I'm studying an application developed by our company. It uses the Apache HttpClient library. In the source code it uses the HttpClient
class to create instances to connect to a server.
I want to learn about Apache HttpClient and I've gone trough this set of examples. All the examples use CloseableHttpClient
instead of HttpClient
. So I think CloseableHttpClient
is an extended version of HttpClient
. If this is the case I have two questions:
This question is related to
java
apache
http
apache-httpclient-4.x
CloseableHttpClient
is the base class of the httpclient library, the one all implementations use. Other subclasses are for the most part deprecated.
The HttpClient
is an interface for this class and other classes.
You should then use the CloseableHttpClient
in your code, and create it using the HttpClientBuilder
. If you need to wrap the client to add specific behaviour you should use request and response interceptors instead of wrapping with the HttpClient
.
This answer was given in the context of httpclient-4.3.
In the next major version of the library HttpClient
interface is going to extend Closeable
. Until then it is recommended to use CloseableHttpClient
if compatibility with earlier 4.x versions (4.0, 4.1 and 4.2) is not required.
HttpClient
is not a class, it is an interface. You cannot use it for development in the way you mean.
What you want is a class that implements the HttpClient
interface, and that is CloseableHttpClient
.
Jon skeet said:
The documentation seems pretty clear to me: "Base implementation of HttpClient that also implements Closeable" - HttpClient is an interface; CloseableHttpClient is an abstract class, but because it implements AutoCloseable you can use it in a try-with-resources statement.
But then Jules asked:
@JonSkeet That much is clear, but how important is it to close HttpClient instances? If it's important, why is the close() method not part of the basic interface?
Answer for Jules
close need not be part of basic interface since underlying connection is released back to the connection manager automatically after every execute
To accommodate the try-with-resources statement. It is mandatory to implement Closeable. Hence included it in CloseableHttpClient.
Note:
close method in AbstractHttpClient which is extending CloseableHttpClient is deprecated, I was not able to find the source code for that.
Had the same question. The other answers don't seem to address why close() is really necessary? Also, Op seemed to be struggling to figure out the preferred way to work with HttpClient, et al.
According to Apache:
// The underlying HTTP connection is still held by the response object
// to allow the response content to be streamed directly from the network socket.
// In order to ensure correct deallocation of system resources
// the user MUST call CloseableHttpResponse#close() from a finally clause.
In addition, the relationships go as follows:
HttpClient
(interface)implemented by:
CloseableHttpClient
- ThreadSafe.
DefaultHttpClient
- ThreadSafe BUT deprecated, useHttpClientBuilder
instead.
HttpClientBuilder
- NOT ThreadSafe, BUT creates ThreadSafeCloseableHttpClient
.
- Use to create CUSTOM
CloseableHttpClient
.
HttpClients
- NOT ThreadSafe, BUT creates ThreadSafeCloseableHttpClient
.
- Use to create DEFAULT or MINIMAL
CloseableHttpClient
.
The preferred way according to Apache:
CloseableHttpClient httpclient = HttpClients.createDefault();
The example they give does httpclient.close()
in the finally
clause, and also makes use of ResponseHandler
as well.
As an alternative, the way mkyong does it is a bit interesting, as well:
HttpClient client = HttpClientBuilder.create().build();
He doesn't show a client.close()
call but I would think it is necessary, since client
is still an instance of CloseableHttpClient
.
The other answers don't seem to address why close()
is really necessary? * 2
It is mentioned in old 3.x httpcomponents doc, which is long back and has a lot difference from 4.x HC. Besides the explanation is so brief that doesn't say what this underlying resource is.
I did some research on 4.5.2 release source code, found the implementations of CloseableHttpClient:close()
basically only closes its connection manager.
(FYI) That's why when you use a shared PoolingClientConnectionManager
and call client close()
, exception java.lang.IllegalStateException: Connection pool shut down
will occur. To avoid, setConnectionManagerShared
works.
CloseableHttpClient:close()
after every single requestI used to create a new http client instance when doing request and finally close it. In this case, it'd better not to call close()
. Since, if connection manager doesn't have "shared" flag, it'll be shutdown, which is too expensive for a single request.
In fact, I also found in library clj-http, a Clojure wrapper over Apache HC 4.5, doesn't call close()
at all. See func request
in file core.clj
Source: Stackoverflow.com