Groovy built-in REST/HTTP client?

Native Groovy GET and POST

// GET
def get = new URL("https://httpbin.org/get").openConnection();
def getRC = get.getResponseCode();
println(getRC);
if (getRC.equals(200)) {
    println(get.getInputStream().getText());
}

// POST
def post = new URL("https://httpbin.org/post").openConnection();
def message = '{"message":"this is a message"}'
post.setRequestMethod("POST")
post.setDoOutput(true)
post.setRequestProperty("Content-Type", "application/json")
post.getOutputStream().write(message.getBytes("UTF-8"));
def postRC = post.getResponseCode();
println(postRC);
if (postRC.equals(200)) {
    println(post.getInputStream().getText());
}

If your needs are simple and you want to avoid adding additional dependencies you may be able to use the getText() methods that Groovy adds to the java.net.URL class:

new URL("http://stackoverflow.com").getText()

// or

new URL("http://stackoverflow.com")
        .getText(connectTimeout: 5000, 
                readTimeout: 10000, 
                useCaches: true, 
                allowUserInteraction: false, 
                requestProperties: ['Connection': 'close'])

If you are expecting binary data back there is also similar functionality provided by the newInputStream() methods.