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本篇文章为大家展示了怎么在mybatisplus 中使用SQL拦截器实现关联查询功能，内容简明扼要并且容易理解，绝对能使你眼前一亮，通过这篇文章的详细介绍希望你能有所收获。
环境信息
jdk： 1.8
springboot： 2.3.4.RELEASE
mybatisplus： 3.4.2
lombok：1.18.12
代码涉及四个关键的类：
JoinBuilder
这是一个建造者类，主要适用于生成关联查询的语句
CaseBuilder
这也是一个建造者类，主要是用来生成连接查询的条件语句
MyQueryWrapper
这是查询器，这里继承了官方的QueryWrapper，然后扩展了一些功能。添加了关联查询的功能
JoinQueryInterceptor
这是SQL拦截器，在上面使用自定义的查询器添加了关联查询之后就可以使用SQL拦截器进行sql的构造
类关系图如下：
实现连接条件构造器
package com.jenkin.common.config;
import cn.hutool.core.util.ArrayUtil;
import com.baomidou.mybatisplus.core.toolkit.StringUtils;
import lombok.extern.slf4j.Slf4j;
import net.sf.jsqlparser.JSQLParserException;
import net.sf.jsqlparser.parser.CCJSqlParserManager;
import net.sf.jsqlparser.statement.select.Join;
import net.sf.jsqlparser.statement.select.PlainSelect;
import net.sf.jsqlparser.statement.select.Select;
import java.io.StringReader;
import java.util.HashSet;
import java.util.Set;
/**
* @author jenkin
* @Since 2021年4月12日14:45:58
* @Version 1.0
* @Description : 关联查询join构造器
*/
@Slf4j
public class JoinBuilder {
private StringBuilder sb = new StringBuilder();
/**
* 关联表里面的查询字段，比如要查询关联的用户表里面的用户名称
*/
private String[] selectFields;
/**
* 关联表
*/
private String joinTable;
/**
* 查询字段去重
*/
Set<String> set = new HashSet<>();
/**
* 主表
*/
private String mainTable;
/**
* 关联类型
*/
private String joinType;
private static final String LEFT_BRACKET = " ( ";
private static final String RIGHT_BRACKET = " ) ";
private static final String AND = " AND ";
private static final String OR = " OR ";
/**
* 左连接
*/
public static final String LEFT = " left ";
/**
* 右连接
*/
public static final String RIGHT = " right ";
/**
* 内连接
*/
public static final String INNER = " inner ";
public JoinBuilder selectField(String... fields) {
this.selectFields = fields;
if (!ArrayUtil.isEmpty(this.selectFields)) {
for (int i = 0; i < this.selectFields.length; i++) {
this.selectFields[i] = StringUtils.camelToUnderline(this.selectFields[i]);
set.add(this.selectFields[i].toUpperCase());
}
}
return this;
}
public Set<String> getSelectFields() {
return set;
}
public String getMainTable() {
return mainTable;
}
public String getSubTable() {
return this.joinTable;
}
/**
* @param joinType 关联类型 JoinBuilder.LEFT,JoinBuilder.RIGHT,JoinBuilder.INNER
* @param mainTable 主表
* @param joinTable 关联表
* @return
*/
public JoinBuilder join(String joinType, String mainTable, String joinTable) {
mainTable = StringUtils.camelToUnderline(mainTable);
;
joinTable = StringUtils.camelToUnderline(joinTable);
;
this.joinTable = joinTable;
this.mainTable = mainTable;
this.joinType = joinType;
return this;
}
public static JoinBuilder build() {
return new JoinBuilder();
}
public JoinBuilder and() {
sb.append(AND);
return this;
}
public JoinBuilder or() {
sb.append(OR);
return this;
}
public StringBuilder getSql() {
return sb;
}
public JoinBuilder on(CaseBuilder builder) {
sb.append(LEFT_BRACKET).append(builder.getSql()).append(RIGHT_BRACKET);
return this;
}
public Join getJoin() {
CCJSqlParserManager pm = new CCJSqlParserManager();
String sql = "select * from " + mainTable + " " + joinType + " join " + joinTable + " on " + sb;
try {
net.sf.jsqlparser.statement.Statement parse = pm.parse(new StringReader(sql));
if (parse instanceof Select) {
return ((PlainSelect) ((Select) parse).getSelectBody()).getJoins().get(0);
}
return null;
} catch (JSQLParserException e) {
log.warn(sql);
e.printStackTrace();
