HttpClient 是接口，DefaultHttpClient是實現這個接口的子類

public interface HttpClient {/*** Obtains the parameters for this client.* These parameters will become defaults for all requests being* executed with this client, and for the parameters of* dependent objects in this client.** @return  the default parameters*/HttpParams getParams();/*** Obtains the connection manager used by this client.** @return  the connection manager*/ClientConnectionManager getConnectionManager();/*** Executes a request using the default context.** @param request   the request to execute** @return  the response to the request. This is always a final response,*          never an intermediate response with an 1xx status code.*          Whether redirects or authentication challenges will be returned*          or handled automatically depends on the implementation and*          configuration of this client.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/HttpResponse execute(HttpUriRequest request)throws IOException, ClientProtocolException;/*** Executes a request using the given context.* The route to the target will be determined by the HTTP client.** @param request   the request to execute* @param context   the context to use for the execution, or*                  <code>null</code> to use the default context** @return  the response to the request. This is always a final response,*          never an intermediate response with an 1xx status code.*          Whether redirects or authentication challenges will be returned*          or handled automatically depends on the implementation and*          configuration of this client.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/HttpResponse execute(HttpUriRequest request, HttpContext context)throws IOException, ClientProtocolException;/*** Executes a request to the target using the default context.** @param target    the target host for the request.*                  Implementations may accept <code>null</code>*                  if they can still determine a route, for example*                  to a default target or by inspecting the request.* @param request   the request to execute** @return  the response to the request. This is always a final response,*          never an intermediate response with an 1xx status code.*          Whether redirects or authentication challenges will be returned*          or handled automatically depends on the implementation and*          configuration of this client.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/HttpResponse execute(HttpHost target, HttpRequest request)throws IOException, ClientProtocolException;/*** Executes a request to the target using the given context.** @param target    the target host for the request.*                  Implementations may accept <code>null</code>*                  if they can still determine a route, for example*                  to a default target or by inspecting the request.* @param request   the request to execute* @param context   the context to use for the execution, or*                  <code>null</code> to use the default context** @return  the response to the request. This is always a final response,*          never an intermediate response with an 1xx status code.*          Whether redirects or authentication challenges will be returned*          or handled automatically depends on the implementation and*          configuration of this client.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/HttpResponse execute(HttpHost target, HttpRequest request,HttpContext context)throws IOException, ClientProtocolException;/*** Executes a request using the default context and processes the* response using the given response handler.** @param request   the request to execute* @param responseHandler the response handler** @return  the response object as generated by the response handler.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/<T> T execute(HttpUriRequest request, ResponseHandler<? extends T> responseHandler)throws IOException, ClientProtocolException;/*** Executes a request using the given context and processes the* response using the given response handler.** @param request   the request to execute* @param responseHandler the response handler** @return  the response object as generated by the response handler.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/<T> T execute(HttpUriRequest request, ResponseHandler<? extends T> responseHandler,HttpContext context)throws IOException, ClientProtocolException;/*** Executes a request to the target using the default context and * processes the response using the given response handler.** @param target    the target host for the request.*                  Implementations may accept <code>null</code>*                  if they can still determine a route, for example*                  to a default target or by inspecting the request.* @param request   the request to execute* @param responseHandler the response handler** @return  the response object as generated by the response handler.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/<T> T execute(HttpHost target, HttpRequest request,ResponseHandler<? extends T> responseHandler)throws IOException, ClientProtocolException;/*** Executes a request to the target using the given context and * processes the response using the given response handler.** @param target    the target host for the request.*                  Implementations may accept <code>null</code>*                  if they can still determine a route, for example*                  to a default target or by inspecting the request.* @param request   the request to execute* @param responseHandler the response handler* @param context   the context to use for the execution, or*                  <code>null</code> to use the default context** @return  the response object as generated by the response handler.* @throws IOException in case of a problem or the connection was aborted* @throws ClientProtocolException in case of an http protocol error*/<T> T execute(HttpHost target, HttpRequest request,ResponseHandler<? extends T> responseHandler, HttpContext context)throws IOException, ClientProtocolException;} // interface HttpClient