Java: Display request of an HttpURLConnection before sending

I want to make some API calls to a server using HttpURLConnection. But the requests are not successful, returning:

<error>
  <http_status>400 Bad Request</http_status>
  <message>Unexpected request Content-Type header ''. Expecting 'application/x-www-form-urlencoded'.</message>
</error>

So I want to check what the "real" content is that is being sent to the server. By real content 开发者_JAVA百科I mean the exact HTTP request.

Any ideas how I can see this?

Edit: Based on the first answers here I should clarify my problem: I want to avoid using an external program like HTTP sniffer or anything and I was hoping that there is a function or a property or whatever that holds the information I am looking for.

If that is not the case, does someone know if this information can be manually rebuilt (for example by calling several functions like getRequestMethod(), etc.)

I am facing this problem kinda often so that it's worth the effort to build such functionality myself. Just need to know how :)

You can put the HttpURLConnection in debug mode by enabling java.logging with

-Djava.util.logging.config.file=logging.properties

and put in logging.properties (by default in JRE_HOME\lib) the following property

sun.net.www.protocol.http.HttpURLConnection.level = ALL

tcpdump will work, but it can be hard to make it do what you want. NetCat is more user-friendly (here's the project page: http://netcat.sourceforge.net/ - most Unix platforms already include it).

nc -l 9999

This will listen on TCP port 9999, and when an HTTP client connects, it'll print out the full text of the request.

The accepted solution did not work for me. But what did was

static {
    ConsoleHandler handler = new ConsoleHandler();
    handler.setLevel(Level.ALL);
    Logger log = LogManager.getLogManager().getLogger("");
    log.addHandler(handler);
    log.setLevel(Level.ALL);
    System.setProperty("javax.net.debug","all"); 
}

Use something like tcpdump, which can dump the actual network packets that are emitted or received by your computer.

On JDK 11 I was able to log all the http connections, setting java.util.logging.ConsoleHandler.level to FINEST and adding the following lines in the file logging.properties which is by default in %JAVA_HOME%/conf:

sun.net.www.protocol.http.HttpURLConnection.level = FINEST
sun.net.www.protocol.http.HttpURLConnection.handlers = java.util.logging.ConsoleHandler