本篇对jdbc的获取连接进行解析
1.这里是直接上代码哈，它的getConnection会通过相对应调用的重载方法来进行不同的操作，我们主要看我们常规的连接方式。
@CallerSensitive
public static Connection getConnection(String url,
String user, String password) throws SQLException {
//存放相关配置属性
java.util.Properties info = new java.util.Properties();
if (user != null) {
info.put("user", user);
}
if (password != null) {
info.put("password", password);
}
//会继续调用本类getConnection方法,并传入反射的getCallerClass方法。
return (getConnection(url, info, Reflection.getCallerClass()));
}
继续之前需要解释一下@CallerSensitive
@CallerSensitive开发者正常调用是不起作用的，因为java并不希望我们调用该方法。（它也是可以被调用的， -Xbootclasspath/a: path 在jvm启动进行配置）它必须有启动类ClassLoader加载（比如rt.jar才能被识别)。
Reflection.getCallerClass()这个方法就必须加上@CallerSensitive配合使用。
它这里是做什么用的？？
2.然后我们继续往下看（看注解哦）
// Worker method called by the public getConnection() methods.
private static Connection getConnection(
String url, java.util.Properties info, Class<?> caller) throws SQLException {
/*
* When callerCl is null, we should check the application's
* (which is invoking this class indirectly)
* classloader, so that the JDBC driver class outside rt.jar
* can be loaded from here.
*/
//这里的话就基本大概知道为什么了，它在这里的话就会通过它来获取的我们的数据库连接的驱动。
ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
synchronized(DriverManager.class) {
// synchronize loading of the correct classloader.
if (callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
}
if(url == null) {
throw new SQLException("The url cannot be null", "08001");
}
println("DriverManager.getConnection(\"" + url + "\")");
// Walk through the loaded registeredDrivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;
//这里的话就会获取完后并通过配置进行连接,最后返回Connection。
for(DriverInfo aDriver : registeredDrivers) {
// If the caller does not have permission to load the driver then
// skip it.
if(isDriverAllowed(aDriver.driver, callerCL)) {
try {
println(" trying " + aDriver.driver.getClass().getName());
Connection con = aDriver.driver.connect(url, info);
if (con != null) {
// Success!
println("getConnection returning " + aDriver.driver.getClass().getName());
return (con);
}
} catch (SQLException ex) {
if (reason == null) {
reason = ex;
}
}
} else {
println(" skipping: " + aDriver.getClass().getName());
}
}
//如果走到这里就是百分百有问题啦，就是报错的原因会在这抛出。
// if we got here nobody could connect.
if (reason != null) {
println("getConnection failed: " + reason);
throw reason;
}
//走到这里的话就是找不到合适的驱动
println("getConnection: no suitable driver found for "+ url);
throw new SQLException("No suitable driver found for "+ url, "08001");
}
