400 Error HTTP GET Request between Docker Containers with HTTPURLConnection - api

Question

I got two Containers defined in a docker-compose file:
tomcat_webserver_api:
image: tomcat:8
volumes:
- ./API/Docker/API.war:/usr/local/tomcat/webapps/API.war
ports:
- "8080:8080"
depends_on:
- mysql_database
tomcat_webserver_anwendung:
image: tomcat:8
ports:
- "8081:8080"
volumes:
- ./Anwendung/Docker/Anwendung.war:/usr/local/tomcat/webapps/Anwendung.war
depends_on:
- tomcat_webserver_api
environment:
API_HOST: tomcat_webserver_api
API_PORT: 8080
Now i want to access the URL http://tomcat_webserver_api:8080/API/restaurants/Wochentag from Inside the Java Web Application with an HttpURLConnection.
Issue: It returns an 400 Error
java.io.IOException: Server returned HTTP response code: 400 for URL: http://tomcat_webserver_api:8080/API/restaurants/Wochentag
The Code is like that (The Headers are nearly the same when i try to connect to the URL via curl - this works inside the container huh):
URL api = UriBuilder.fromUri("http://" + "tomcat_webserver_api" + ":" + "8080" +"/API/restaurants/RestaurantSpeisen").build().toURL();
System.setProperty("http.agent", "curl/7.52.1");
HttpURLConnection connection = (HttpURLConnection) api.openConnection();
connection.setRequestMethod("GET");
connection.setRequestProperty("Host", "localhost");
connection.setRequestProperty("User-Agent", "curl/7.52.1");
connection.connect();
BufferedReader in = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));
If i try to connect to http://172.20.0.3:8080/API/restaurants/Wochentag i got an 200-ok HTTP Response Code and the JSON-Data.
If i exec the API Container and inspect the logs i can see the 400 GET-Request.
Why is this happen?
http://172.20.0.3:8080/API/restaurants/Wochentag - Works
http://tomcat_webserver_api:8080/API/restaurants/Wochentag - Won't Work but not with an 404 Error

Answer 1

I have had the same issue as you, apparently underscore are not allowed as virtualhost, try to remove it, for example, use just tomcatwebserverapi, that should fix your problem.
See Can (domain name) subdomains have an underscore "_" in it? for more information about valid letters in hostnames.

Answer 2

Please give explicit container names a try:
tomcat_webserver_api:
image: tomcat:8
container_name: tomcat_webserver_api
volumes:
- ./API/Docker/API.war:/usr/local/tomcat/webapps/API.war
ports:
- "8080:8080"
depends_on:
- mysql_database
tomcat_webserver_anwendung:
container_name: tomcat_webserver_app
image: tomcat:8
ports:
- "8081:8080"
volumes:
- ./Anwendung/Docker/Anwendung.war:/usr/local/tomcat/webapps/Anwendung.war
depends_on:
- tomcat_webserver_api
environment:
API_HOST: tomcat_webserver_api
API_PORT: 8080
The "local only" configuration needs explicit container names to activate Docker's name lookup mechanism. In Swarm mode you wouldn't need to set container names.