}
return null;
}
/**
* @author jenkin
* @Since 2021年4月12日14:45:58
* @Version 1.0
* @Description : 条件构造器，局限于关联查询
*/
public static class CaseBuilder {
/**
* SQL语句
*/
private StringBuilder sb = new StringBuilder();
private static final String LEFT_BRACKET = " ( ";
private static final String RIGHT_BRACKET = " ) ";
private static final String EQ = "=";
private static final String NE = "<>";
private static final String GT = ">";
private static final String LT = "<";
private static final String GT_EQ = ">=";
private static final String LT_EQ = "<=";
private static final String AND = " AND ";
private static final String OR = " OR ";
public static CaseBuilder build() {
return new CaseBuilder();
}
public StringBuilder getSql() {
return sb;
}
/**
* 把条件表达式用括号包裹起来
*
* @param builder
* @return
*/
public CaseBuilder brackets(CaseBuilder builder) {
sb.append(LEFT_BRACKET).append(builder.sb).append(RIGHT_BRACKET);
return this;
}
public CaseBuilder and() {
sb.append(AND);
return this;
}
public CaseBuilder or() {
sb.append(OR);
return this;
}
/**
* 规定左侧为主表的列
* ，右侧为从表的列，不可以写反
* 注意，在使用定值查询的时候 例如 on a.name = b.name and age = 1
* 这个时候一样要遵循左边为主表，右边为关联表的规则.
* 例如
* <p>
* 1、 and里面的条件 age字段是存在在主表里面的 那么就写成 eq("age",1)
* 2、如果age字段是在关联表里面的，那么应该写成 eq(1,"age")
* <p>
* 其他的条件语句例如，ne，gt，lt等等也适用这个逻辑
*
* @param left 左侧列名称
* @param right 右侧列名称
* @return
*/
public CaseBuilder eq(Object left, Object right) {
if (left instanceof String) {
left = StringUtils.camelToUnderline((String) left);
}
if (right instanceof String) {
right = StringUtils.camelToUnderline(String.valueOf(right));
}
sb.append(left).append(EQ).append(right);
return this;
}
/**
* 规定左侧为主表的列
* ，右侧为从表的列，不可以写反
*
* @param left 左侧列名称
* @param right 右侧列名称
* @return
*/
public CaseBuilder ne(String left, Object right) {
left = StringUtils.camelToUnderline(left);
if (right instanceof String) {
right = StringUtils.camelToUnderline(String.valueOf(right));
}
sb.append(left).append(NE).append(right);
return this;
}
/**
* 关联查询一般是列关联，如果条件里面有值等式，要做特殊处理，目前还不支持
*
* @param left
* @param right
* @return
*/
@Deprecated
public CaseBuilder gt(String left, Object right) {
sb.append(left).append(GT).append(right);
return this;
}
/**
* 关联查询一般是列关联，如果条件里面有值等式，要做特殊处理，目前还不支持
*
* @param left
* @param right
* @return
*/
@Deprecated
public CaseBuilder gtEq(String left, Object right) {
sb.append(left).append(GT_EQ).append(right);
return this;
}
/**
* 关联查询一般是列关联，如果条件里面有值等式，要做特殊处理，目前还不支持
*
* @param left
* @param right
* @return
*/
@Deprecated
public CaseBuilder lt(String left, Object right) {
sb.append(left).append(LT).append(right);
return this;
}
/**
* 关联查询一般是列关联，如果条件里面有值等式，要做特殊处理，目前还不支持
*
* @param left
* @param right
* @return
*/
@Deprecated
public CaseBuilder ltEq(String left, Object right) {
sb.append(left).append(LT_EQ).append(right);
return this;
}
}
}定制化QueryWrapper
在这个定制化的查询器里面添加了一个addJoin的方法用来添加关联查询
package com.jenkin.common.config;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.core.toolkit.CollectionUtils;
import com.baomidou.mybatisplus.core.toolkit.StringUtils;
import com.jenkin.common.entity.qos.Sort;
import java.util.ArrayList;
import java.util.List;
/**
* @author ：jenkin
* @date ：Created at 2020/3/13 12:07
* @description：条件构造器，重写字符串转换方法
* @modified By：
* @version: 1.0
*/
public class
MyQueryWrapper<T> extends QueryWrapper<T> {
/**
* 关联查询构造器
*/
private final List<JoinBuilder> joinBuilder = new ArrayList<>();
/**
* 获取 columnName
*
* @param column
*/
