在Java中有多少个JDBC连接？ [英] How many JDBC connections in Java?

问题描述

我有一个由大约15个方法组成的Java程序。并且，这些方法在程序的执行期间被非常频繁地调用。目前，我在每个方法中创建一个新的连接并调用它们上的语句（数据库在网络上的另一台机器上设置）。

我想知道的是：我应该在main方法中只创建一个连接，并将它作为参数传递给所有需要连接对象的方法，因为它将显着减少程序中连接对象的数量，而不是在每种方法中非常频繁地创建和关闭连接。

我怀疑我目前的设计并没有非常有效地使用这些资源，并且有很多改进的空间，考虑到这个程序可能在未来。

解决方案

是的，您应该考虑重新使用连接，而不是每次都创建一个新连接。通常的过程是：

• 猜测你的数据库可以处理多少同时连接（例如，以每个CPU 2或3开始在数据库机器上，直到你发现这太少或太多 - 它往往取决于磁盘绑定你的查询是什么）


• 创建池：实质上是一个类，你可以在每个方法开始时请求下一个空闲连接，然后在每个方法结束时回传到池中。


• 您的getFreeConnection（）方法需要返回一个空闲连接（如果有），否则（1）创建一个新连接，直到您决定允许的最大连接数，或者（2）if


• 我建议使用Semaphore类来管理连接;我实际上在我的网站上有一篇关于使用信号量管理资源池的文章




几个实用注意事项：






• 为了获得最佳性能，您需要小心不要在您实际上不使用连接来运行查询时截断连接。如果您从池中取出一次连接，然后将其传递给各种方法，则需要确保不会意外执行此操作。


• 不要忘记将连接返回游泳池！ （try / finally是您的朋友在这里...）


• 在许多系统上，您无法保持连接打开永远：O / S将在一些最大时间后关闭它们。因此，在您的返回连接池方法中，您需要考虑退役已存在很长时间的连接（构建某种机制来记住，例如通过


• 您可以考虑使用预准备的语句。​​
>

• 随着时间的推移，您可能需要调整连接池大小





I have a Java program consisting of about 15 methods. And, these methods get invoked very frequently during the exeuction of the program. At the moment, I am creating a new connection in every method and invoking statements on them (Database is setup on another machine on the network).

What I would like to know is: Should I create only one connection in the main method and pass it as an argument to all the methods that require a connection object since it would significantly reduce the number of connections object in the program, instead of creating and closing connections very frequently in every method.

I suspect I am not using the resources very efficiently with the current design, and there is a lot of scope for improvement, considering that this program might grow a lot in the future.

解决方案

Yes, you should consider re-using connections rather than creating a new one each time. The usual procedure is:

• make some guess as to how many simultaneous connections your database can sensibly handle (e.g. start with 2 or 3 per CPU on the database machine until you find out that this is too few or too many-- it'll tend to depend on how disk-bound your queries are)
• create a pool of this many connections: essentially a class that you can ask for "the next free connection" at the beginning of each method and then "pass back" to the pool at the end of each method
• your getFreeConnection() method needs to return a free connection if one is available, else either (1) create a new one, up to the maximum number of connections you've decided to permit, or (2) if the maximum are already created, wait for one to become free
• I'd recommend the Semaphore class to manage the connections; I actually have a short article on my web site on managing a resource pool with a Semaphore with an example I think you could adapt to your purpose

A couple of practical considerations:

• For optimum performance, you need to be careful not to "hog" a connection while you're not actually using it to run a query. If you take a connection from the pool once and then pass it to various methods, you need to make sure you're not accidentally doing this.
• Don't forget to return your connections to the pool! (try/finally is your friend here...)
• On many systems, you can't keep connections open 'forever': the O/S will close them after some maximum time. So in your 'return a connection to the pool' method, you'll need to think about 'retiring' connections that have been around for a long time (build in some mechanism for remembering, e.g. by having a wrapper object around an actual JDBC Connection object that you can use to store metrics such as this)
• You may want to consider using prepared statements.
• Over time, you'll probably need to tweak the connection pool size

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