Implementing a Custom Predicate Evaluator for the Query Builder implementing-a-custom-predicate-evaluator-for-the-query-builder
This section describes how to extend the Query Builder by implementing a custom predicate evaluator.
Overview overview
The Query Builder offers an easy way of querying the content repository. CQ ships with a set of predicate evaluators that helps you deal with your data.
However you might want to simplify your queries by implementing a custom predicate evaluator that hides some complexity and ensures better semantics.
A custom predicate could also perform other things not directly possible with XPath, for example:
- looking up some data from some service
- custom filtering based on calculation
CODE ON GITHUB
You can find the code of this page on GitHub.
- Download the project as
Predicate Evaluator in Detail predicate-evaluator-in-detail
A predicate evaluator handles the evaluation of certain predicates, which are the defining constraints of a query.
It maps a higher-level search constraint (such as “width > 200”) to a specific JCR query that fits the actual content model (for example, metadata/@width > 200). Or it can manually filter nodes and check their constraints.
PredicateEvaluator
and the com.day.cq.search
package, see the .Implementing a Custom Predicate Evaluator for Replication Metadata implementing-a-custom-predicate-evaluator-for-replication-metadata
As an example this section describes how to create a custom predicate evaluator that helps data based on the replication metadata:
-
cq:lastReplicated
that stores the date of the last replication action -
cq:lastReplicatedBy
that stores the id of the user who triggered the last replication action -
cq:lastReplicationAction
that stores the last replication action (for example, Activation, Deactivation)
Querying Replication Metadata with Default Predicate Evaluators querying-replication-metadata-with-default-predicate-evaluators
The following query fetches the list of nodes in /content
branch that have been activated by admin
since the beginning of the year.
path=/content
1_property=cq:lastReplicatedBy
1_property.value=admin
2_property=cq:lastReplicationAction
2_property.value=Activate
daterange.property=cq:lastReplicated
daterange.lowerBound=2013-01-01T00:00:00.000+01:00
daterange.lowerOperation=>=
This query is valid but hard to read and does not highlight the relationship between the three replication properties. Implementing a custom predicate evaluator reduces the complexity and improve the semantic of this query.
Objectives objectives
The goal of the ReplicationPredicateEvaluator
is to support the above query using the following syntax.
path=/content
replic.by=admin
replic.since=2013-01-01T00:00:00.000+01:00
replic.action=Activate
Grouping replication metadata predicates with a custom predicate evaluator helps to create a meaningful query.
Updating Maven Dependencies updating-maven-dependencies
First, update the Maven dependencies of your project. The PredicateEvaluator
is part of the cq-search
artifact so it must be added to your Maven pom.xml file.
cq-search
dependency is set to provided
because cq-search
is provided by the OSGi
container.pom.xml
The following snippet shows the differences in
@@ -120,6 +120,12 @@
<scope>provided</scope>
<dependency>
+ <groupid>com.day.cq</groupid>
+ <artifactid>cq-search</artifactid>
+ <version>5.6.4</version>
+ <scope>provided</scope>
+ </dependency>
+ <dependency>
<groupid>junit</groupid>
<artifactid>junit</artifactid>
<version>3.8.1</version></dependency>
-
Writing The ReplicationPredicateEvaluator writing-the-replicationpredicateevaluator
The cq-search
project contains the AbstractPredicateEvaluator
abstract class. This can be extended with a few steps to implement your own custom predicate evaluator (PredicateEvaluator
).
Xpath
expression to filter data. Another option would be to implement the includes
method that selects data on a row basis. See the for more information.-
Create a Java™ class which extends
com.day.cq.search.eval.AbstractPredicateEvaluator
-
Annotate your class with a
@Component
like the followingsrc/main/java/com/adobe/aem/docs/search/ReplicationPredicateEvaluator.java
The following snippet shows the differences in
@@ -19,8 +19,11 @@
*/
package com.adobe.aem.docs.search;
+import org.apache.felix.scr.annotations.Component;
+
import com.day.cq.search.eval.AbstractPredicateEvaluator;
+@Component(metatype = false, factory = "com.day.cq.search.eval.PredicateEvaluator/repli")
public class ReplicationPredicateEvaluator extends AbstractPredicateEvaluator {
}
-
factory
must be a unique string starting with com.day.cq.search.eval.PredicateEvaluator/
and ending with the name of your custom PredicateEvaluator
.PredicateEvaluator
is the predicate name, which is used when building queries.-
Override:
code language-java public String getXPathExpression(Predicate predicate, EvaluationContext context)
In the override method, you build a
Xpath
expression based on thePredicate
given in argument.
Example of a Custom Predicate Evaluator for Replication Metadata example-of-a-custom-predicate-evalutor-for-replication-metadata
The complete implementation of this PredicateEvaluator
might be similar to the following class.
src/main/java/com/adobe/aem/docs/search/ReplicationPredicateEvaluator.java
/*
* #%L
* aem-docs-custom-predicate-evaluator
* %%
* Copyright (C) 2013 ۶Ƶ Research
* %%
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
* #L%
*/
package com.adobe.aem.docs.search;
import org.apache.felix.scr.annotations.Component;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import com.day.cq.search.Predicate;
import com.day.cq.search.eval.AbstractPredicateEvaluator;
import com.day.cq.search.eval.EvaluationContext;
@Component(metatype = false, factory = "com.day.cq.search.eval.PredicateEvaluator/repli")
public class ReplicationPredicateEvaluator extends AbstractPredicateEvaluator {
static final String PE_NAME = "replic";
static final String PN_LAST_REPLICATED_BY = "cq:lastReplicatedBy";
static final String PN_LAST_REPLICATED = "cq:lastReplicated";
static final String PN_LAST_REPLICATED_ACTION = "cq:lastReplicationAction";
static final String PREDICATE_BY = "by";
static final String PREDICATE_SINCE = "since";
static final String PREDICATE_SINCE_OP = " >= ";
static final String PREDICATE_ACTION = "action";
Logger log = LoggerFactory.getLogger(getClass());
/**
* Returns a XPath expression filtering by replication metadata.
*
* @see com.day.cq.search.eval.AbstractPredicateEvaluator#getXPathExpression(com.day.cq.search.Predicate,
* com.day.cq.search.eval.EvaluationContext)
*/
@Override
public String getXPathExpression(Predicate predicate,
EvaluationContext context) {
log.debug("predicate {}", predicate);
String date = predicate.get(PREDICATE_SINCE);
String user = predicate.get(PREDICATE_BY);
String action = predicate.get(PREDICATE_ACTION);
StringBuilder sb = new StringBuilder();
if (date != null) {
sb.append(PN_LAST_REPLICATED).append(PREDICATE_SINCE_OP);
sb.append("xs:dateTime('").append(date).append("')");
}
if (user != null) {
addAndOperator(sb);
sb.append(PN_LAST_REPLICATED_BY);
sb.append("='").append(user).append("'");
}
if (action != null) {
addAndOperator(sb);
sb.append(PN_LAST_REPLICATED_ACTION);
sb.append("='").append(action).append("'");
}
String xpath = sb.toString();
log.debug("xpath **{}**", xpath);
return xpath;
}
/**
* Add an and operator if the builder is not empty.
*
* @param sb a {@link StringBuilder} containing the query under construction
*/
private void addAndOperator(StringBuilder sb) {
if (sb.length() != 0) {
sb.append(" and ");
}
}
}
-