问题的引出
为什么我们通过DriverManager.getConnection(url, username, password)获取到的是Connection是一个接口,却能调用方法?不是说接口只是一个抽象的东西?不能做具体的事情么?
那我们通过DriverManager获取到的Connection接口为什么可以做获取Statement对象之类的事情?
首先我们看我们获取到的对象是怎么样获取的:
Connection conn = DriverManager.getConnection(url, username, password);
这种获取方法是不是很眼熟?我们看一下从小学到大的一个创建对象的写法:
Person person=new Student();
这样来看是不是很熟悉?是不是我们的父类引用指向子类对象?
所以我们实际上通过DriverManager.getConnection(url, username, password)获取到的是实现了Connection接口的实体对象
我们接着看DriverManager是怎么获取到Connection实体对象的,先通过DriverManager.getConnection(url, username, password)跳到源码位置
DvierManamger的方法
public static Connection getConnection(String url,
String user, String password) throws SQLException {
java.util.Properties info = new java.util.Properties();//创建了一个Properties对象来存放信息
if (user != null) {
info.put("user", user);//如果user不为空,将user信息存入info对象
}
if (password != null) {
info.put("password", password);//如果password不为空,将password信息存入info对象
}
//再通过另一个重载的getConnection方法获取连接
return (getConnection(url, info, Reflection.getCallerClass()));
}
我们在第一次跳转进来的方法中是没法直接获取实现了Connection接口的实体类的,这里还要通过DriverManger的重载方法继续做做事情,接着我们进到getConnection(url, info, Reflection.getCallerClass())方法
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.
*/
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;
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());
}
}
// if we got here nobody could connect.
if (reason != null) {
println("getConnection failed: " + reason);
throw reason;
}
println("getConnection: no suitable driver found for "+ url);
throw new SQLException("No suitable driver found for "+ url, "08001");
}
这一整个方法我们不用全部关心,找到切入点,我们现在知道自己在找Connection,和Connection息息相关的是Driver,那我们现在就针对这两个对象来查找,可以看到这个方法里的这段关键代码
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());
}
}
这个增强for循环中有一个registeredDrivers对象,我们通过查看可以知道他是一个ArrayList,存在这个List的对象是DriverInfo,现在先看一下DriverInfo
final Driver driver;
DriverAction da;
DriverInfo(Driver driver, DriverAction action) {
this.driver = driver;
da = action;
}
可以看到他接收了一个成员变量driver,那我们初步可以判断,DriverManager是通过registeredDrivers这个集合来获取已经注册好的驱动,那registeredDrivers这个集合是在什么地方添加对象的呢?
public static synchronized void registerDriver(java.sql.Driver driver,
DriverAction da)
throws SQLException {
/* Register the driver if it has not already been added to our list */
if(driver != null) {
registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
} else {
// This is for compatibility with the original DriverManager
throw new NullPointerException();
}
println("registerDriver: " + driver);
}
根据关键字"registeredDrivers"查找,我们发现了registerDriver这个方法,如果driver对象不为空就尝试进行添加,如果没有添加过,我们就会将该driver对象加入registerDriver集合中,而注册驱动是我们第一步做的事情,所以在我们注册驱动的时候,已经将com.mysql.jdbc.Driver加到了该集合中
再回到增强for循环看他做了什么事
Connection con = aDriver.driver.connect(url, info);
if (con != null) {
// Success!
println("getConnection returning " + aDriver.driver.getClass().getName());
return (con);
}
DriverManger通过每次遍历到的aDriver获取里面的driver对象,再调用它的connect方法,将url(数据库地址),info(访问用户名,访问密码)拿来使用,获取实现了Connection接口的实现类,到此,我们初步可以看到DriverManager是如何获取Connection实现类的了
去到我们注册的com.mysql.jdbc.Driver类中找connect方法,发现没找到,那子类没有的方法一般父类会有,我们继续往父类com.mysql.jdbc.NonRegisteringDriver那查找connect(url,info)方法
我们在父类com.mysql.jdbc.NonRegisteringDriver中可以找到下面方法:
public java.sql.Connection connect(String url, Properties info) throws SQLException
在这个方法里我们就能看到它在返回ConnectionImpl对象了