Spring Boot 3 - How can I perform client API GET request with request body using RestTemplate?

The solution for your problem is to extend existing RestTemplate class.

RestTemplate class provides execute() method that can be used to perform GET request with body.

I the below you can find practical example and ExtendedRestTemplate class that contains required API extension for your problem - GET method + Payload/Body.

Practical example

Example ExtendedRestTemplate class usage:

ExtendedRestTemplate restTemplate = new ExtendedRestTemplate();

SomeRequest someRequest = new SomeRequest(1L, "John", "john@email.com");
SomeResponse someResponse = restTemplate.getForObject("http://localhost/api/some-path", someRequest, SomeResponse.class);  // <--- GET method request with body

Example ExtendedRestTemplate.java file:

package com.dirask.example;

import java.net.URI;
import java.util.List;
import java.util.Map;

import org.springframework.http.HttpMethod;
import org.springframework.http.client.ClientHttpRequestFactory;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.lang.Nullable;
import org.springframework.web.client.HttpMessageConverterExtractor;
import org.springframework.web.client.RequestCallback;
import org.springframework.web.client.RestClientException;
import org.springframework.web.client.RestTemplate;

public class ExtendedRestTemplate extends RestTemplate {

	public ExtendedRestTemplate() {
		super();
	}

	public ExtendedRestTemplate(ClientHttpRequestFactory requestFactory) {
		super(requestFactory);
	}

	public ExtendedRestTemplate(List<HttpMessageConverter<?>> messageConverters) {
		super(messageConverters);
	}

	@Nullable
	public <T> T getForObject(URI url, @Nullable Object requestObject, Class<T> responseType) throws RestClientException {
		List<HttpMessageConverter<?>> messageConverters = super.getMessageConverters();
		RequestCallback requestCallback = super.httpEntityCallback(requestObject, responseType);
		HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor<>(responseType, messageConverters);
		return super.execute(url, HttpMethod.GET, requestCallback, responseExtractor);
	}

	@Nullable
	public <T> T getForObject(String url, @Nullable Object requestObject, Class<T> responseType, Object... uriVariables) throws RestClientException {
		List<HttpMessageConverter<?>> messageConverters = super.getMessageConverters();
		RequestCallback requestCallback = httpEntityCallback(requestObject, responseType);
		HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor<>(responseType, messageConverters);
		return super.execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables);
	}

	@Nullable
	public <T> T getForObject(String url, @Nullable Object requestObject, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException {
		List<HttpMessageConverter<?>> messageConverters = super.getMessageConverters();
		RequestCallback requestCallback = super.httpEntityCallback(requestObject, responseType);
		HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor<>(responseType, messageConverters);
		return super.execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables);
	}
}

See also

1. Spring Boot 3 - RestTemplate and GET method request with body (JSON payload)

2. Spring Boot 3 - RestTemplate and GET method request with body (JSON payload)