JDBC事务2

Class.forName("com.mysql.jdbc.Driver");

public class Driver extends NonRegisteringDriver implements java.sql.Driver { 
    static {
        try {
            java.sql.DriverManager.registerDriver(new Driver());   //先执行DriverManager的初始化，再registerDriver方法，静态代码只走一次。
        }  
    }

DriverManager类初始化：
static {
        loadInitialDrivers();   //静态代码只走一次
    }

private static void loadInitialDrivers() {
        String drivers;
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
                public String run() {
                    return System.getProperty("jdbc.drivers");   //null
                }
            });
        }  
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() { 
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);    //loadedDrivers  = java.sql.Driver接口
                Iterator<Driver> driversIterator = loadedDrivers.iterator(); 
                try{
                    while(driversIterator.hasNext()) {   //加载META-INF/services/java.sql.Driver，找到mysql-connector-java-5.1.45.jar!/META-INF/services/java.sql.Driver返回[com.mysql.jdbc.Driver, com.mysql.fabric.jdbc.FabricMySQLDriver]
                        driversIterator.next();
//class com.mysql.jdbc.Driver反射出对象时候不会去执行java.sql.DriverManager.registerDriver(new Driver());因为上面已经走了一次，com.mysql.jdbc.Driver此时不在DriverManager的registeredDrivers里面，
com.mysql.jdbc.Driver是在DriverManager静态代码执行完之后通过registerDriver方法加到registeredDrivers里面去的。
//com.mysql.fabric.jdbc.FabricMySQLDriver反射时候去执行java.sql.DriverManager.registerDriver(new Driver());把自己加到DriverManager的registeredDrivers里面。但是DriverManager的静态代码不会执行。因为执行过一次。
                    }
                } 
                return null;
            }
        });  
    }

public class FabricMySQLDriver extends NonRegisteringDriver implements Driver { 
    static {
        try {
            DriverManager.registerDriver(new FabricMySQLDriver());    //静态代码只走一次。
        }  
    }
}