# Advanced topics

## Custom client connections

In certain situations it may be necessary to customize the way HTTP messages get transmitted across the wire beyond what is possible using HTTP parameters in order to be able to deal non-standard, non-compliant behaviours. For instance, for web crawlers it may be necessary to force HttpClient into accepting malformed response heads in order to salvage the content of the messages.

Usually the process of plugging in a custom message parser or a custom connection implementation involves several steps:

- Provide a custom LineParser / LineFormatter interface implementation. Implement message parsing / formatting logic as required.

      class MyLineParser extends BasicLineParser {

          @Override
          public Header parseHeader(
                  CharArrayBuffer buffer) throws ParseException {
              try {
                  return super.parseHeader(buffer);
              } catch (ParseException ex) {
                  // Suppress ParseException exception
                  return new BasicHeader(buffer.toString(), null);
              }
          }

      }

- Provide a custom HttpConnectionFactory implementation. Replace default request writer and / or response parser with custom ones as required.

      HttpConnectionFactory<HttpRoute, ManagedHttpClientConnection> connFactory =
              new ManagedHttpClientConnectionFactory(
                  new DefaultHttpRequestWriterFactory(),
                  new DefaultHttpResponseParserFactory(
                          new MyLineParser(), new DefaultHttpResponseFactory()));

- Configure HttpClient to use the custom connection factory.

      PoolingHttpClientConnectionManager cm = new PoolingHttpClientConnectionManager(
          connFactory);
      CloseableHttpClient httpclient = HttpClients.custom()
              .setConnectionManager(cm)
              .build();

## Stateful HTTP connections

While HTTP specification assumes that session state information is always embedded in HTTP messages in the form of HTTP cookies and therefore HTTP connections are always stateless, this assumption does not always hold true in real life. There are cases when HTTP connections are created with a particular user identity or within a particular security context and therefore cannot be shared with other users and can be reused by the same user only. Examples of such stateful HTTP connections are `NTLM` authenticated connections and SSL connections with client certificate authentication.

### User token handler

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();

### Persistent stateful connections

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();
    }

## Using the FutureRequestExecutionService

Using the FutureRequestExecutionService, you can schedule http calls and treat the response as a Future. This is useful when e.g. making multiple calls to a web service. The advantage of using the FutureRequestExecutionService is that you can use multiple threads to schedule requests concurrently, set timeouts on the tasks, or cancel them when a response is no longer necessary.

FutureRequestExecutionService wraps the request with a HttpRequestFutureTask, which extends FutureTask. This class allows you to cancel the task as well as keep track of various metrics such as request duration.

### Creating the FutureRequestExecutionService

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);

### Scheduling requests

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();

### Canceling tasks

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

### Callbacks

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());

### Metrics

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