@Override
protected String columnToString(String column) {
return StringUtils.camelToUnderline(column);
}
public static<T> MyQueryWrapper<T> query(){
return new MyQueryWrapper<T>();
}
/**
* 关联查询构造
* @param builder
* @return
*/
public MyQueryWrapper<T> addJoin(JoinBuilder builder){
this.joinBuilder.add(builder);
return this;
}
public List<JoinBuilder> getJoinBuilder() {
return joinBuilder;
}
/**
* 排序
* @param sorts
* @return
*/
public QueryWrapper<T> sort(List<Sort> sorts){
if(!CollectionUtils.isEmpty(sorts)){
sorts.forEach(item->{
orderBy(item.getSortField()!=null,"asc".equals(item.getSortValue()),item.getSortField());
});
}
return this;
}
}定义SQL拦截器
通过自定义的SQL拦截器去拦截我们写好的关联查询，然后生成对应的SQL
package com.jenkin.common.config;
import com.alibaba.nacos.client.naming.utils.CollectionUtils;
import com.baomidou.mybatisplus.core.toolkit.PluginUtils;
import com.baomidou.mybatisplus.extension.parser.JsqlParserSupport;
import com.baomidou.mybatisplus.extension.plugins.inner.InnerInterceptor;
import net.sf.jsqlparser.expression.Alias;
import net.sf.jsqlparser.expression.BinaryExpression;
import net.sf.jsqlparser.expression.Expression;
import net.sf.jsqlparser.expression.Parenthesis;
import net.sf.jsqlparser.expression.operators.relational.Between;
import net.sf.jsqlparser.expression.operators.relational.IsNullExpression;
import net.sf.jsqlparser.schema.Column;
import net.sf.jsqlparser.schema.Table;
import net.sf.jsqlparser.statement.delete.Delete;
import net.sf.jsqlparser.statement.insert.Insert;
import net.sf.jsqlparser.statement.select.*;
import net.sf.jsqlparser.statement.update.Update;
import org.apache.ibatis.binding.MapperMethod;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
/**
* @author jenkin
* @className JoinQueryInterceptor
* @description TODO
* @date 2021/4/12 14:58
*/
public class JoinQueryInterceptor extends JsqlParserSupport implements InnerInterceptor {
/**
* 保存我们的关联查询的上下文信息
*/
static ThreadLocal<List<JoinBuilder>> joinBuilderThreadLocal = new ThreadLocal<>();
/**
* 操作前置处理
* <p>
* 改改sql啥的
*
* @param executor Executor(可能是代理对象)
* @param ms MappedStatement
* @param parameter parameter
* @param rowBounds rowBounds
* @param resultHandler resultHandler
* @param boundSql boundSql
*/
@Override
public void beforeQuery(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds,
ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
if (parameter instanceof MapperMethod.ParamMap) {
for (Object value : ((MapperMethod.ParamMap) parameter).values()) {
if (value instanceof MyQueryWrapper) {
List<JoinBuilder> joinBuilders = ((MyQueryWrapper) value).getJoinBuilder();
if(!CollectionUtils.isEmpty(joinBuilders)){
joinBuilderThreadLocal.set(joinBuilders);
try {
logger.debug("开始添加关联查询SQL");
String s = this.parserSingle(boundSql.getSql(), parameter);
logger.debug("加了关联查询的SQL ： "+ s);
PluginUtils.MPBoundSql mpBs = PluginUtils.mpBoundSql(boundSql);
mpBs.sql(s);
}finally {
joinBuilderThreadLocal.remove();
}
return;
}
}
}
}
}
/**
* 查询
*/
@Override
protected void processSelect(Select select, int index, String sql, Object obj) {
List<JoinBuilder> joinBuilders = joinBuilderThreadLocal.get();
PlainSelect selectBody = (PlainSelect) select.getSelectBody();
if (selectBody.getFromItem().getAlias()==null) {
selectBody.getFromItem().setAlias(new Alias("mainjointable"));
}
setJoins(selectBody,joinBuilders);
}
private void setJoins(PlainSelect selectBody,List<JoinBuilder> joinBuilders) {
