• JdbcTemplate使用小结


      org.springframework.jdbc.core.JdbcTemplate.query(String sql, Object[] args, RowMapper<StaffUnionVO> rowMapper) throws DataAccessException

    1.自定义rowMapper

    public class StaffUnionVO implements RowMapper<StaffUnionVO>, Serializable {
        private static final long serialVersionUID = 1544023434308856628L;
    
        public StaffUnionVO() {
        }
      private String code;// 员工编码
      
        private String name;// 员工姓名
    
      
      public String getCode() {
            return code;
        }
    
        public void setCode(String code) {
            this.code = code;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }private boolean isExistColumn(ResultSet rs, String columnName) {
            try {
                if (rs.findColumn(columnName) > 0) {
                    return true;
                }
            } catch (SQLException e) {
                return false;
            }
    
            return false;
        }
    
        @Override
        public StaffUnionVO mapRow(ResultSet rs, int row) throws SQLException {
            StaffUnionVO vo = new StaffUnionVO();     
     
        if (isExistColumn(rs, "code")) vo.setCode(rs.getString("code"));
        if (isExistColumn(rs, "name")) vo.setName(rs.getString("name"));
      
        return vo; } }

      示例: List<StaffUnionVO> vos = JdbcTemplate.query(sql, new Object[0], new StaffUnionVO() );

    2.使用BeanPropertyRowMapper

    public class StaffUnionVO implements Serializable {
        private static final long serialVersionUID = 1544023434308856628L;
    
        public StaffUnionVO() {
        }
    
        private String code;// 员工编码
    
        private String name;// 员工姓名
    
        public String getCode() {
            return code;
        }
    
        public void setCode(String code) {
            this.code = code;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    }

      示例:List<StaffUnionVO> vos = JdbcTemplate.query(sql, new Object[0], BeanPropertyRowMapper.newInstance(StaffUnionVO.class));

      看一下 BeanPropertyRowMapper.java 的源码,可以学到不少东西。

    /*** Eclipse Class Decompiler plugin, copyright (c) 2016 Chen Chao (cnfree2000@hotmail.com) ***/
    package org.springframework.jdbc.core;
    
    import java.beans.PropertyDescriptor;
    import java.sql.ResultSet;
    import java.sql.ResultSetMetaData;
    import java.sql.SQLException;
    import java.util.HashMap;
    import java.util.HashSet;
    import java.util.Map;
    import java.util.Set;
    import org.apache.commons.logging.Log;
    import org.apache.commons.logging.LogFactory;
    import org.springframework.beans.BeanUtils;
    import org.springframework.beans.BeanWrapper;
    import org.springframework.beans.NotWritablePropertyException;
    import org.springframework.beans.PropertyAccessorFactory;
    import org.springframework.beans.TypeMismatchException;
    import org.springframework.dao.DataRetrievalFailureException;
    import org.springframework.dao.InvalidDataAccessApiUsageException;
    import org.springframework.jdbc.support.JdbcUtils;
    import org.springframework.util.Assert;
    import org.springframework.util.StringUtils;
    
    public class BeanPropertyRowMapper<T> implements RowMapper<T> {
        protected final Log logger = LogFactory.getLog(super.getClass());
        private Class<T> mappedClass;
        private boolean checkFullyPopulated = false;
    
        private boolean primitivesDefaultedForNullValue = false;
        private Map<String, PropertyDescriptor> mappedFields;
        private Set<String> mappedProperties;
    
        public BeanPropertyRowMapper() {
        }
    
        public BeanPropertyRowMapper(Class<T> mappedClass) {
            initialize(mappedClass);
        }
    
        public BeanPropertyRowMapper(Class<T> mappedClass, boolean checkFullyPopulated) {
            initialize(mappedClass);
            this.checkFullyPopulated = checkFullyPopulated;
        }
    
        public void setMappedClass(Class<T> mappedClass) {
            if (this.mappedClass == null) {
                initialize(mappedClass);
            } else if (!(this.mappedClass.equals(mappedClass)))
                throw new InvalidDataAccessApiUsageException(
                        new StringBuilder().append("The mapped class can not be reassigned to map to ").append(mappedClass)
                                .append(" since it is already providing mapping for ").append(this.mappedClass).toString());
        }
    
        protected void initialize(Class<T> 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(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());
                }
        }
      //在大写的字符前加入下滑线
        private String underscoreName(String name) {
            if (!(StringUtils.hasLength(name))) {
                return "";
            }
            StringBuilder result = new StringBuilder();
            result.append(name.substring(0, 1).toLowerCase());
            for (int i = 1; i < name.length(); ++i) {
                String s = name.substring(i, i + 1);
                String slc = s.toLowerCase();
                if (!(s.equals(slc))) {
                    result.append("_").append(slc);
                } else {
                    result.append(s);
                }
            }
            return result.toString();
        }
    
