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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;
} 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> {
@Override
public Map mapRow(ResultSet rs, int rowNum) throws SQLException {
ResultSetMetaData rsmd = rs.getMetaData();
int columnCount = rsmd.getColumnCount();
Map mapOfColValues = createColumnMap(columnCount);
for (int i = 1; i <= columnCount; i++) {
String key = getColumnKey(JdbcUtils.lookupColumnName(rsmd, i));
Object obj = getColumnValue(rs, i);
mapOfColValues.put(key, obj);
}
return mapOfColValues;
}
/**
* Create a Map instance to be used as column map.
* By default, a linked case-insensitive Map will be created.
* @param columnCount the column count, to be used as initial
* capacity for the Map
* @return the new Map instance
* @see org.springframework.util.LinkedCaseInsensitiveMap
*/
protected Map createColumnMap(int columnCount) {
return new LinkedCaseInsensitiveMap
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);
}
}
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;
}
} 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);
} 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);
} 有没有发现使用了BeanPropertyRowMapper更为简洁。