What is the difference between CloseableHttpClient and HttpClient in Apache HttpClient API?

Question

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:

• What is the difference between these two?
• Which class is recommended to use for my new development?

Answer 1

• The main entry point of the HttpClient API is the HttpClient interface.
• The most essential function of HttpClient is to execute HTTP methods.
• Execution of an HTTP method involves one or several HTTP request / HTTP response exchanges, usually handled internally by HttpClient.

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• CloseableHttpClient is an abstract class which is the base implementation of HttpClient that also implements java.io.Closeable.

• Here is an example of request execution process in its simplest form:

  CloseableHttpClient httpclient = HttpClients.createDefault(); HttpGet httpget = new HttpGet("http://localhost/"); CloseableHttpResponse response = httpclient.execute(httpget); try {     //do something } finally {     response.close(); }

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• HttpClient resource deallocation: When an instance CloseableHttpClient is no longer needed and is about to go out of scope the connection manager associated with it must be shut down by calling the CloseableHttpClient#close() method.

  CloseableHttpClient httpclient = HttpClients.createDefault(); try {     //do something } finally {     httpclient.close(); }

see the Reference to learn fundamentals.

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@Scadge Since Java 7, Use of try-with-resources statement ensures that each resource is closed at the end of the statement. It can be used both for the client and for each response

try(CloseableHttpClient httpclient = HttpClients.createDefault()){      // e.g. do this many times     try (CloseableHttpResponse response = httpclient.execute(httpget)) {     //do something     }      //do something else with httpclient here } 

Answer 2

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.

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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, use HttpClientBuilder instead.

HttpClientBuilder - NOT ThreadSafe, BUT creates ThreadSafe CloseableHttpClient.

• Use to create CUSTOM CloseableHttpClient.

HttpClients - NOT ThreadSafe, BUT creates ThreadSafe CloseableHttpClient.

• 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.

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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.