《Http Client 官方文档》7. 高级主题

HttpClient依赖于’UserTokenHandler’接口来确定给定的执行上下文是否为用户特定的。如果是，则该处理程序返回的令牌对象应该唯一标识当前用户，如果上下文不包含当前用户特有的任何资源或详细信息，则该对象将为null。 用户令牌将用于确保用户特定资源不会被其他用户共享或重复使用。

‘UserTokenHandler’接口的默认实现类作为Principal类的实例来表示HTTP连接的状态对象，如果它可以从给定的执行上下文获得的话。
‘DefaultUserTokenHandler’将使用基于连接的认证方案（例如’NTLM’）或使用客户端认证的SSL会话的用户主体。 如果两者都不可用，将返回NULL令牌。

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();
}

如果默认设置不能满足需要，用户可以提供自定义实现：

UserTokenHandler userTokenHandler = new UserTokenHandler() {

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

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

请注意，只有当执行请求时相同的状态对象绑定到执行上下文时，携带状态对象的持久连接才可重复使用。因此，确保用户执行后续HTTP请求时重用了相同的上下文或用户令牌在请求执行之前就被绑定到上下文，是非常重要的。

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();
}

futureRequestExecutionService类的构造函数接受任何已有的httpClient实例和ExecutorService实例。 当配置这两者时，重要的是将最大连接数与要使用的线程数一致。当线程数超过了最大连接数时，因为没有了可用的连接，连接将会进入超时。而出现相反的情况时，futureRequestExecutionService将不会使用它们

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

要计划请求，只需提供一个HttpUriRequest，HttpContext和一个ResponseHandler。 因为请求由执行器服务处理，所以会强制使用ResponseHandler。

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();

计划任务有可能被取消。如果任务一直就在队列中等待执行，那么它将永远不会执行。如果它正在执行中并且mayInterruptIfRunning参数为true，那么将在请求上调用abort（）; 否则虽然会正常完成请求，但是会忽略响应。任何对task.get（）的后续调用都将失败并返回IllegalStateException异常。 应当注意，取消任务仅会释放客户端资源。 该请求实际上可以在服务器端正常处理。

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

如果要代替手动调用task.get（），还可以使用FutureCallback实例在请求完成时获取回调， 它与HttpAsyncClient中使用的接口相同。

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通常用于进行大量Web服务调用的应用程序中。 为了方便监控或配置调整等，FutureRequestExecutionService会追踪多项数据。

每个HttpRequestFutureTask提供了获取任务调度，启动和结束时间的方法。 此外，请求和任务持续时间也可用。这些数据标准在FutureRequestExecutionService中的FutureRequestExecutionMetrics实例中聚合，可通过FutureRequestExecutionService.metrics（）访问。

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