Using Spring REST template, either creating too many connections or slow

Actually Spring Boot is not leaking connections. What you're seeing here is standard behavior of the Linux kernel (and every major OS). All sockets that are closed from the machine go to a TIME_WAIT state for some duration of time. This is to prevent the next socket that uses that ephemeral port from receiving packets that were actually intended for the previous socket on that port. The difference you're seeing between the two is a result of the connection pooling approaches each one takes.

More specifically, RestTemplate does not use connection pooling by default. This means every rest call opens a new local ephemeral port and a new connection to the server. If your service is very fast, it will blow through its available local port range in no time at all. With the Apache HttpClient, you are taking advantage of connection pooling. This will prevent your application from seeing the problem that you described. However, given that your service is able to respond faster than the Linux kernel is taking sockets out of TIME_WAIT, connection pooling will make your client slower no matter what you do (if it didn't slow anything down - then you'd run out of local ephemeral ports again).

While it's possible to enable TCP reuse in the Linux kernel, it's can get dangerous (packets can get delayed and you could get ephemeral ports receiving random packets they don't understand which could cause all kinds of problems). The solution here is to use connection pooling as you have in the second example, with sufficiently high numbers to achieve close to the performance you're looking for.

To help you tune your connection pool, you'll want to tweak the maxConnPerRoute and maxConnTotal parameters. maxConnPerRoute limits the number of connections that will be made to a single IP:Port pair, and maxTotal limits the number of total connections that will ever be opened. In your case, since it appears all requests are made to the same location, you could set them to the same (high) value.