mybatis-plus 自定义QueryWrapper（一）实现查询函数

以下所有代码和观点来源均为

        
            org.mybatis.spring.boot
            mybatis-spring-boot-starter
            2.0.1
        

mybatis-plus 作为一个强大的 mybatis阵营的orm框架，很多功能已经用的飞起，但是呢在查询方面还是欠缺部分api，比如要进行如下查询时无法用到 lamdba表达式

    select min(column) as n from table 

这时候是无法用自带的LambdaQueryWrapper实现的，此时就需要我们手动改造下

核心源码其实就两个方法一个属性

// 存放sql语句
private SharedString sqlSelect = new SharedString();


@SafeVarargs
@Override
public final LambdaQueryWrapper select(SFunction... columns) {
    if (ArrayUtils.isNotEmpty(columns)) {
        this.sqlSelect.setStringValue(columnsToString(false, columns));
    }
    return typedThis;
}



@Override
public String getSqlSelect() {
    return sqlSelect.getStringValue();
}

SharedString 这个类本质上也是一个string只是多了几个方法而已

public class SharedString implements Serializable {
    private static final long serialVersionUID = -1536422416594422874L;

    
    private String stringValue;

    
    public static SharedString emptyString() {
        return new SharedString(StringPool.EMPTY);
    }

    
    public void toEmpty() {
        stringValue = StringPool.EMPTY;
    }

    
    public void toNull() {
        stringValue = null;
    }
}

所以在使用中多个字段的查询不能函数调用

此时我们自己实现 LambdaQueryWrapper,然后将SharedString 改成一个由list实现的类

自定义的LambdaQueryWrapper 命名为 MyLambdaQueryWrapper

public class MyLambdaQueryWrapper extends AbstractLambdaWrapper>
        implements Query, T, SFunction> {

    private SelectBuilder sqlSelect = new SelectBuilder();

    
    public MyLambdaQueryWrapper() {
        this((T) null);
    }

    
    public MyLambdaQueryWrapper(T entity) {
        super.setEntity(entity);
        super.initNeed();
    }

    
    public MyLambdaQueryWrapper(Class entityClass) {
        super.setEntityClass(entityClass);
        super.initNeed();
    }

    
    MyLambdaQueryWrapper(T entity, Class entityClass, SelectBuilder sqlSelect, AtomicInteger paramNameSeq,
                       Map paramNamevaluePairs, MergeSegments mergeSegments,
                       SharedString lastSql, SharedString sqlComment, SharedString sqlFirst) {
        super.setEntity(entity);
        super.setEntityClass(entityClass);
        this.paramNameSeq = paramNameSeq;
        this.paramNamevaluePairs = paramNamevaluePairs;
        this.expression = mergeSegments;
        this.sqlSelect = sqlSelect;
        this.lastSql = lastSql;
        this.sqlComment = sqlComment;
        this.sqlFirst = sqlFirst;
    }

    
    @SafeVarargs
    @Override
    public final MyLambdaQueryWrapper select(SFunction... columns) {
        if (ArrayUtils.isNotEmpty(columns)) {
            this.sqlSelect.getParts().add(columnsToString(columns));
        }
        return typedThis;
    }

    
    @Override
    public MyLambdaQueryWrapper select(Class entityClass, Predicate predicate) {
        if (entityClass == null) {
            entityClass = getEntityClass();
        } else {
            setEntityClass(entityClass);
        }
        Assert.notNull(entityClass, "entityClass can not be null");
        this.sqlSelect.getParts().add(TableInfoHelper.getTableInfo(entityClass).chooseSelect(predicate));
        return typedThis;
    }

    @Override
    public String getSqlSelect() {
        return sqlSelect.getStringValue();
    }

    
    @Override
    protected MyLambdaQueryWrapper instance() {
        return new MyLambdaQueryWrapper<>(getEntity(), getEntityClass(), null, paramNameSeq, paramNamevaluePairs,
                new MergeSegments(), SharedString.emptyString(), SharedString.emptyString(), SharedString.emptyString());
    }

    @Override
    public void clear() {
        super.clear();
        sqlSelect.toNull();
    }

// =================== 上面的代码都是照抄的   =====================

    public MyLambdaQueryWrapper min(SFunction column){
        if (column != null) {
            this.sqlSelect.getParts().add(String.format("min(%s)", columnToString(column)));
        }
        return typedThis;
    }

    public MyLambdaQueryWrapper max(SFunction column){
        if (column != null) {
            this.sqlSelect.getParts().add(String.format("max(%s)", columnToString(column)));
        }
        return typedThis;
    }

    public MyLambdaQueryWrapper sum(SFunction column){
        if (column != null) {
            this.sqlSelect.getParts().add(String.format("sum(%s)", columnToString(column)));
        }
        return typedThis;
    }

    public MyLambdaQueryWrapper as(String asName){
        if (StringUtils.isEmpty(asName)) {
            return typedThis;
        }

        List parts = this.sqlSelect.getParts();

        if (CollectionUtil.isEmpty(parts)) {
            return typedThis;
        }

        String lastPart = parts.remove(parts.size() - 1);
        lastPart += " as " + asName;

        parts.add(lastPart);


        return typedThis;
    }



}

还有一个暂存查询列的list组装对象

@Getter
@Setter
public class SelectBuilder implements Serializable {
    private static final long serialVersionUID = -1536422416594422874L;

    private List parts;

    public SelectBuilder() {
        this.parts = new ArrayList<>();
    }

    public void toEmpty() {
        parts.clear();
    }

    
    public void toNull() {
        parts = null;
    }

    public String getStringValue(){
        if (CollectionUtil.isNotEmpty(parts)) {
            return String.join(",", parts);
        }else {
            return null;
        }
    }

}

MyLambdaQueryWrapper myLambdaQuery = getMyLambdaQuery();
myLambdaQuery.min(SignConfig::getLongitude).as("logg");
myLambdaQuery.sum(SignConfig::getLongitude).as("lo");

return baseMapper.selectObjs(myLambdaQuery);

对应的sql

SELECT min(longitude) as logg,sum(longitude) as lo FROM work_sign_config