappedFields.put(underscoredName, pd);                }                this.mappedProperties.add(pd.getName());            }        }    }    @Override    public T mapRow(ResultSet rs, int rowNumber) throws SQLException {        Assert.state(this.mappedClass != null, "Mapped class was not specified");        T mappedObject = BeanUtils.instantiateClass(this.mappedClass);        BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(mappedObject);        initBeanWrapper(bw);        ResultSetMetaData rsmd = rs.getMetaData();        int columnCount = rsmd.getColumnCount();        Set
         
           populatedProperties = (isCheckFullyPopulated() ? new HashSet
          
           () : null); for (int index = 1; index <= columnCount; index++) { String column = JdbcUtils.lookupColumnName(rsmd, index); String field = lowerCaseName(column.replaceAll(" ", "")); PropertyDescriptor pd = this.mappedFields.get(field); if (pd != null) { try { Object value = getColumnValue(rs, index, pd); if (rowNumber == 0 && logger.isDebugEnabled()) { logger.debug("Mapping column '" + column + "' to property '" + pd.getName() + "' of type '" + ClassUtils.getQualifiedName(pd.getPropertyType()) + "'"); } try { bw.setPropertyValue(pd.getName(), value); } catch (TypeMismatchException ex) { if (value == null && this.primitivesDefaultedForNullValue) { if (logger.isDebugEnabled()) { logger.debug("Intercepted TypeMismatchException for row " + rowNumber + " and column '" + column + "' with null value when setting property '" + pd.getName() + "' of type '" + ClassUtils.getQualifiedName(pd.getPropertyType()) + "' on object: " + mappedObject, ex); } } else { throw ex; } } if (populatedProperties != null) { populatedProperties.add(pd.getName()); } } catch (NotWritablePropertyException ex) { throw new DataRetrievalFailureException( "Unable to map column '" + column + "' to property '" + pd.getName() + "'", ex); } } else { // No PropertyDescriptor found if (rowNumber == 0 && logger.isDebugEnabled()) { logger.debug("No property found for column '" + column + "' mapped to field '" + field + "'"); } } } 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.getName() + "]: " + this.mappedProperties); } return mappedObject; }}复制代码
          
         
        
       
      
     
    

初始化构造map的时候，存的key是下划线的

如果数据库字段命名跟实体类一致，或者是下划线变驼峰的这种，那么可以直接使用这个

实例

• 使用BeanPropertyRowMapper的版本

  public Book findById(Integer id){     return jdbcTemplate.query("select * from book where book_id=?",new Object[]{id},new BeanPropertyRowMapper
        
         (Book.class)).get(0);  }复制代码
        

• 不使用BeanPropertyRowMapper的版本

  public Book findById2(Integer id){      return jdbcTemplate.query("select * from book where book_id=?",new Object[]{id},new RowMapper
        
         () {          @Override          public Book mapRow(ResultSet rs, int rowNum) throws SQLException {              Book book = new Book();              book.setBookId(rs.getInt("book_id"));              book.setTitle(rs.getString("title"));              book.setCreatedAt(rs.getTimestamp("created_at"));              return book;          }      }).get(0);  }复制代码
        

有没有发现使用了BeanPropertyRowMapper更为简洁。