        public final Class<T> getMappedClass() {
            return this.mappedClass;
        }
    
        public void setCheckFullyPopulated(boolean checkFullyPopulated) {
            this.checkFullyPopulated = checkFullyPopulated;
        }
    
        public boolean isCheckFullyPopulated() {
            return this.checkFullyPopulated;
        }
    
        public void setPrimitivesDefaultedForNullValue(boolean primitivesDefaultedForNullValue) {
            this.primitivesDefaultedForNullValue = primitivesDefaultedForNullValue;
        }
    
        public boolean isPrimitivesDefaultedForNullValue() {
            return this.primitivesDefaultedForNullValue;
        }
    
        public T mapRow(ResultSet rs, int rowNumber) throws SQLException {
            Assert.state(this.mappedClass != null, "Mapped class was not specified");
            Object mappedObject = BeanUtils.instantiate(this.mappedClass);
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(mappedObject);
            initBeanWrapper(bw);
    
            ResultSetMetaData rsmd = rs.getMetaData();
            int columnCount = rsmd.getColumnCount();
            Set populatedProperties = (isCheckFullyPopulated()) ? new HashSet() : null;//稀少的特性
    
            for (int index = 1; index <= columnCount; ++index) {
                String column = JdbcUtils.lookupColumnName(rsmd, index);
                PropertyDescriptor pd = (PropertyDescriptor) this.mappedFields
                        .get(column.replaceAll(" ", "").toLowerCase());
                if (pd == null)
                    continue;
                try {
                    Object value = getColumnValue(rs, index, pd);
                    if ((this.logger.isDebugEnabled()) && (rowNumber == 0))
                        this.logger.debug(
                                new StringBuilder().append("Mapping column '").append(column).append("' to property '")
                                        .append(pd.getName()).append("' of type ").append(pd.getPropertyType()).toString());
                    try {
                        bw.setPropertyValue(pd.getName(), value);
                    } catch (TypeMismatchException e) {
                        if ((value == null) && (this.primitivesDefaultedForNullValue)) {
                            this.logger.debug(new StringBuilder().append("Intercepted TypeMismatchException for row ")
                                    .append(rowNumber).append(" and column '").append(column).append("' with value ")
                                    .append(value).append(" when setting property '").append(pd.getName())
                                    .append("' of type ").append(pd.getPropertyType()).append(" on object: ")
                                    .append(mappedObject).toString());
                        } else {
                            throw e;
                        }
                    }
                    if (populatedProperties != null) {
                        populatedProperties.add(pd.getName());
                    }
                } catch (NotWritablePropertyException ex) {
                    throw new DataRetrievalFailureException(new StringBuilder().append("Unable to map column ")
                            .append(column).append(" to property ").append(pd.getName()).toString(), ex);
                }
    
            }
    
            if ((populatedProperties != null) && (!(populatedProperties.equals(this.mappedProperties)))) {
                throw new InvalidDataAccessApiUsageException(new StringBuilder()
                        .append("Given ResultSet does not contain all fields necessary to populate object of class [")
                        .append(this.mappedClass).append("]: ").append(this.mappedProperties).toString());
            }
    
            return mappedObject;
        }
    
        protected void initBeanWrapper(BeanWrapper bw) {
        }
    
        protected Object getColumnValue(ResultSet rs, int index, PropertyDescriptor pd) throws SQLException {
            return JdbcUtils.getResultSetValue(rs, index, pd.getPropertyType());
        }
    
        public static <T> BeanPropertyRowMapper<T> newInstance(Class<T> mappedClass) {
            BeanPropertyRowMapper newInstance = new BeanPropertyRowMapper();
            newInstance.setMappedClass(mappedClass);
            return newInstance;
        }
    }

      1.通过PropertyDescriptor反映射调用set和get方法

      2.HashSet、TreeSet equals方法

      AbstractSet.java

      public
    boolean equals(Object o) { if (o == this) return true; if (!(o instanceof Set)) return false; Collection<?> c = (Collection<?>) o; if (c.size() != size()) return false; try { return containsAll(c); } catch (ClassCastException unused) { return false; } catch (NullPointerException unused) { return false; } }

      3.BeanPropertyRowMapper checkFullyPopulated 默认是false,这样的话如果 sql结果集中的字段 和 DTO 字段不匹配,就会抛异常。可以手动设置这个值。

      4.PO BO VO DTO POJO DAO概念及其作用(附转换图)

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  • 原文地址:https://www.cnblogs.com/hujunzheng/p/6306769.html
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