Java 里面的SPI

今天在看web.xml为什么逐渐淡出我们的视野，重温了下JDK的SPI机制（Service Provider Interface）。

SPI 是什么？

面向接口编程+配置文件！
对，我总结的就是一句话。看看我们日常使用不同数据库驱动，不同日志框架，是不是都在使用SPI机制呢？下面我用数据库驱动来举例。

JDBC驱动使用SPI

一、概述

我们都知道数据库驱动 java.sql.Driver，针对不同数据库厂商有不同的实现，翻翻我们mysql驱动jar包，依次找到META-INF/services/java.sql.Driver，看看里面有啥？

# mysql 驱动实现类
com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver

我们看看Driver类的static方法

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

二、加载驱动，获取连接

注册Driver到DriverManager -> DriverManager.getConnection(…) 获取连接

新（SPI机制）、老（反射）2种方式

1、新方式，SPI机制，省略Class.forName(“xxx.xxx.xxx”)方式（jar包下所需文件如概述描述）

驱动从 5.x 开始jar包下必须要有 META-INF/services/java.sql.Driver。

// 会调用DriverManager的static方法，里面会寻找驱动jar包下面META-INF/services/java.sql.Driver 文件
// 初始化Driver，当然也就调用了Driver的static方法
Connection connection = DriverManager.getConnection(URL, username, pwd);

2、老方式，通过Class.forName(“xxx.xxx.xxx”)反射

驱动4.x一般这样来写

// 反射，会调用Driver里static方法，注册Driver到DriverManager
Class.forName("com.mysql.jdbc.Driver");
// 从DriverManager 里面获取连接
Connection connection = DriverManager.getConnection(URL, username, pwd);

2种方式讲解

• 新的

static {
       loadInitialDrivers();
       println("JDBC DriverManager initialized");
   }

ServiceLoader实现spi机制

private static void loadInitialDrivers() {
     String drivers;
     try {
         drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
             public String run() {
                 return System.getProperty("jdbc.drivers");
             }
         });
     } catch (Exception ex) {
         drivers = null;
     }
     AccessController.doPrivileged(new PrivilegedAction<Void>() {
         public Void run() {
             //查找mysql驱动jar下META-INF/services/java.sql.Driver
             //初始化它，调用 static方法，注册驱动(同下分析)
             ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
             Iterator<Driver> driversIterator = loadedDrivers.iterator();
             try{
                 while(driversIterator.hasNext()) {
                     driversIterator.next();
                 }
             } catch(Throwable t) {
             }
             return null;
         }
     });
     println("DriverManager.initialize: jdbc.drivers = " + drivers);
     if (drivers == null || drivers.equals("")) {
         return;
     }
     String[] driversList = drivers.split(":");
     println("number of Drivers:" + driversList.length);
     for (String aDriver : driversList) {
         try {
             println("DriverManager.Initialize: loading " + aDriver);
             Class.forName(aDriver, true,
                     ClassLoader.getSystemClassLoader());
         } catch (Exception ex) {
             println("DriverManager.Initialize: load failed: " + ex);
         }
     }
 }

后续流程同下面老的讲解

• 老的

//Driver里static方法
  static {
        try {
        //向DriverManager 注册Driver
            java.sql.DriverManager.registerDriver(new Driver());
        } catch (SQLException E) {
            throw new RuntimeException("Can't register driver!");
        }
    }
 //  registerDriver 方法   
  public static synchronized void registerDriver(java.sql.Driver driver)
        throws SQLException {

        registerDriver(driver, null);
    }
    
 // 最终方法，注册Driver到DriverManager
 public static synchronized void registerDriver(java.sql.Driver driver,DriverAction da)
        throws SQLException {
        if(driver != null) {
            registeredDrivers.addIfAbsent(new DriverInfo(driver, da));
        } else {
            throw new NullPointerException();
        }
        println("registerDriver: " + driver);
    }  
   // 获取连接    
    @CallerSensitive
    public static Connection getConnection(String url,
        String user, String password) throws SQLException {
        java.util.Properties info = new java.util.Properties();
        if (user != null) {
            info.put("user", user);
        }
        if (password != null) {
            info.put("password", password);
        }
        //最终调用
        return (getConnection(url, info, Reflection.getCallerClass()));
    }    
    
 private static Connection getConnection(
        String url, java.util.Properties info, Class<?> caller) throws SQLException {
        ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
        synchronized(DriverManager.class) {
            if (callerCL == null) {
                callerCL = Thread.currentThread().getContextClassLoader();
            }
        }
        if(url == null) {
            throw new SQLException("The url cannot be null", "08001");
        }
        println("DriverManager.getConnection(\"" + url + "\")");
        SQLException reason = null;
        // 从registeredDrivers获取连接信息
        for(DriverInfo aDriver : registeredDrivers) {
            if(isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    println("    trying " + aDriver.driver.getClass().getName());
                   // 获取连接
                    Connection con = aDriver.driver.connect(url, info);
                    if (con != null) {
                        println("getConnection returning " + aDriver.driver.getClass().getName());
                        return (con);
                    }
                } catch (SQLException ex) {
                    if (reason == null) {
                        reason = ex;
                    }
                }
            } else {
                println("    skipping: " + aDriver.getClass().getName());
            }
        }
        if (reason != null)    {
            println("getConnection failed: " + reason);
            throw reason;
        }
        println("getConnection: no suitable driver found for "+ url);
        throw new SQLException("No suitable driver found for "+ url, "08001");
    }

我们可以看到SPI机制核心在于：ServiceLoader、META-INF/services/需要实现接口全定限名 文件。ServiceLoader位于java.util包下，其load方法就是实现SPI的关键。有兴趣可以自己实现个SPI，试验下！