HttpClient relies on UserTokenHandler interface to determine if the given execution context is user specific or not. The token object returned by this handler is expected to uniquely identify the current user if the context is user specific or to be null if the context does not contain any resources or details specific to the current user. The user token will be used to ensure that user specific resources will not be shared with or reused by other users.

The default implementation of the UserTokenHandler interface uses an instance of Principal class to represent a state object for HTTP connections, if it can be obtained from the given execution context. DefaultUserTokenHandler will use the user principle of connection based authentication schemes such as NTLM or that of the SSL session with client authentication turned on. If both are unavailable, null token will be returned.

CloseableHttpClient httpclient = HttpClients.createDefault();
HttpClientContext context = HttpClientContext.create();
HttpGet httpget = new HttpGet("http://localhost:8080/");
CloseableHttpResponse response = httpclient.execute(httpget, context);
try {
    Principal principal = context.getUserToken(Principal.class);
    System.out.println(principal);
} finally {
    response.close();
}

Users can provide a custom implementation if the default one does not satisfy their needs:

UserTokenHandler userTokenHandler = new UserTokenHandler() {

    public Object getUserToken(HttpContext context) {
        return context.getAttribute("my-token");
    }

};
CloseableHttpClient httpclient = HttpClients.custom()
        .setUserTokenHandler(userTokenHandler)
        .build();

Please note that a persistent connection that carries a state object can be reused only if the same state object is bound to the execution context when requests are executed. So, it is really important to ensure the either same context is reused for execution of subsequent HTTP requests by the same user or the user token is bound to the context prior to request execution.

CloseableHttpClient httpclient = HttpClients.createDefault();
HttpClientContext context1 = HttpClientContext.create();
HttpGet httpget1 = new HttpGet("http://localhost:8080/");
CloseableHttpResponse response1 = httpclient.execute(httpget1, context1);
try {
    HttpEntity entity1 = response1.getEntity();
} finally {
    response1.close();
}
Principal principal = context1.getUserToken(Principal.class);

HttpClientContext context2 = HttpClientContext.create();
context2.setUserToken(principal);
HttpGet httpget2 = new HttpGet("http://localhost:8080/");
CloseableHttpResponse response2 = httpclient.execute(httpget2, context2);
try {
    HttpEntity entity2 = response2.getEntity();
} finally {
    response2.close();
}

The constructor for the futureRequestExecutionService takes any existing httpClient instance and an ExecutorService instance. When configuring both, it is important to align the maximum number of connections with the number of threads you are going to use. When there are more threads than connections, the connections may start timing out because there are no available connections. When there are more connections than threads, the futureRequestExecutionService will not use all of them

HttpClient httpClient = HttpClientBuilder.create().setMaxConnPerRoute(5).build();
ExecutorService executorService = Executors.newFixedThreadPool(5);
FutureRequestExecutionService futureRequestExecutionService =
    new FutureRequestExecutionService(httpClient, executorService);

To schedule a request, simply provide a HttpUriRequest, HttpContext, and a ResponseHandler. Because the request is processed by the executor service, a ResponseHandler is mandatory.

private final class OkidokiHandler implements ResponseHandler<Boolean> {
    public Boolean handleResponse(
            final HttpResponse response) throws ClientProtocolException, IOException {
        return response.getStatusLine().getStatusCode() == 200;
    }
}

HttpRequestFutureTask<Boolean> task = futureRequestExecutionService.execute(
    new HttpGet("http://www.google.com"), HttpClientContext.create(),
    new OkidokiHandler());
// blocks until the request complete and then returns true if you can connect to Google
boolean ok=task.get();

Scheduled tasks may be cancelled. If the task is not yet executing but merely queued for execution, it simply will never execute. If it is executing and the mayInterruptIfRunning parameter is set to true, abort() will be called on the request; otherwise the response will simply be ignored but the request will be allowed to complete normally. Any subsequent calls to task.get() will fail with an IllegalStateException. It should be noticed that canceling tasks merely frees up the client side resources. The request may actually be handled normally on the server side.

task.cancel(true)
task.get() // throws an Exception

Instead of manually calling task.get(), you can also use a FutureCallback instance that gets callbacks when the request completes. This is the same interface as is used in HttpAsyncClient

private final class MyCallback implements FutureCallback<Boolean> {

    public void failed(final Exception ex) {
        // do something
    }

    public void completed(final Boolean result) {
        // do something
    }

    public void cancelled() {
        // do something
    }
}

HttpRequestFutureTask<Boolean> task = futureRequestExecutionService.execute(
    new HttpGet("http://www.google.com"), HttpClientContext.create(),
    new OkidokiHandler(), new MyCallback());

FutureRequestExecutionService is typically used in applications that make large amounts of web service calls. To facilitate e.g. monitoring or configuration tuning, the FutureRequestExecutionService keeps track of several metrics.

Each HttpRequestFutureTask provides methods to get the time the task was scheduled, started, and ended. Additionally, request and task duration are available as well. These metrics are aggregated in the FutureRequestExecutionService in a FutureRequestExecutionMetrics instance that may be accessed through FutureRequestExecutionService.metrics().

task.scheduledTime() // returns the timestamp the task was scheduled
task.startedTime() // returns the timestamp when the task was started
task.endedTime() // returns the timestamp when the task was done executing
task.requestDuration // returns the duration of the http request
task.taskDuration // returns the duration of the task from the moment it was scheduled

FutureRequestExecutionMetrics metrics = futureRequestExecutionService.metrics()
metrics.getActiveConnectionCount() // currently active connections
metrics.getScheduledConnectionCount(); // currently scheduled connections
metrics.getSuccessfulConnectionCount(); // total number of successful requests
metrics.getSuccessfulConnectionAverageDuration(); // average request duration
metrics.getFailedConnectionCount(); // total number of failed tasks
metrics.getFailedConnectionAverageDuration(); // average duration of failed tasks
metrics.getTaskCount(); // total number of tasks scheduled
metrics.getRequestCount(); // total number of requests
metrics.getRequestAverageDuration(); // average request duration
metrics.getTaskAverageDuration(); // average task duration