HttpClient interface represents the most essential contract for HTTP request execution. It imposes no restrictions or particular details on the request execution process and leaves the specifics of connection management, state management, authentication and redirect handling up to individual implementations. This should make it easier to decorate the interface with additional functionality such as response content caching.

DefaultHttpClient is the default implementation of the HttpClient interface. This class acts as a facade to a number of special purpose handler or strategy interface implementations responsible for handling of a particular aspect of the HTTP protocol such as redirect or authentication handling or making decision about connection persistence and keep alive duration. This enables the users to selectively replace default implementation of those aspects with custom, application specific ones.

DefaultHttpClient httpclient = new DefaultHttpClient();

httpclient.setKeepAliveStrategy(new DefaultConnectionKeepAliveStrategy() {

    @Override
    public long getKeepAliveDuration(
            HttpResponse response,
            HttpContext context) {
        long keepAlive = super.getKeepAliveDuration(response, context);
        if (keepAlive == -1) {
            // Keep connections alive 5 seconds if a keep-alive value 
            // has not be explicitly set by the server 
            keepAlive = 5000;
        }
        return keepAlive;
    }
    
});

DefaultHttpClient also maintains a list of protocol interceptors intended for processing outgoing requests and incoming responses and provides methods for managing those interceptors. New protocol interceptors can be introduced to the protocol processor chain or removed from it if needed. Internally protocol interceptors are stored in a simple java.util.ArrayList. They are executed in the same natural order as they are added to the list.

DefaultHttpClient httpclient = new DefaultHttpClient();
httpclient.removeRequestInterceptorByClass(RequestUserAgent.class);
httpclient.addRequestInterceptor(new HttpRequestInterceptor() {

    public void process(
            HttpRequest request, HttpContext context)
            throws HttpException, IOException {
        request.setHeader(HTTP.USER_AGENT, "My-own-client");
    }
    
});

DefaultHttpClient is thread safe. It is recommended that the same instance of this class is reused for multiple request executions. When an instance of DefaultHttpClient 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 ClientConnectionManager#shutdown() method.

HttpClient httpclient = new DefaultHttpClient();
// Do something useful
httpclient.getConnectionManager().shutdown();

These are parameters that be used to customize the behaviour of the default HttpClient implementation:

HttpClient handles all types of redirects automatically, except those explicitly prohibited by the HTTP specification as requiring user intervention. See Other (status code 303) redirects on POST and PUT requests are converted to GET requests as required by the HTTP specification.

The DefaultHttpClient treats HTTP requests as immutable objects that are never supposed to change in the course of request execution. Instead, it creates a private mutable copy of the original request object, whose properties can be updated depending on the execution context. Therefore the final request properties such as the target host and request URI can be determined by examining the content of the local HTTP context after the request has been executed.

DefaultHttpClient httpclient = new DefaultHttpClient();

HttpContext localContext = new BasicHttpContext();
HttpGet httpget = new HttpGet("http://localhost:8080/"); 
HttpResponse response = httpclient.execute(httpget, localContext);
HttpHost target = (HttpHost) localContext.getAttribute(
        ExecutionContext.HTTP_TARGET_HOST);
HttpUriRequest req = (HttpUriRequest) localContext.getAttribute(
        ExecutionContext.HTTP_REQUEST);

System.out.println("Target host: " + target);
System.out.println("Final request URI: " + req.getURI());
System.out.println("Final request method: " + req.getMethod());