1.框架介绍
服务提供者框架是指这样一个系统：多个服务提供者实现一个服务，系统为服务提供者的客服端提供多个实现，并把他们从多个实现中解耦出来。
服务框架中有四个组件：
- 服务接口：用来提供者实现的
- 提供者注册API：系统用来注册实现，让客服端访问它们
- 服务访问API：客户端用来获取服务的实例
- 服务提供者接口（可选）：负责创建其服务实现的实例
2.JDBC的实现
JDBC连接数据库重要的两步就是加载驱动和获取连接
1.加载驱动，通过执行驱动中静态代码块，进行注册到DiverManager中。
#加载驱动
Class.forName("com.mysql.cj.jdbc.Driver");
private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
#加载mysql驱动的时候，完成注册
public class Driver extends NonRegisteringDriver implements java.sql.Driver {
public Driver() throws SQLException {
}
static {
try {
#注册服务实现
DriverManager.registerDriver(new Driver());
} catch (SQLException var1) {
throw new RuntimeException("Can't register driver!");
}
}
}
public static synchronized void registerDriver(java.sql.Driver driver)
throws SQLException {
registerDriver(driver, null);
}
public static synchronized void registerDriver(java.sql.Driver driver,
DriverAction da)
throws SQLException {
// 如果不存在，注册驱动到集合中
if(driver != null) {
registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
} else {
throw new NullPointerException();
}
println("registerDriver: " + driver);
}
2.获取连接，找到注册的驱动，调用驱动实现的connect方法
Connection conn=DriverManager.getConnection(url, user, password);
public static Connection getConnection(String url,
String user, String password) throws SQLException {
java.util.Properties info = new java.util.Properties();
if (user != null) {
info.put("user", user);
}
if (password != null) {
info.put("password", password);
}
return (getConnection(url, info, Reflection.getCallerClass()));
}
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");
}
对于JDBC来说，Connection是服务接口，DriverManager.registerDriver提供者注册API，DriverManager.getConnection是服务访问API，Driver是服务提供者接口。