到底什么是消息队列？Java中如何实现消息队列

所谓队列，就是按照队首先出的规则建立的数据结构，消息队列就是根据消息到来后按照一定的规则进行排序，但一定是队首的消息先得到应答的队列。
比如：

public static String do_post(String url, List name_value_pair) throws IOException {
String body = "{}";
DefaultHttpClient httpclient = new DefaultHttpClient();
try {
HttpPost httpost = new HttpPost(url);
httpost.setEntity(new UrlEncodedFormEntity(name_value_pair, StandardCharsets.UTF_8));
HttpResponse response = httpclient.execute(httpost);
HttpEntity entity = response.getEntity();
body = EntityUtils.toString(entity);
} finally {
httpclient.getConnectionManager().shutdown();
}
return body;
}
public static String do_get(String url) throws ClientProtocolException, IOException {
String body = "{}";
DefaultHttpClient httpclient = new DefaultHttpClient();
try {
HttpGet httpget = new HttpGet(url);
HttpResponse response = httpclient.execute(httpget);
HttpEntity entity = response.getEntity();
body = EntityUtils.toString(entity);
} finally {
httpclient.getConnectionManager().shutdown();
}
return body;
}