kafkaeagle使用教程，Java SPI机制分析

1.准备驱动包

mysql

mysql-connector-java

5.1.47

org.postgresql

postgresql

42.2.2

com.microsoft.sqlserver

mssql-jdbc

7.0.0.jre8

分别准备了mysql，postgresql和sqlserver，可以打开jar，发现每个jar包的META-INF/services/都存在一个java.sql.Driver文件，文件里面存在一个或多个类名，比如mysql：

com.mysql.jdbc.Driver

com.mysql.fabric.jdbc.FabricMySQLDriver

提供的每个驱动类占据一行，解析的时候会按行读取，具体使用哪个会根据url来决定；

2.简单实例

String url = “jdbc:mysql://localhost:3306/db3”;

String username = “root”;

String password = “root”;

String sql = “update travelrecord set name=‘bbb’ where id=1”;

Connection con = DriverManager.getConnection(url, username, password);

类路径下存在多个驱动包，具体在使用DriverManager.getConnection应该使用哪个驱动类会解析url来识别，不同的数据库有不同的url前缀；

3.驱动类加载分析

具体META-INF/services/下的驱动类是什么时候加载的，DriverManager有一个静态代码块：

static {

loadInitialDrivers();

println(“JDBC DriverManager initialized”);

}

private static void loadInitialDrivers() {

String drivers;

try {

drivers = AccessController.doPrivileged(new PrivilegedAction() {

public String run() {

return System.getProperty(“jdbc.drivers”);

}

});

} catch (Exception ex) {

drivers = null;

}

// If the driver is packaged as a Service Provider, load it.

// Get all the drivers through the classloader

// exposed as a java.sql.Driver.class service.

// ServiceLoader.load() replaces the sun.misc.Providers()

AccessController.doPrivileged(new PrivilegedAction() {

public Void run() {

ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);

Iterator driversIterator = loadedDrivers.iterator();

/* Load these drivers, so that they can be instantiated.

• It may be the case that the driver class may not