List<SelectItem> selectItems = selectBody.getSelectItems();
Expression where = selectBody.getWhere();
List<Join> joins = new ArrayList<>();
for (int i = 0; i < joinBuilders.size(); i++) {
JoinBuilder joinBuilder = joinBuilders.get(i);
Join builderJoin = joinBuilder.getJoin();
Set<String> selectFields = new HashSet<>(joinBuilder.getSelectFields());
Join join = new Join();
join.setLeft(builderJoin.isLeft());
join.setRight(builderJoin.isRight());
join.setInner(builderJoin.isInner());
Table table = new Table(joinBuilder.getSubTable());
table.setAlias( new Alias("subjointable"+i));
setSelectItems(table,selectFields,selectItems,selectBody);
join.setRightItem(table);
// Expression expression = getOnExpressionWithTable(joinBuilder);
Expression onExpression = joinBuilder.getJoin().getOnExpression();
selectFields = new HashSet<>(joinBuilder.getSelectFields());
setOnCase(onExpression,table,selectFields,(Table) selectBody.getFromItem(),false);
join.setOnExpression(onExpression);
joins.add(join);
selectFields = new HashSet<>(joinBuilder.getSelectFields());
setWhere(where,table,selectFields,(Table) selectBody.getFromItem());
}
selectBody.setJoins(joins);
}
// private Expression getOnExpressionWithTable(JoinBuilder joinBuilder) {
setWhere(joinBuilder.getJoin().getOnExpression(),);
// return joinBuilder.getJoin().getOnExpression();
// }
private void setSelectItems(Table table, Set<String> selectFields, List<SelectItem> selectItems, PlainSelect selectBody) {
for (SelectItem selectItem : selectItems) {
if (selectItem instanceof SelectExpressionItem) {
if (((SelectExpressionItem) selectItem).getExpression() instanceof Column && selectBody.getFromItem() instanceof Table) {
Column expression = (Column) ((SelectExpressionItem) selectItem).getExpression();
if (expression.getTable()==null&&selectFields.contains(expression.getColumnName().toUpperCase())){
expression.setTable(table);
selectFields.remove(expression.getColumnName().toUpperCase());
}else if(expression.getTable()==null){
expression.setTable((Table) selectBody.getFromItem());
}
}
}
}
if (!selectFields.isEmpty()){
for (String selectField : selectFields) {
SelectExpressionItem selectExpressionItem = new SelectExpressionItem();
Column column = new Column();
column.setTable(table);
column.setColumnName(selectField);
selectExpressionItem.setExpression(column);
selectItems.add(selectExpressionItem);
}
}
}
/**
*
* @param on
* @param subTable
* @param joinSelectFields
* @param sourceTable
* @param isLeft 是否是左侧列，如果是那么就是主表，如果false，那么就是关联表，如果为null，那么就需要根据join字段判断
*/
private void setOnCase(Object on, Table subTable, Set<String> joinSelectFields , Table sourceTable,Boolean isLeft) {
if (on==null) {
return;
}
if (on instanceof Column) {
Column column = (Column) on;
if((column).getTable()==null &&isLeft!=null){
(column).setTable(isLeft?sourceTable:subTable);
}
if (isLeft==null&&column.getTable()==null){
(column).setTable(joinSelectFields.contains(column.getColumnName().toUpperCase())?subTable:sourceTable);
}
}else if (on instanceof BinaryExpression){
setOnCase(((BinaryExpression) on).getLeftExpression(),subTable,joinSelectFields,sourceTable,true);
setOnCase(((BinaryExpression) on).getRightExpression(),subTable,joinSelectFields,sourceTable,false);
}else if (on instanceof Parenthesis){
setOnCase(((Parenthesis) on).getExpression(),subTable,joinSelectFields,sourceTable,false);
}else if(on instanceof IsNullExpression){
