Conditionally create bean if Map of properties is not empty with SpEL - spring

Question

I have a class with a method that is periodically invoked with the #Scheduled annotation. The method does some bulk operations on a given set of properties.
If there are no properties set, I don't need the scheduled method invocation nor the instantiated class. Therefore, I've added this SpEL expression to check whether the properties are set:
#Service
#ConditionalOnExpression("#(T(java.util.Map)('${myproperties.people:{:}}')).size() > 0")
public class PeopleService { ... }
Example values in the application.yml could be:
myproperties:
people:
uuid1:
name: Mark
age: 32
uuid2:
name: Jeff
age: 36
Unfortunately I get this error message:
Caused by: org.springframework.expression.spel.SpelParseException: Expression [#(T(java.util.Map)('{:}')).size() > 0] #1: EL1043E: Unexpected token. Expected 'identifier' but was 'lparen(()'
Note that I came up with {:} for an empty map as default value here: https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#expressions-inline-maps
If I use this SpEL, I get the following error: "#(T(java.util.Map)(${myproperties.people:})).size() > 0"
Caused by: org.springframework.expression.spel.SpelParseException: Expression [#(T(java.util.Map)()).size() > 0] #1: EL1043E: Unexpected token. Expected 'identifier' but was 'lparen(()'
What is the correct way to accomplish this?

Answer 1

This annotation:
#ConditionalOnExpression("#(T(java.util.Map)('${myproperties.people:{:}}')).size() > 0")
Doesn't work because Spring read LITERALLY all your properties like that:
myproperties.people.uuid1.name=Mark
myproperties.people.uuid1.age=32
myproperties.people.uuid2.name=Jeff
myproperties.people.uuid2.age=36
If you check Spring Boot documentation, this page explains that "the condition matches if spring.example.values is present in the Environment but does not match if spring.example.values[0] is present.".
So for Spring, property "myproperties.people" just doesn't exist. On the same page, they explain that "it is better to use a custom condition for such cases."
A workaround is to define a property class (but it's not wasted, because you will use this class to parse your yaml as a map):
#ConfigurationProperties(prefix = "myproperties")
public class MyPeopleProperties {
private Map<String, String> people;
public Map<String, String> getPeople() {
return people;
}
public void setPeople(Map<String, String> people) {
this.people = people;
}
}
And on your main class:
...
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.context.annotation.Conditional;
...
#Service
#Conditional(PeopleService.PeopleServiceCondition.class)
public class PeopleService {
...
public static class PeopleServiceCondition implements Condition {
#Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
MyPeopleProperties config = Binder.get(context.getEnvironment())
.bind("myproperties", MyPeopleProperties.class).orElse(null);
return config != null && config.getPeople() != null && !config.getPeople().isEmpty();
}
}
}
So myproperties.people will be bound as map, then we check if this map exists on the bean declaration condition.

Answer 2

Circling back around to the SpEL implementation, you actually can do it entirely in SpEL but it's really ugly:
#ConditionalOnExpression(
"(" +
"T(org.springframework.boot.context.properties.bind.Binder)" +
".get(environment)" +
".bind('myproperties.people', T(java.util.Map))" +
".orElse(null)" +
"?.size()" +
"?: 0" +
") > 0"
)
This will work even for deeply-nested config subtrees - the simplistic binding to the Map class just creates a bunch of denormalized/flattened <String,String> entries with keys like uuid1.name. That means that if you are just testing for the existence of any sub-tree elements under the myproperties.people namespace, the above will work fine. It is, in effect, exactly what you probably would have meant when writing #ConditionalOnProperty("myproperties.people")