Java spi机制

private static Connection getConn() {
  String driver = "com.mysql.jdbc.Driver";
  String url = "jdbc:mysql://localhost:33066/mysql";
  String username = "root";
  String password = "root";
  Connection conn = null;
  try {
    Class.forName(driver); //classLoader,加载对应驱动
    conn = DriverManager.getConnection(url, username, password);
  } catch (Exception e) {
    e.printStackTrace();
  }
  return conn;
}

package com.mysql.jdbc;

import java.sql.DriverManager;
import java.sql.SQLException;

public class Driver extends NonRegisteringDriver implements java.sql.Driver {
  public Driver() throws SQLException {}
  static {
    try {
      DriverManager.registerDriver(new Driver());
    } catch (SQLException var1) {
      throw new RuntimeException("Can't register driver!");
    }
  }
}

conn = DriverManager.getConnection(url, username, password);

public static Connection getConnection(String url,String user, String password) throws SQLException {
  java.util.Properties info = new java.util.Properties();
  // 获取当前的类加载器
  ClassLoader callerCL = DriverManager.getCallerClassLoader();

  if (user != null) {
    info.put("user", user);
  }
  if (password != null) {
    info.put("password", password);
  }
  return (getConnection(url, info, callerCL));
}

//  Worker method called by the public getConnection() methods.
private static Connection getConnection(String url, java.util.Properties info, ClassLoader callerCL) throws SQLException {
  java.util.Vector drivers = null;
  /*
  * 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.
  */
  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 + "\")");
  if (!initialized) {
    //重点在这个初始化方法
    initialize();
  }

  ...省略

}

// Class initialization.
static void initialize() {
  if (initialized) {
      return;
  }
  initialized = true;
  //继续扒这个方法
  loadInitialDrivers();
  println("JDBC DriverManager initialized");
}

private static void loadInitialDrivers() {
  String drivers;

  try {
    drivers = (String) java.security.AccessController.doPrivileged(new sun.security.action.GetPropertyAction("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.

  DriverService ds = new DriverService();

  // Have all the privileges to get all the 
  // implementation of java.sql.Driver
  //调用实现的run方法进行接口的所有实现自注册
  java.security.AccessController.doPrivileged(ds);

  println("DriverManager.initialize: jdbc.drivers = " + drivers);
  if (drivers == null) {
    return;
  }
  ...省略
}

public interface PrivilegedAction<T> {
    /**
     * Performs the computation.  This method will be called by
     * <code>AccessController.doPrivileged</code> after enabling privileges.
     *
     * @return a class-dependent value that may represent the results of the
     *	       computation. Each class that implements
     *         <code>PrivilegedAction</code>
     *	       should document what (if anything) this value represents.
     * @see AccessController#doPrivileged(PrivilegedAction)
     * @see AccessController#doPrivileged(PrivilegedAction,
     *                                     AccessControlContext)
     */
    T run();
}

public Object run() {
  // uncomment the followin line before mustang integration 	
  // Service s = Service.lookup(java.sql.Driver.class);
  // ps = s.iterator();
  ps = Service.providers(java.sql.Driver.class);

	/* Load these drivers, so that they can be instantiated. 
	 * It may be the case that the driver class may not be there
         * i.e. there may be a packaged driver with the service class
         * as implementation of java.sql.Driver but the actual class
         * may be missing. In that case a sun.misc.ServiceConfigurationError
         * will be thrown at runtime by the VM trying to locate 
	 * and load the service.
         * 
	 * Adding a try catch block to catch those runtime errors
         * if driver not available in classpath but it's 
	 * packaged as service and that service is there in classpath.
	 */
		
  try {
    while (ps.hasNext()) {
      ps.next();
    } // end while
  } catch(Throwable t) {
    // Do nothing
  }
  return null;
}

public boolean hasNext() throws ServiceConfigurationError {
  if (this.nextName != null) {
    return true;
  } else {
    if (this.configs == null) {
      try {
        //根据拼接文件路径 最终结果是 META-INF/services/java.sql.Driver
        String var1 = "META-INF/services/" + this.service.getName();
        if (this.loader == null) {
            this.configs = ClassLoader.getSystemResources(var1);
        } else {
            //类加载器加载对应的文件
            this.configs = this.loader.getResources(var1);
        }
      } catch (IOException var2) {
        Service.fail(this.service, ": " + var2);
      }
    }

    while(this.pending == null || !this.pending.hasNext()) {
      if (!this.configs.hasMoreElements()) {
        return false;
      }

      this.pending = Service.parse(this.service, (URL)this.configs.nextElement(), this.returned);
    }

    this.nextName = (String)this.pending.next();
    return true;
  }
}

public Object next() throws ServiceConfigurationError {
  if (!this.hasNext()) {
    throw new NoSuchElementException();
  } else {
    String var1 = this.nextName;
    this.nextName = null;
    Class var2 = null;

    try {
      //将实现类的初始化放到这里 系统自动进行初始化,实现驱动的自动注册.
      var2 = Class.forName(var1, false, this.loader);
    } catch (ClassNotFoundException var5) {
      Service.fail(this.service, "Provider " + var1 + " not found");
    }

    if (!this.service.isAssignableFrom(var2)) {
      Service.fail(this.service, "Provider " + var1 + " not a subtype");
    }

    try {
      return this.service.cast(var2.newInstance());
    } catch (Throwable var4) {
      Service.fail(this.service, "Provider " + var1 + " could not be instantiated: " + var4, var4);
      return null;
    }
  }
}

public static void main(String[] args) {
  ServiceLoader<DemoService> services = ServiceLoader.load(DemoService.class);
  Iterator<DemoService> it = services.iterator();
  while (it.hasNext()) {
    DemoService service = it.next();
    service.sayHello();
  }
}

DemoService service = new Demo1ServiceImpl();
service.sayHello();