如何从Spring查询dsl谓词中提取表达式的路径和值？

Question

I'm using Spring query DSL on one of my projects. Query DSL predicates are created automatically on Controller endpoint, like this

public ResponseEntity<MyDTO> agentWinLoseSA(
        @QuerydslPredicate(root = MyObject.class) Predicate filter, Pageable pageable) throws URISyntaxException

It is supplied via query params like this

https://myhost:port/api/myobj?person.name=Dave

Is there any way to extract expressions (path=value) from created Predicate?

I found how to extract Expression path from Predicate object, but I can't find meaningful way to extract value under this path.

Path can be extracted like this (and for current example it will be person.name)

    List<Expression<?>> expressions = ((PredicateOperation) filter).getArgs();
    for (Expression<?> expression : expressions)
    {
        String expressionPath = expression.accept(PathExtractor.DEFAULT, null).toString();
    }

I assume, this is somehow should be done via visitors with extracted path, but I can't figure out how.

UPDATE . I found a solution:

import com.querydsl.core.types.Constant;
import com.querydsl.core.types.Expression;
import com.querydsl.core.types.FactoryExpression;
import com.querydsl.core.types.Operation;
import com.querydsl.core.types.ParamExpression;
import com.querydsl.core.types.Path;
import com.querydsl.core.types.SubQueryExpression;
import com.querydsl.core.types.TemplateExpression;
import com.querydsl.core.types.Visitor;


public class ConstantExtractor implements Visitor<Constant<?>, Void>
{

    public static final ConstantExtractor DEFAULT = new ConstantExtractor();


    private ConstantExtractor()
    {
    }


    @Override
    public Constant<?> visit(Constant<?> expr, Void context)
    {
        return expr;
    }


    @Override
    public Constant<?> visit(FactoryExpression<?> expr, Void context)
    {
        return visit(expr.getArgs());
    }


    @Override
    public Constant<?> visit(Operation<?> expr, Void context)
    {
        return visit(expr.getArgs());
    }


    @Override
    public Constant<?> visit(ParamExpression<?> expr, Void context)
    {
        return null;
    }


    @Override
    public Constant<?> visit(Path<?> expr, Void context)
    {
        return null;
    }


    @Override
    public Constant<?> visit(SubQueryExpression<?> expr, Void context)
    {
        return null;
    }


    @Override
    public Constant<?> visit(TemplateExpression<?> expr, Void context)
    {
        return visit(expr.getArgs());
    }


    private Constant<?> visit(List<?> exprs)
    {
        for (Object e : exprs)
        {
            if (e instanceof Expression)
            {
                Constant<?> constant = ((Expression<?>) e).accept(this, null);
                if (constant != null)
                {
                    return constant;
                }
            }
        }

        return null;
    }

   }

Usage is quite simple:

Constant constant = expression.accept(ConstantExtractor.DEFAULT, null);

Answer 1

I found a solution . Code below extracts first Constant value of expression, it uses it own visitor. Solution can be easily modified to work with some collection of Constants (because sometimes there are more then one value under the same path, for example, in date ranges)

import com.querydsl.core.types.Constant;
import com.querydsl.core.types.Expression;
import com.querydsl.core.types.FactoryExpression;
import com.querydsl.core.types.Operation;
import com.querydsl.core.types.ParamExpression;
import com.querydsl.core.types.Path;
import com.querydsl.core.types.SubQueryExpression;
import com.querydsl.core.types.TemplateExpression;
import com.querydsl.core.types.Visitor;


public class ConstantExtractor implements Visitor<Constant<?>, Void>
{

    public static final ConstantExtractor DEFAULT = new ConstantExtractor();


    private ConstantExtractor()
    {
    }


    @Override
    public Constant<?> visit(Constant<?> expr, Void context)
    {
        return expr;
    }


    @Override
    public Constant<?> visit(FactoryExpression<?> expr, Void context)
    {
        return visit(expr.getArgs());
    }


    @Override
    public Constant<?> visit(Operation<?> expr, Void context)
    {
        return visit(expr.getArgs());
    }


    @Override
    public Constant<?> visit(ParamExpression<?> expr, Void context)
    {
        return null;
    }


    @Override
    public Constant<?> visit(Path<?> expr, Void context)
    {
        return null;
    }


    @Override
    public Constant<?> visit(SubQueryExpression<?> expr, Void context)
    {
        return null;
    }


    @Override
    public Constant<?> visit(TemplateExpression<?> expr, Void context)
    {
        return visit(expr.getArgs());
    }


    private Constant<?> visit(List<?> exprs)
    {
        for (Object e : exprs)
        {
            if (e instanceof Expression)
            {
                Constant<?> constant = ((Expression<?>) e).accept(this, null);
                if (constant != null)
                {
                    return constant;
                }
            }
        }

        return null;
    }

   }

And usage:

List<Expression<?>> expressions = ((PredicateOperation) filter).getArgs();
for (Expression<?> expression : expressions)
{
    String expressionPath = expression.accept(PathExtractor.DEFAULT, null).toString();
    Constant constant = expression.accept(ConstantExtractor.DEFAULT, null);
}