[java] What is the best Java library to use for HTTP POST, GET etc.?

May I recommend you corn-httpclient. It's simple,fast and enough for most cases.

HttpForm form = new HttpForm(new URI("http://localhost:8080/test/formtest.jsp"));
//Authentication form.setCredentials("user1", "password");
form.putFieldValue("input1", "your value");
HttpResponse response = form.doPost();
assertFalse(response.hasError());
assertNotNull(response.getData());
assertTrue(response.getData().contains("received " + val));

maven dependency

<dependency>
    <groupId>net.sf.corn</groupId>
    <artifactId>corn-httpclient</artifactId>
    <version>1.0.0</version>
</dependency>