Specify spring expression language filter for KubernetesDiscoveryClient in Spring Boot Admin

I've specified a spring expression language filter in my spring boot admin application:

filter: "'.*string.*'"

And I get this error:

org.springframework.expression.spel.SpelEvaluationException: EL1001E: Type conversion problem, cannot convert from java.lang.String to java.lang.Boolean
    at org.springframework.expression.spel.support.StandardTypeConverter.convertValue(StandardTypeConverter.java:75) ~[spring-expression-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.expression.common.ExpressionUtils.convertTypedValue(ExpressionUtils.java:57) ~[spring-expression-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:377) ~[spring-expression-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.cloud.kubernetes.discovery.KubernetesDiscoveryClient.lambda$getServices$7(KubernetesDiscoveryClient.java:266) ~[spring-cloud-kubernetes-discovery-1.1.0.RELEASE.jar:1.1.0.RELEASE]
    at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:176) ~[na:na]
    at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1655) ~[na:na]
    at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484) ~[na:na]
    at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474) ~[na:na]
    at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913) ~[na:na]
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:na]
    at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578) ~[na:na]
    at org.springframework.cloud.kubernetes.discovery.KubernetesDiscoveryClient.getServices(KubernetesDiscoveryClient.java:280) ~[spring-cloud-kubernetes-discovery-1.1.0.RELEASE.jar:1.1.0.RELEASE]
    at org.springframework.cloud.kubernetes.discovery.KubernetesDiscoveryClient.getServices(KubernetesDiscoveryClient.java:274) ~[spring-cloud-kubernetes-discovery-1.1.0.RELEASE.jar:1.1.0.RELEASE]
    at org.springframework.cloud.client.discovery.composite.CompositeDiscoveryClient.getServices(CompositeDiscoveryClient.java:67) ~[spring-cloud-commons-2.2.0.RELEASE.jar:2.2.0.RELEASE]
    at de.codecentric.boot.admin.server.cloud.discovery.InstanceDiscoveryListener.discover(InstanceDiscoveryListener.java:126) ~[spring-boot-admin-server-cloud-2.2.2.jar:2.2.2]
    at de.codecentric.boot.admin.server.cloud.discovery.InstanceDiscoveryListener.onApplicationReady(InstanceDiscoveryListener.java:100) ~[spring-boot-admin-server-cloud-2.2.2.jar:2.2.2]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
    at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
    at org.springframework.context.event.ApplicationListenerMethodAdapter.doInvoke(ApplicationListenerMethodAdapter.java:305) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.event.ApplicationListenerMethodAdapter.processEvent(ApplicationListenerMethodAdapter.java:190) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.event.ApplicationListenerMethodAdapter.onApplicationEvent(ApplicationListenerMethodAdapter.java:153) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.event.SimpleApplicationEventMulticaster.doInvokeListener(SimpleApplicationEventMulticaster.java:172) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:165) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:404) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:361) ~[spring-context-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.boot.context.event.EventPublishingRunListener.running(EventPublishingRunListener.java:103) ~[spring-boot-2.2.8.RELEASE.jar:2.2.8.RELEASE]
    at org.springframework.boot.SpringApplicationRunListeners.running(SpringApplicationRunListeners.java:77) ~[spring-boot-2.2.8.RELEASE.jar:2.2.8.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:330) ~[spring-boot-2.2.8.RELEASE.jar:2.2.8.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1226) ~[spring-boot-2.2.8.RELEASE.jar:2.2.8.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1215) ~[spring-boot-2.2.8.RELEASE.jar:2.2.8.RELEASE]
    at com.coremedia.blueprint.boot.admin.SpringBootAdminApp.main(SpringBootAdminApp.java:29) ~[classes/:na]
Caused by: org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [java.lang.Boolean] for value '.*string.*'; nested exception is java.lang.IllegalArgumentException: Invalid boolean value '.*string.*'
    at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:47) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.core.convert.support.GenericConversionService.convert(GenericConversionService.java:191) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.expression.spel.support.StandardTypeConverter.convertValue(StandardTypeConverter.java:70) ~[spring-expression-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    ... 33 common frames omitted
Caused by: java.lang.IllegalArgumentException: Invalid boolean value '.*string.*'
    at org.springframework.core.convert.support.StringToBooleanConverter.convert(StringToBooleanConverter.java:63) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.core.convert.support.StringToBooleanConverter.convert(StringToBooleanConverter.java:31) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.core.convert.support.GenericConversionService$ConverterAdapter.convert(GenericConversionService.java:385) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    at org.springframework.core.convert.support.ConversionUtils.invokeConverter(ConversionUtils.java:41) ~[spring-core-5.2.7.RELEASE.jar:5.2.7.RELEASE]
    ... 35 common frames omitted

Why do I get this error?
How should I specify a filter in spring expression language here?

Solution

The el filter must return a boolean value. The discovered service, which should be filtered, is injected as #root in the expression. So #root.metadata.name retrieves the actual name of the kubernetes service.

This part of code of KubernetesDiscoveryClient does the filtering:

Expression filterExpr = this.parser.parseExpression(spelExpression);
filteredServices = (Service instance) -> {
    Boolean include = filterExpr.getValue(this.evalCtxt, instance, Boolean.class);
    ...
};

This is my full spring boot admin configuration:

spring:
    cloud:
        kubernetes:
            discovery:
                namespace: myname
                all-namespaces: true
                filter: "#root.metadata.name matches '.*string.*'"
                service-labels:
                    "[app.kubernetes.io/part-of]": myapp