How can I analyze the header and body posted by my API call

In Java, using httpclient5.1.3 and httpcore5.1.3 I'm trying to do a POST request to an API URL. However, I keep receiving a 400 Bad request. Are there any tools so I can debug how the request is exactly received by the API? To see how the headers and body are received exactly?

You could use Postman or Advanced Rest Client (A plugin for Chrome) that allows you to send HTTP requests and see the responses. There is a tool called Fidler that shows HHTP traffic.

Also there are some other HTTP clients you could use:

1. Apache Http client - very popular and well known 3d party Http Client
2. OK Http client - Open-source Http client. Here is tutorial
3. MgntUtils Http client - very simple 3d party HttpClient: Provided in MgntUtils Open source library (written by me). Very simple in use. Take a look at Javadoc. Library itself provided as Maven artifacts and on Git (including source code and Javadoc).