Preemptive Basic authentication with Apache HttpClient 4

This is the same solution as Mat's Mannion's, but you don't have to put localContext to each request. It's simpler, but it adds authentication to ALL requests. Useful, if you don't have control over individual requests, as in my case when using Apache Solr, which uses HttpClient internally.

import org.apache.http.HttpException;
import org.apache.http.HttpHost;
import org.apache.http.HttpRequest;
import org.apache.http.HttpRequestInterceptor;
import org.apache.http.auth.AuthScope;
import org.apache.http.auth.AuthState;
import org.apache.http.auth.Credentials;
import org.apache.http.client.CredentialsProvider;
import org.apache.http.client.protocol.HttpClientContext;
import org.apache.http.impl.auth.BasicScheme;
import org.apache.http.protocol.HttpContext;
import org.apache.http.protocol.HttpCoreContext;

httpClient.addRequestInterceptor(new PreemptiveAuthInterceptor(), 0);

(...)

static class PreemptiveAuthInterceptor implements HttpRequestInterceptor {

    public void process(final HttpRequest request, final HttpContext context) throws HttpException, IOException {
         AuthState authState = (AuthState) context.getAttribute(HttpClientContext.TARGET_AUTH_STATE);

         // If no auth scheme available yet, try to initialize it
         // preemptively
         if (authState.getAuthScheme() == null) {
             CredentialsProvider credsProvider = (CredentialsProvider) context.getAttribute(HttpClientContext.CREDS_PROVIDER);
             HttpHost targetHost = (HttpHost) context.getAttribute(HttpCoreContext.HTTP_TARGET_HOST);
             Credentials creds = credsProvider.getCredentials(new AuthScope(targetHost.getHostName(), targetHost.getPort()));
             if (creds == null) {
                 throw new HttpException("No credentials for preemptive authentication");
             }
             authState.update(new BasicScheme(), creds);
         }
     }
}

Of course, you have to set the credentials provider:

httpClient.getCredentialsProvider().setCredentials(
                new AuthScope(url.getHost(), url.getPort()),
                new UsernamePasswordCredentials(username, password))

The AuthScope must not contain realm, as it is not known in advance.

A lot of the answers above use deprecated code. I am using Apache SOLRJ version 5.0.0. My code consists of

private HttpSolrClient solrClient; 

private void initialiseSOLRClient() {
            URL solrURL = null;
            try {
                solrURL = new URL(urlString);
            } catch (MalformedURLException e) {
                LOG.error("Cannot parse the SOLR URL!!" + urlString);
                throw new SystemException("Cannot parse the SOLR URL!! " + urlString, e);
            }
            String host = solrURL.getHost();
            int port = solrURL.getPort();
            AuthScope authScope = new AuthScope(host, port);

    BasicTextEncryptor textEncryptor = new BasicTextEncryptor();
    textEncryptor.setPassword("red bananas in the spring");
    String decryptPass = textEncryptor.decrypt(pass);
    UsernamePasswordCredentials creds = new UsernamePasswordCredentials(userName, decryptPass);

    CredentialsProvider credsProvider = new BasicCredentialsProvider();
    credsProvider.setCredentials(
            authScope,
            creds);

    HttpClientBuilder builder = HttpClientBuilder.create();
    builder.addInterceptorFirst(new PreemptiveAuthInterceptor());
    builder.setDefaultCredentialsProvider(credsProvider);
    CloseableHttpClient httpClient = builder.build();

    solrClient = new HttpSolrClient(urlString, httpClient);
}

The PreemptiveAuthInterceptor is now as follows:-

static class PreemptiveAuthInterceptor implements HttpRequestInterceptor {

    public void process(final HttpRequest request, final HttpContext context) throws HttpException, IOException {
        AuthState authState = (AuthState) context.getAttribute(HttpClientContext.TARGET_AUTH_STATE);
        // If no auth scheme available yet, try to initialize it
        // preemptively
        if (authState.getAuthScheme() == null) {
            CredentialsProvider credsProvider = (CredentialsProvider) 
                        context.getAttribute(HttpClientContext.CREDS_PROVIDER);
            HttpHost targetHost = (HttpHost) context.getAttribute(HttpCoreContext.HTTP_TARGET_HOST);
            AuthScope authScope = new AuthScope(targetHost.getHostName(), targetHost.getPort());
            Credentials creds = credsProvider.getCredentials(authScope);
            if(creds == null){

            }
            authState.update(new BasicScheme(), creds);
        }

    }
}

It's difficult to do this without passing a context through every time, but you can probably do it by using a request interceptor. Here is some code that we use (found from their JIRA, iirc):

// Pre-emptive authentication to speed things up
BasicHttpContext localContext = new BasicHttpContext();

BasicScheme basicAuth = new BasicScheme();
localContext.setAttribute("preemptive-auth", basicAuth);

httpClient.addRequestInterceptor(new PreemptiveAuthInterceptor(), 0);

(...)

static class PreemptiveAuthInterceptor implements HttpRequestInterceptor {

    public void process(final HttpRequest request, final HttpContext context) throws HttpException, IOException {
        AuthState authState = (AuthState) context.getAttribute(ClientContext.TARGET_AUTH_STATE);

        // If no auth scheme avaialble yet, try to initialize it
        // preemptively
        if (authState.getAuthScheme() == null) {
            AuthScheme authScheme = (AuthScheme) context.getAttribute("preemptive-auth");
            CredentialsProvider credsProvider = (CredentialsProvider) context.getAttribute(ClientContext.CREDS_PROVIDER);
            HttpHost targetHost = (HttpHost) context.getAttribute(ExecutionContext.HTTP_TARGET_HOST);
            if (authScheme != null) {
                Credentials creds = credsProvider.getCredentials(new AuthScope(targetHost.getHostName(), targetHost.getPort()));
                if (creds == null) {
                    throw new HttpException("No credentials for preemptive authentication");
                }
                authState.setAuthScheme(authScheme);
                authState.setCredentials(creds);
            }
        }

    }

}

If you are looking to force HttpClient 4 to authenticate with a single request, the following will work:

String username = ...
String password = ...
UsernamePasswordCredentials creds = new UsernamePasswordCredentials(username, password);

HttpRequest request = ...
request.addHeader(new BasicScheme().authenticate(creds, request));