详细介绍在：http://hc.apache.org/httpclient-3.x/logging.html
一般使用context logging基本够用，context logging解释原文如下：
context logging
Context logging contains information about the internal operation of HttpClient as it performs HTTP requests. Each class has its own log named according to the class's fully qualified name. For example the class HttpClient
has a log named org.apache.commons.httpclient.HttpClient. Since all classes follow this convention it is possible to configure context logging for all classes using the single log named org.apache.commons.httpclient.
Example
Commons Logging comes with a basic logger called SimpleLog. This logger writes all logged messages to System.err. The following examples show how to configure Commons Logging via system properties to use SimpleLog.
配置代码如下：