1 /**管理一个集合JDBC驱动的基础服务  2 注意：在新的JDBC2.0api中实现了新的DataSource接口，提供了另一种链接数据源的方式。  3 使用DataSource的对象是首选方案  4 */  5 public class DriverManager {  6   7     // 注册了JDBC驱动的集合  8     private final static CopyOnWriteArrayList
     
       registeredDrivers = new CopyOnWriteArrayList<>();  9    10     /** 11     通过检查jdcb.properties加载初始化JDBC驱动 12      */ 13     static { 14         loadInitialDrivers(); 15         println("JDBC DriverManager initialized"); 16     } 17  18 /* 19 *对外使用提供数据库连接的方法， 20 *重点说明Reflection.getCallerClass()方法，该方法返回调用者的类 21 */ 22     @CallerSensitive 23     public static Connection getConnection(String url) 24         throws SQLException { 25  26         java.util.Properties info = new java.util.Properties(); 27         return (getConnection(url, info, Reflection.getCallerClass())); 28     } 29  30     //  用于根据url、对应的属性信息在registeredDrivers中找到合适的注册的驱动创建数据库链接 31     private static Connection getConnection( 32         String url, java.util.Properties info, Class
       caller) throws SQLException { 33         /* 34          * When callerCl is null, we should check the application's 35          * (which is invoking this class indirectly) 36          * classloader, so that the JDBC driver class outside rt.jar 37          * can be loaded from here. 38          */ 39         ClassLoader callerCL = caller != null ? caller.getClassLoader() : null; 40         synchronized(DriverManager.class) { 41             // synchronize loading of the correct classloader. 42             if (callerCL == null) { 43                 callerCL = Thread.currentThread().getContextClassLoader(); 44             } 45         } 46  47         if(url == null) { 48             throw new SQLException("The url cannot be null", "08001"); 49         } 50  51         println("DriverManager.getConnection(\"" + url + "\")"); 52  53         // Walk through the loaded registeredDrivers attempting to make a connection. 54         // Remember the first exception that gets raised so we can reraise it. 55         SQLException reason = null; 56  57         for(DriverInfo aDriver : registeredDrivers) { 58             // If the caller does not have permission to load the driver then 59             // skip it. 60          //一个应用中有可能会有多个数据库驱动，需要判断数据库连接的调用者（调用getConnection()方法的对象）是否与驱动相匹配 61             if(isDriverAllowed(aDriver.driver, callerCL)) { 62                 try { 63                     println("    trying " + aDriver.driver.getClass().getName()); 64                     Connection con = aDriver.driver.connect(url, info); 65                     if (con != null) { 66                         // Success! 67                         println("getConnection returning " + aDriver.driver.getClass().getName()); 68                         return (con); 69                     } 70                 } catch (SQLException ex) { 71                     if (reason == null) { 72                         reason = ex; 73                     } 74                 } 75  76             } else { 77                 println("    skipping: " + aDriver.getClass().getName()); 78             } 79  80         } 81  82         // if we got here nobody could connect. 83         if (reason != null)    { 84             println("getConnection failed: " + reason); 85             throw reason; 86         } 87  88         println("getConnection: no suitable driver found for "+ url); 89         throw new SQLException("No suitable driver found for "+ url, "08001"); 90     } 91 } 92  93 //一个应用中有可能会有多个数据库驱动，需要判断数据库连接的调用者（调用getConnection()方法的对象）是否与驱动相匹配 94 private static boolean isDriverAllowed(Driver driver, ClassLoader classLoader) { 95         boolean result = false; 96         if(driver != null) { 97             Class
       aClass = null; 98             try { 99                 aClass =  Class.forName(driver.getClass().getName(), true, classLoader);100             } catch (Exception ex) {101                 result = false;102             }103 104              result = ( aClass == driver.getClass() ) ? true : false;105         }106 107         return result;108     }109 110 //根据URL从registeredDrivers 中获得驱动，根据Reflection.getCallerClass()返回的调用者类，在registeredDrivers 中进行匹配111   @CallerSensitive112     public static Driver getDriver(String url)113         throws SQLException {114 115         println("DriverManager.getDriver(\"" + url + "\")");116 117         Class
       callerClass = Reflection.getCallerClass();118 119         // Walk through the loaded registeredDrivers attempting to locate someone120         // who understands the given URL.121         for (DriverInfo aDriver : registeredDrivers) {122             // If the caller does not have permission to load the driver then123             // skip it.124             if(isDriverAllowed(aDriver.driver, callerClass)) {125                 try {126                     if(aDriver.driver.acceptsURL(url)) {127                         // Success!128                         println("getDriver returning " + aDriver.driver.getClass().getName());129                     return (aDriver.driver);130                     }131 132                 } catch(SQLException sqe) {133                     // Drop through and try the next driver.134                 }135             } else {136                 println("    skipping: " + aDriver.driver.getClass().getName());137             }138 139         }140 141         println("getDriver: no suitable driver");142         throw new SQLException("No suitable driver", "08001");143     }144 145 146 /*147 *注册数据驱动，使用同步方式，最终保存在CopyOnWriteArrayList的集合中148 */149     public static synchronized void registerDriver(java.sql.Driver driver,150             DriverAction da)151         throws SQLException {152 153         /* Register the driver if it has not already been added to our list */154         if(driver != null) {155             registeredDrivers.addIfAbsent(new DriverInfo(driver, da));156         } else {157             // This is for compatibility with the original DriverManager158             throw new NullPointerException();159         }160 161         println("registerDriver: " + driver);162 163     }