SpringBoot JDBC 源码分析之——NamedParameterJdbcTemplate 查询数据返回bean对象

1，NamedParameterJdbcTemplate 查询列表

 /***测试***/

    public void   queyBeanTest(){
        String s = "select * from PT_USER ";

        List<PtUser> list = namedJdbcTemplate.query(s, new BeanPropertyRowMapper<PtUser>(PtUser.class));


        System.out.println(list);

    }

　　

2，如果有参数，期间会把  参数绑定的 例如： :name 替换成 ? 

源码：getPreparedStatementCreator 方法。 所以最后还是调用  JdbcTemplate  模板。

3，按源码一直走下去。下面是一个抽象类实现的模板方法。

/**
     * Query using a prepared statement, allowing for a PreparedStatementCreator
     * and a PreparedStatementSetter. Most other query methods use this method,
     * but application code will always work with either a creator or a setter.
     * @param psc Callback handler that can create a PreparedStatement given a
     * Connection
     * @param pss object that knows how to set values on the prepared statement.
     * If this is null, the SQL will be assumed to contain no bind parameters.
     * @param rse object that will extract results.
     * @return an arbitrary result object, as returned by the ResultSetExtractor
     * @throws DataAccessException if there is any problem
     */
    public <T> T query(
            PreparedStatementCreator psc, final PreparedStatementSetter pss, final ResultSetExtractor<T> rse)
            throws DataAccessException {

        Assert.notNull(rse, "ResultSetExtractor must not be null");
        logger.debug("Executing prepared SQL query");

        return execute(psc, new PreparedStatementCallback<T>() {
            @Override
            public T doInPreparedStatement(PreparedStatement ps) throws SQLException {
                ResultSet rs = null;
                try {
                    if (pss != null) {
                        pss.setValues(ps);
                    }
                    rs = ps.executeQuery();
                    ResultSet rsToUse = rs;
                    if (nativeJdbcExtractor != null) {
                        rsToUse = nativeJdbcExtractor.getNativeResultSet(rs);
                    }
                    return rse.extractData(rsToUse);
                }
                finally {
                    JdbcUtils.closeResultSet(rs);
                    if (pss instanceof ParameterDisposer) {
                        ((ParameterDisposer) pss).cleanupParameters();
                    }
                }
            }
        });
    }

4，因为一开始 我们 用的 BeanPropertyRowMapper 类，也就是用这个类来装载返回的数据。

　　一开始new 的时候会初始化方法。

　　

    /**
     * Create a new {@code BeanPropertyRowMapper}, accepting unpopulated
     * properties in the target bean.
     * <p>Consider using the {@link #newInstance} factory method instead,
     * which allows for specifying the mapped type once only.
     * @param mappedClass the class that each row should be mapped to
     */
    public BeanPropertyRowMapper(Class<T> mappedClass) {
        initialize(mappedClass);
    }

这个方法，其实是把实体类的属性拆分了来存的，例如：userName 存为user_name.

    /**
     * Initialize the mapping metadata for the given class.
     * @param mappedClass the mapped class
     */
    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(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());
            }
        }
    }

　　下面这个方法是最终实现，会把数据库的字段都转成小写。所以也支持Oracle 。

/**
     * Extract the values for all columns in the current row.
     * <p>Utilizes public setters and result set metadata.
     * @see java.sql.ResultSetMetaData
     */
    @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<String> populatedProperties = (isCheckFullyPopulated() ? new HashSet<String>() : 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;
    }

总结： 因为源码就是这样写死的。所以要约定装配实体类。