在Android中建立与MySQL的池化连接 [英] Establish pooled connection to MySQL in android

问题描述

我需要从我的Android应用访问MySQL数据库.现在所有的工作都通过

I need to access a MySQL db from my android app. Now all the work is done through

DriverManager.getConnection(url);

，依此类推.但是我必须从多个线程访问数据库，因此必须使用连接池.

and so on. But I have to access the db from multiple threads, so I have to use connection pooling.

问题1.是

com.mysql.jdbc.jdbc2.optional.MysqlDataSource ds = new com.mysql.jdbc.jdbc2.optional.MysqlDataSource();

创建数据源的正确方法?

the right way of creating the data source?

问题2.如果我编写了上一行代码，则我的应用可以正常编译并安装在设备(而不是仿真器)上，但很奇怪

Question 2. If I write the previous line of code, my app compiles and installs on the device (not emulator) fine, but I get a weird

java.lang.NoClassDefFoundError: com.mysql.jdbc.jdbc2.optional.MysqlDataSource`

，我无法使用try/catch处理程序进行捕获:

, that I can't catch with a try/catch handler:

try
{
com.mysql.jdbc.jdbc2.optional.MysqlDataSource a = new com.mysql.jdbc.jdbc2.optional.MysqlDataSource();
}
catch (Exception e)
{
I don't get here. The app just crashes, as if I had no try/catch block.
}

问题3.我将mysql-connector-java-5.1.20-bin.jar复制到设备并编写了以下代码:

Question 3. I copied mysql-connector-java-5.1.20-bin.jar to the device and wrote the following code:

try
{
final String str = Environment.getExternalStorageState();
final File sd = getActivity().getExternalFilesDir(null);
final File file = new File(sd, "mysql-connector-java-5.1.20-bin.jar");

boolean b = file.exists();

final URLClassLoader cl = URLClassLoader.newInstance(new URL[] {file.toURI().toURL()} );
cl.loadClass("com.mysql.jdbc.jdbc2.optional.MysqlDataSource"); //$NON-NLS-1$
}

catch (Exception e)
{
new AlertDialog.Builder(getActivity())
.setMessage(ObjectConverter.throwable2String(e))
.show();
}

找到文件，但是

cl.loadClass()

失败

java.lang.NullPointerException
at java.net.URLClassLoader.getPermissions(URLClassLoader.java:614)
at java.security.SecureClassLoader.getPD(SecureClassLoader.java:140)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:93)
at java.net.URLClassLoader.access$600(URLClassLoader.java:55)
at java.net.URLClassLoader$URLJarHandler.createClass(URLClassLoader.java:364)
at java.net.URLClassLoader$URLJarHandler.findClass(URLClassLoader.java:303)
at java.net.URLClassLoader.findClass(URLClassLoader.java:748)
at java.lang.ClassLoader.loadClass(ClassLoader.java:501)
at java.lang.ClassLoader.loadClass(ClassLoader.java:461)
at ru.mypkg.myapp.func(myapp.java:367)
at android.view.View.performClick(View.java:3511)
at android.view.View$PerformClick.run(View.java:14110)
at android.os.Handler.handleCallback(Handler.java:605)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4424)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
at dalvik.system.NativeStart.main(Native Method)

任何帮助都将不胜感激.

Any help greatly appreciated.

推荐答案

这个问题已有6年历史了，马克(Mark)是正确的，在您能想到的几乎每种情况下，Android上的JDBC与尝试使用JDBC一样明智.烤面包机在浴缸里.但是，有时候我们做事情是因为我们可以做而不是因为我们必须做，而不是因为我们必须做，而今天我有正当的理由要这样做(对于在不寻常的环境中运行的特定的非常小众的应用程序)，这就是我找到这个问题的方式.

This question is 6 years old and Mark is correct that, in almost every scenario you can think of, JDBC on Android is about as sensible as trying to use a toaster in the bath. However, sometimes we do things because we can and not because we necessarily should, and today I had a justifiable reason to want to do this (for a specific very niche app running in an unusual environment), which is how I found this question.

首先寻址NoClassDefFoundError，其未被catch块捕获的原因是因为它是Error而不是Exception. Error和Exception都从Throwable继承，因此您可以捕获它:

Addressing the NoClassDefFoundError first, the reason it isn't caught by the catch block is because it's an Error not an Exception. Both Error and Exception inherit from Throwable, so you could catch that instead:

catch (Throwable t)
{
    // This will catch NoClassDefFoundError
}

我相信它不是找不到的MySqlDataSource，而是它所依赖的类或接口之一-在我的情况下是javax.naming.Referenceable. Android不提供javax.naming包，因此尝试为MySQL使用Connector/J JDBC驱动程序的池化功能不会使您走得太远(您可以尝试提供缺少的依赖项，但是那条路很可能导致疯狂).

I believe that it's not MySqlDataSource that it can't find, but one of the classes or interfaces that it depends on - in my case it was javax.naming.Referenceable. Android doesn't provide the javax.naming package so trying to use the pooling features of the Connector/J JDBC driver for MySQL isn't going to get you very far (you could try to provide the missing dependencies but that road likely leads to madness).

相反，第三方连接池实现可能会给您带来更多的运气.有各种Java库可以执行此操作.其中一些将在Android上运行.我已验证确实有效的一种方法是 HikariCP .在此处有专门的配置说明，对于MySQL，特别是

Instead you'll probably have more luck with a third-party connection pool implementation. There are various Java libraries for doing this. Some of them will work on Android. One that I have verified does work is HikariCP. There are instructions for configuring it here and, specifically for MySQL, here.

这篇关于在Android中建立与MySQL的池化连接的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！