JAX-RS ParamConverter with Quarkus

By Rebecca Searls | April 5, 2021

Recently RESTEasy had a request by a user who was implementing their REST service with Quarkus for support of an endpoint input parameter type java.time.Instant.

Lets look at how the user can provide support for java.time.Instant or any other data type in their REST application. The code used in this discussion can be found here.

First lets create a Quarkus getting-started app [1] to use in examining this issue.

  mvn io.quarkus:quarkus-maven-plugin:1.13.0.Final:create \
      -DprojectGroupId=org.acme \
      -DprojectArtifactId=getting-started \
      -DclassName="org.acme.getting.started.GreetingResource" \
  cd getting-started

Edit file ./src/main/java/org/acme/getting/started/GreetingResource.java and add the following endpoint.

    public String helloTime(@QueryParam("name") String name, @QueryParam("time") Instant time) {
        return "Hello " + name + ", " + time;

This is optional. You can also add a test for this endpoint. Edit file, ./src/test/java/org/acme/getting/started/GreetingResourceTest.java and add this code.

    public void testHelloTimeEndpoint() {
                .body(is("Hello abc, 2021-02-12T09:58:39Z"));

Lets run the application and call the endpoint using cURL. Build and run the application with this Quarkus command,

    ./mvnw compile quarkus:dev

YIKES what just happened? I get a InvocationTargetException and a stacktrace. Looking through the stacktrace I see the root cause is

  Caused by: java.lang.RuntimeException: RESTEASY003875: Unable to find a 
  constructor that takes a String param or a valueOf() or fromString() method 
  for javax.ws.rs.QueryParam("time") on public java.lang.String 
  for basetype: java.time.Instant

JAX-RS enables this issue to be resolved with a ParamConverter. Interfaces ParamConverterProvider and ParamConverter are provided by the specification just for this situation. Lets create an implementation for each of these interfaces. In directory src/main/java/org/acme/getting/started create file InstantParamConverterProvider

package org.acme.getting.started;

import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.time.Instant;

import javax.ws.rs.ext.ParamConverter;
import javax.ws.rs.ext.ParamConverterProvider;
import javax.ws.rs.ext.Provider;

public class InstantParamConverterProvider implements ParamConverterProvider {
    public <T> ParamConverter<T> getConverter(Class<T> rawType,
                                              Type genericType,
                                              Annotation[] annotations) {
        if (rawType.isAssignableFrom(Instant.class)) {
            return (ParamConverter<T>) new InstantParamConverter();
        return null;

And file InstantParamConverter

package org.acme.getting.started;

import javax.ws.rs.ext.ParamConverter;
import javax.ws.rs.ext.Provider;
import java.time.Instant;
import java.time.format.DateTimeParseException;

public class InstantParamConverter implements ParamConverter<Instant> {

    public Instant fromString(String value){
        try {
            String x = Instant.parse(value).toString();
            return Instant.parse(value);
        } catch (DateTimeParseException e) {

        return null;

    public String toString(Instant value){
        return value.toString();

Now run the application.

  ./mvnw compile quarkus:dev

In a separate terminal window run cURL command.

    curl -w "\n" "http://localhost:8080/hello/time?name=abc&time=2021-02-12T09:58:39Z"

Result is

  Hello abc, 2021-02-12T09:58:39Z

The test can be run with this command.

  ./mvnw test

If you want to walk through the code with the debugger, set your debugger to port 5005 and run the application with either command

    ./mvnw compile quarkus:dev


    ./mvnw compile quarkus:dev -Dsuspend=true

In conclusion all elements of the JAX-RS specification still apply when writing a RESTful service with Quarkus.

References [1] Quarkus getting-started