提出问题
DriverManager类的简单学习???
解决问题
DriverManager在java.sql这个包里面,管理一组 JDBC 驱动程序的基本服务
package com.evada.de;
import java.sql.Connection;
import java.sql.Driver;
import java.sql.DriverManager;
import java.util.Enumeration;
/**
* 用静态工厂方法管理一个唯一的可重用的连接
* Created by Ay on 2016/5/1.
*/
public class ConnUtils{
private ConnUtils(){}
private static Connection con;
//在静态代码块中创建与数据库的连接
static{
try{
//postgresql驱动名称
Class.forName("org.postgresql.Driver");
//数据库连接路径
String url = "jdbc:postgresql://127.0.0.1:5432/postgres";
//用户名和密码
con = DriverManager.getConnection(url, "postgres", "888888");
}catch(Exception e){
throw new RuntimeException(e.getMessage(),e);
}
}
//使用一个静态方法-静态工厂方法,返回connection实例
public static Connection getCon(){
return con;
}
//main函数测试用
public static void main(String[] args) {
//获取所有的数据库驱动
Enumeration<Driver> driverEnum = DriverManager.getDrivers();
//打印出所有驱动信息
while(driverEnum.hasMoreElements()){
System.out.println(driverEnum.nextElement());
}
}
}
运行结果:
org.mariadb.jdbc.Driver@7cef4e59
com.alibaba.druid.proxy.DruidDriver@3ada9e37
com.alibaba.druid.mock.MockDriver@4ec6a292
org.postgresql.Driver@71c7db30
解释,因为我们的项目包里有如下图片的数据库驱动,所以会打印下面信息
这两者的区别:
DriverManager.registerDriver(new Driver());
Class.forName(“com.mysql.jdbc.Driver”);
org.postgresql.Driver类源码如下,可以看出postgresql是通过单例形式,new一个驱动Driver,也就是说在连接数据库是只new了一个Driver
static {
sharedTimer = new SharedTimer(logger);
try {
//注册
register();
} catch (SQLException var1) {
throw new ExceptionInInitializerError(var1);
}
protocols = new String[]{"jdbc", "postgresql"};
}
public static void register() throws SQLException {
if(isRegistered()) {
throw new IllegalStateException("Driver is already registered......);
} else {
Driver registeredDriver = new Driver();
DriverManager.registerDriver(registeredDriver);
registeredDriver = registeredDriver;
}
}
再来看看DriverManager的源码,它会把Driver包装到DriverInfo中,
//
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) {
//将驱动包装到DriverInfo中
registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
} else {
// This is for compatibility with the original DriverManager
throw new NullPointerException();
}
println("registerDriver: " + driver);
}
Class.forName(“com.mysql.jdbc.Driver”);
结论:DriverManager.registerDriver(new Driver()); 多new一个Driver浪费;而Class.forName(“com.mysql.jdbc.Driver”); 单例形式,更推荐……