笔者做了一个小的系统辅助功能，可以周期性访问某个URL、执行某个SQL语句 or 执行某个系统命令。
执行SQL语句和系统命令比较简单，这里不再详述，主要说一下访问某个URL。

实际上JDK自身已有工具类用于创建HTTP请求，类名是：java.net.HttpURLConnection，但考虑到基础类通常比较粗糙，很多情况要自己考虑和处理，就转头去Google了下，发现果然有开源的工具包可以使用，几个工具包中以HttpClient较为常用，而且是apache的东东，于是决定采用HttpClient。

从apache上down了包commons-httpclient-3.1.jar和commons-codec-1.3.jar两个包，后者是HttpClient依赖的包。
帮助写的很好，即便是像我这样英文很烂，也能很快上手。

public boolean visitURL(/tech-oracle/String url/index.html) {
// Commons HttpClient 3.1
HttpClient client = new HttpClient();
HttpMethod method = new GetMethod(url);
// Provide custom retry handler is necessary
method.getParams().setParameter(HttpMethodParams.RETRY_HANDLER, new DefaultHttpMethodRetryHandler(3, false));
boolean rs = false;
try {
// Execute the method.
int statusCode = client.executeMethod(method);
if (statusCode != HttpStatus.SC_OK) {
logger.error("Method failed: " + method.getStatusLine());
}
else {
rs = true;
}
} catch (HttpException e) {
logger.error("Fatal protocol violation: " + e.getMessage());
} catch (IOException e) {
logger.error("Fatal transport error: " + e.getMessage());
} finally {
// Release the connection.
method.releaseConnection();
}
return rs;
}
本机Tomcat下run一下，工作正常，随即丢到服务器（Oracle IAS环境）上测试，程序应该出乎意料的报了个错。
09/03/16 19:03:43 java.lang.NoClassDefFoundError
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpMethodBase.writeRequestLine(HttpMethodBase.java:2015)
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpMethodBase.writeRequest(HttpMethodBase.java:1864)
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpMethodBase.execute(HttpMethodBase.java:975)
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:368)
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:164)
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:437)
09/03/16 19:03:43 at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:324)
09/03/16 19:03:43 at com.zbht.util.TimerTaskManager.runURLTask(TimerTaskManager.java:237)
09/03/16 19:03:43 at _system._timer__task._test._jspService(_test.java:182)
09/03/16 19:03:43 at com.orionserver.http.OrionHttpJspPage.service(OrionHttpJspPage.java:59)
09/03/16 19:03:43 at oracle.jsp.runtimev2.JspPageTable.service(JspPageTable.java:462)
09/03/16 19:03:43 at oracle.jsp.runtimev2.JspServlet.internalService(JspServlet.java:594)
09/03/16 19:03:43 at oracle.jsp.runtimev2.JspServlet.service(JspServlet.java:518)
09/03/16 19:03:43 at javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
09/03/16 19:03:43 at com.evermind.server.http.ServletRequestDispatcher.invoke(ServletRequestDispatcher.java:713)
09/03/16 19:03:43 at com.evermind.server.http.ServletRequestDispatcher.forwardInternal(ServletRequestDispatcher.java:370)
09/03/16 19:03:43 at com.evermind.server.http.HttpRequestHandler.doProcessRequest(HttpRequestHandler.java:871)
09/03/16 19:03:43 at com.evermind.server.http.HttpRequestHandler.processRequest(HttpRequestHandler.java:453)
09/03/16 19:03:43 at com.evermind.server.http.AJPRequestHandler.run(AJPRequestHandler.java:302)
09/03/16 19:03:43 at com.evermind.server.http.AJPRequestHandler.run(AJPRequestHandler.java:190)
09/03/16 19:03:43 at oracle.oc4j.network.ServerSocketReadHandler$SafeRunnable.run(ServerSocketReadHandler.java:260)
09/03/16 19:03:43 at com.evermind.util.ReleasableResourcePooledExecutor$MyWorker.run(ReleasableResourcePooledExecutor.java:303)
09/03/16 19:03:43 at java.lang.Thread.run(Thread.java:595)
错误信息看上去比较低级：NoClassDefFoundError，类没找到，迅速了检查了一下本机和服务器上的jar包是否相同，“一模一样”！这就奇怪了。
检查本机的开发环境，只添加了这两个jar，其他的都没有动过，又检查服务器的运行环境，一样没有变化。于是删掉本机开发环境下的这两个jar，问题浮出来了，类中对httpclient的7、8个引用中只有1个提示未找到指定的类，看来Oracle自己的某个包中已经包含某个较低版本的httpclient，jar包冲突的问题是件让人沮丧的事情，尝试解决这种问题会所耗费的时间也许是其他方法的N倍，无心恋战。
其实此处要进行的操作很简单，就是访问指定的URL，根据返回的内容检查是否成功，HttpClient是完整模拟浏览器，考虑了很多种问题，使用起来反倒是复杂了，决定转用JDK的基础类：java.net.HttpURLConnection
事情出奇的顺利，空间里找到了之前写的一个方法，正好解决这个问题，以下是代码清单：
private boolean visitURL(/tech-oracle/String strUrl, String successFlag/index.html) {
boolean rs = false;
HttpURLConnection jconn = null;
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
try {
URL url = new URL(/tech-oracle/strUrl/index.html);
jconn = (HttpURLConnection) url.openConnection();
jconn.setDoOutput(true);
jconn.setDoInput(true);
jconn.connect();
InputStream in = jconn.getInputStream();
byte[] buf = new byte[4096];
int bytesRead;
while ((bytesRead = in.read(buf)) != -1) {
byteArrayOutputStream.write(buf, 0, bytesRead);
}
String strRead = new String(byteArrayOutputStream.toByteArray());
logger.debug(strRead);
strRead = StringUtil.NVL(strRead);
if(strRead.indexOf(successFlag) != -1) {
logger.info("Visit URL  success !");
rs = true;
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
jconn.disconnect();
try {
byteArrayOutputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
return rs;
}