Spring源码学习-JdbcTemplate queryForObject_JAVA_编程开发_程序员俱乐部

JdbcTemplate中有两个可能会混淆的queryForObject方法：
1.
Object queryForObject(String sql, Object[] args, Class requiredType)
2.
Object queryForObject(String sql, Object[] args, RowMapper rowMapper)

第1个方法是只查一列的，参数“requiredType”不可以是自定义的类
如果要把查询结果封装为自定义的类，需要采用第2个方法

例如：

class="java" name="code">
	//只查询一列：name
	String sql = "SELECT NAME FROM CUSTOMER WHERE CUST_ID = ?";
 
	String name = (String)getJdbcTemplate().queryForObject(
			sql, new Object[] { custId }, String.class);
 
	return name;
	
	//查询返回自定义的类
	String sql = "SELECT * FROM CUSTOMER WHERE CUST_ID = ?";
 
	Customer customer = (Customer)getJdbcTemplate().queryForObject(
			sql, new Object[] { custId }, 
			new BeanPropertyRowMapper(Customer.class));
 
	return customer;

分析一下第2个方法的源码

先看看BeanPropertyRowMapper
其实不用看源码都知道思路了：
new BeanPropertyRowMapper(Customer.class)
mapRow(ResultSet rs, int rowNumber)
遍历ResultSet，每一行对应一个Customer对象
遍历每一行时，顺序遍历各列，取得该列的值通过反射调用对应的setter方法，赋值给到Customer对象
注意到无论是取列名还是取列的值，都是通过index（1~columnCount）来取的

public T mapRow(ResultSet rs, int rowNumber) throws SQLException {
		T mappedObject = BeanUtils.instantiate(this.mappedClass);
		BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(mappedObject);

		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);
				    bw.setPropertyValue(pd.getName(), value);
					if (populatedProperties != null) {
						populatedProperties.add(pd.getName());
					}
				}
			}
		}

		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;
	}

注意到mapRow方法里面，populatedProperties变量是用来检查是否所有的property都正确地实现了赋值
再看看BeanPropertyRowMapper的构造函数：

	public BeanPropertyRowMapper(Class<T> mappedClass) {
		initialize(mappedClass);
	}
	
	protected void initialize(Class<T> mappedClass) {
		this.mappedClass = mappedClass;
		
		/*保存field
		注意到对于驼峰式命名的field：
		例如，对于gameId, mappedFields 会同时保存"gameId"和"game_id"
		因此在sql语句中，
		select  id as game_id, name from game和
		select  id as gameId, name from game的效果是一样的
		*/
		this.mappedFields = new HashMap<String, PropertyDescriptor>();
		
		//保存property
		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());
			}
		}
	}
	
	//1. "game", return "game", unchanged
	//2. "gameId", return "game_id"
	private String underscoreName(String name) {
		//......
	}
	

看完了BeanPropertyRowMapper，回到queryForObject
调用过程：
queryForObject(String sql, Object[] args, RowMapper<T> rowMapper)
query(sql, args, new RowMapperResultSetExtractor<T>(rowMapper, 1))
query(sql, newArgPreparedStatementSetter(args), resultSetExtractor)
query(new SimplePreparedStatementCreator(sql), argPreparedStatementSetter, resultSetExtractor)
query(PreparedStatementCreator psc, final PreparedStatementSetter argPreparedStatementSetter, final ResultSetExtractor<T>  resultSetExtractor)
整个过程跟JdbcTemplate的batchUpdate方法是类似的
1.
对于参数'sql'，它最终的作用是生成一个PreparedStatement：
connection.prepareStatement(sql);
2.
对sql语句中“?”的赋值，是通过PreparedStatementSetter
需要两个数据：
一是“?”所对应的值，也就是Object[] args
二是PreparedStatement
对第一个数据：
args是在ArgPreparedStatementSetter的构造函数中传入
对第二个数据：
在最后的query方法中，把创建好的PreparedStatement作为匿名内部类PreparedStatementCallback.doInPreparedStatement方法的参数
最后得以传给ArgPreparedStatementSetter：

public <T> T query(
			PreparedStatementCreator psc, final PreparedStatementSetter argPreparedStatementSetter, final ResultSetExtractor<T> rse) {
					return execute(psc, 
										new PreparedStatementCallback<T>() {
											public T doInPreparedStatement(PreparedStatement ps) {
												//...
												argPreparedStatementSetter.setValues(ps);
												ResultSet rsToUse = ps.executeQuery();
												return rse.extractData(rsToUse);
												//...
											}
										}
					);
}

ArgPreparedStatementSetter的setValues方法：

public void setValues(PreparedStatement ps) throws SQLException {
		if (this.args != null) {
			for (int i = 0; i < this.args.length; i++) {
				Object arg = this.args[i];
				
				/*
					doSetValue方法会根据arg的实际类型去调用ps.setSpecificType(index, arg)方法，例如：
					if (isStringValue(arg.getClass())) {
						ps.setString(paramIndex, arg.toString());
					}
				*/
				doSetValue(ps, i + 1, arg);
			}
		}
	}

最后看一下RowMapperResultSetExtractor
它有两个字段：
private final RowMapper<T> rowMapper;
private final int rowsExpected;
因为是queryForObject，因此rowsExpected=1
extractData方法很直观，是在上面的doInPreparedStatement调用的：

	public List<T> extractData(ResultSet rs) throws SQLException {
		List<T> results = (this.rowsExpected > 0 ? new ArrayList<T>(this.rowsExpected) : new ArrayList<T>());
		int rowNum = 0;
		
		//rs.next()迭代每一行
		while (rs.next()) {
			results.add(this.rowMapper.mapRow(rs, rowNum++));
		}
		return results;
	}