How to use LocalDateTime RequestParam in Spring? I get "Failed to convert String to LocalDateTime"

You did everything correct :) . Here is an example that shows exactly what you are doing. Just Annotate your RequestParam with @DateTimeFormat. There is no need for special GenericConversionService or manual conversion in the controller. This blog post writes about it.

@RestController
@RequestMapping("/api/datetime/")
final class DateTimeController {

    @RequestMapping(value = "datetime", method = RequestMethod.POST)
    public void processDateTime(@RequestParam("datetime") 
                                @DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME) LocalDateTime dateAndTime) {
        //Do stuff
    }
}

I guess you had an issue with the format. On my setup everything works well.

TL;DR - you can capture it as a string with just @RequestParam, or you can have Spring additionally parse the string into a java date / time class via @DateTimeFormat on the parameter as well.

the @RequestParam is enough to grab the date you supply after the = sign, however, it comes into the method as a String. That is why it is throwing the cast exception.

There are a few ways to achieve this:

1. parse the date yourself, grabbing the value as a string.

@GetMapping("/test")
public Page<User> get(@RequestParam(value="start", required = false) String start){

    //Create a DateTimeFormatter with your required format:
    DateTimeFormatter dateTimeFormat = 
            new DateTimeFormatter(DateTimeFormatter.BASIC_ISO_DATE);

    //Next parse the date from the @RequestParam, specifying the TO type as 
a TemporalQuery:
   LocalDateTime date = dateTimeFormat.parse(start, LocalDateTime::from);

    //Do the rest of your code...
}

2. Leverage Spring's ability to automatically parse and expect date formats:

@GetMapping("/test")
public void processDateTime(@RequestParam("start") 
                            @DateTimeFormat(iso = DateTimeFormat.ISO.DATE_TIME) 
                            LocalDateTime date) {
        // The rest of your code (Spring already parsed the date).
}

I found workaround here.

Spring/Spring Boot only supports the date/date-time format in BODY parameters.

The following configuration class adds support for date/date-time in QUERY STRING (request parameters):

// Since Spring Framwork 5.0 & Java 8+
@Configuration
public class DateTimeFormatConfiguration implements WebMvcConfigurer {

    @Override
    public void addFormatters(FormatterRegistry registry) {
        DateTimeFormatterRegistrar registrar = new DateTimeFormatterRegistrar();
        registrar.setUseIsoFormat(true);
        registrar.registerFormatters(registry);
    }
}

respectively:

// Until Spring Framwork 4.+
@Configuration
public class DateTimeFormatConfiguration extends WebMvcConfigurerAdapter {

    @Override
    public void addFormatters(FormatterRegistry registry) {
        DateTimeFormatterRegistrar registrar = new DateTimeFormatterRegistrar();
        registrar.setUseIsoFormat(true);
        registrar.registerFormatters(registry);
    }
}

It works even if you bind multiple request parameters to some class (@DateTimeFormat annotation helpless in this case):

public class ReportRequest {
    private LocalDate from;
    private LocalDate to;

    public LocalDate getFrom() {
        return from;
    }

    public void setFrom(LocalDate from) {
        this.from = from;
    }

    public LocalDate getTo() {
        return to;
    }

    public void setTo(LocalDate to) {
        this.to = to;
    }
}

// ...

@GetMapping("/api/report")
public void getReport(ReportRequest request) {
// ...