setOnCase(((IsNullExpression) on).getLeftExpression(),subTable,joinSelectFields,sourceTable,null);
}else if (on instanceof Between){
setOnCase(((Between) on).getLeftExpression(),subTable,joinSelectFields,sourceTable,null);
}
//有其他条件再补充
}
private void setWhere(Object where, Table subTable, Set<String> joinSelectFields , Table sourceTable) {
if (where==null) {
return;
}
if (where instanceof Column) {
Column column = (Column) where;
if((column).getTable()==null&&joinSelectFields.contains((column).getColumnName().toUpperCase())){
(column).setTable(subTable);
}else if((column).getTable()==null){
(column).setTable(sourceTable);
}
}else if (where instanceof BinaryExpression){
setWhere(((BinaryExpression) where).getLeftExpression(),subTable,joinSelectFields,sourceTable);
setWhere(((BinaryExpression) where).getRightExpression(),subTable,joinSelectFields,sourceTable);
}else if (where instanceof Parenthesis){
setWhere(((Parenthesis) where).getExpression(),subTable,joinSelectFields,sourceTable);
}else if(where instanceof IsNullExpression){
setWhere(((IsNullExpression) where).getLeftExpression(),subTable,joinSelectFields,sourceTable);
}else if(where instanceof Between){
setWhere(((Between) where).getLeftExpression(),subTable,joinSelectFields,sourceTable);
}
//有其他条件再补充
}
}注入拦截器
紧接着只需要在mybatisplus的配置文件里面注入这个拦截器就可以了
使用示例
使用的过程我们分为两步：
添加字段到主表PO
如图，红框中的部分是我们添加的需要从其他表里面关联查询的字段，注意这些字段需要使用@TableField注解标注，并且 select字段和exist字段都要为false，不然会影响新增和修改操作
queryWrapper构造关联查询
把字段添加好之后就可以开始写关联查询了，下奶的示例应该是一个涵盖了大部分场景的示例了，多表关联，多条件关联，等等
MyQueryWrapper<MenuPo> queryWrapper = new MyQueryWrapper<>(); //添加一个关联表查询，关联用户表 queryWrapper.addJoin( JoinBuilder.build() //查询用户表里面的用户名称和用户邮箱字段 .selectField(MenuPo.Fields.userName, MenuPo.Fields.userEmail) //使用左连接关联 .join(JoinBuilder.LEFT, MenuPo.class, UserPo.class) //设置关联条件 .on(JoinBuilder.CaseBuilder.build() //主表的创建人字段等于关联表的用户编码字段 // 注意，在条件中默认是第一个参数为主表的字段，第二个参数为关联表的字段 .eq(BasePo.Fields.createdBy, UserPo.Fields.userCode) ) //再添加一个关联查询，关联角色表 ).addJoin( JoinBuilder.build() //查血角色表里面的角色名称 .selectField(MenuPo.Fields.roleName) //左连接 .join(JoinBuilder.LEFT,MenuPo.class, RolePo.class) //关联条件 .on(JoinBuilder.CaseBuilder.build() //code等于角色code .eq(MenuPo.Fields.code, RolePo.Fields.roleCode) //并且 .and() //括号 .brackets( //parent =-1 or parent =1 JoinBuilder.CaseBuilder.build() .eq(MenuPo.Fields.parent,-1) .or() .eq(MenuPo.Fields.parent,1) ) ) //外层筛选条件，用户名=jenkin ).eq(MenuPo.Fields.userName,"jenkin"); //执行查询 menuService.list(queryWrapper);
可以在控制台看到执行的SQL：
SELECT mainjointable.id, mainjointable.NAME, mainjointable.CODE, mainjointable.parent, mainjointable.menu_level, mainjointable.permissions, mainjointable.menu_url, mainjointable.menu_icon, mainjointable.menu_order, mainjointable.menu_type, mainjointable.delete_flag, mainjointable.created_by, mainjointable.creation_date, mainjointable.last_update_date, mainjointable.last_updated_by, mainjointable.version_number, subjointable0.USER_EMAIL, subjointable0.USER_NAME, subjointable1.ROLE_NAME FROM lsc_menu AS mainjointable LEFT JOIN lsc_user AS subjointable0 ON ( mainjointable.created_by = subjointable0.user_code ) LEFT JOIN lsc_role AS subjointable1 ON ( mainjointable.CODE = subjointable1.role_code AND ( mainjointable.parent = - 1 OR mainjointable.parent = 1 ) ) WHERE mainjointable.delete_flag = 0 AND ( subjointable0.user_name = ?)
上述内容就是怎么在mybatisplus 中使用SQL拦截器实现关联查询功能，你们学到知识或技能了吗？如果还想学到更多技能或者丰富自己的知识储备，欢迎关注亿速云行业资讯频道。
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