java中的SPI介绍及使用 java中的SPI介绍及使用

SPI介绍
mysql驱动的实现
Driver原理解析

SPI介绍 SPI（Service Provider Interface）即服务提供接口，JDK内置的一种服务提供发现机制,是Java提供的一套用来被第三方实现或者扩展的API，它可以用来启用框架扩展和替换组件。
Java SPI的具体约定如下：
当服务的提供者实现了服务接口后，在classpath下的META-INF/services/目录里同时创建一个以服务接口命名的文件。该文件里就是实现该服务接口的具体实现类。
而当外部程序装配这个模块的时候，就能通过该jar包META-INF/services/里的配置文件找到具体的实现类名，并装载实例化，完成模块的注入。
mysql驱动的实现我们以jdk提供的Driver为例，java提供了SPI类Driver供数据库驱动厂商如mysql、oracle等实现

public interface Driver {Connection connect(String url, java.util.Properties info) throws SQLException; boolean acceptsURL(String url) throws SQLException; DriverPropertyInfo[] getPropertyInfo(String url, java.util.Properties info) throws SQLException; int getMajorVersion(); int getMinorVersion(); boolean jdbcCompliant(); public Logger getParentLogger() throws SQLFeatureNotSupportedException; }

我们在pom.xml中引入mysql的驱动包

mysql mysql-connector-java 8.0.19 >runtime

查看其中实现了Driver的实现类

public class Driver extends NonRegisteringDriver implements java.sql.Driver { // // Register ourselves with the DriverManager // static { try { java.sql.DriverManager.registerDriver(new Driver()); } catch (SQLException E) { throw new RuntimeException("Can't register driver!"); } }/** * Construct a new driver and register it with DriverManager * * @throws SQLException *if a database error occurs. */ public Driver() throws SQLException { // Required for Class.forName().newInstance() } }

public class NonRegisteringDriver implements java.sql.Driver { @Override public java.sql.Connection connect(String url, Properties info) throws SQLException {try { if (!ConnectionUrl.acceptsUrl(url)) { /* * According to JDBC spec: * The driver should return "null" if it realizes it is the wrong kind of driver to connect to the given URL. This will be common, as when the * JDBC driver manager is asked to connect to a given URL it passes the URL to each loaded driver in turn. */ return null; }ConnectionUrl conStr = ConnectionUrl.getConnectionUrlInstance(url, info); switch (conStr.getType()) { case SINGLE_CONNECTION: return com.mysql.cj.jdbc.ConnectionImpl.getInstance(conStr.getMainHost()); case FAILOVER_CONNECTION: case FAILOVER_DNS_SRV_CONNECTION: return FailoverConnectionProxy.createProxyInstance(conStr); case LOADBALANCE_CONNECTION: case LOADBALANCE_DNS_SRV_CONNECTION: return LoadBalancedConnectionProxy.createProxyInstance(conStr); case REPLICATION_CONNECTION: case REPLICATION_DNS_SRV_CONNECTION: return ReplicationConnectionProxy.createProxyInstance(conStr); default: return null; }} catch (UnsupportedConnectionStringException e) { // when Connector/J can't handle this connection string the Driver must return null return null; } catch (CJException ex) { throw ExceptionFactory.createException(UnableToConnectException.class, Messages.getString("NonRegisteringDriver.17", new Object[] { ex.toString() }), ex); } } ...... }

Driver通过NonRegisteringDriver间接实现了java的Driver接口，并且classpath下指定了对应的实现类路径

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我们在当前项目测试下实现java中Driver的SPI实现类有哪些

public static void main( String[] args ) { ServiceLoader serviceLoader=ServiceLoader.load(Driver.class); for(Driver dbd:serviceLoader){ System.out.println(dbd.toString()); } }输出 D:\java\jdk1.8\jdk1.8.0_161\bin\java.exe "-javaagent:D:\java\idea\IntelliJ IDEA com.mysql.cj.jdbc.Driver@34c45dca

Driver原理解析 1、我们先了解一下我们最初连接数据库的操作

Class.forName(“com.mysql.jdbc.Driver”)注册mysql驱动
使用connection = DriverManager.getConnection(“jdbc:mysql://localhost:3306/test”, “root”, “123456”)连接数据库

其中DriverManager是jdk提供的，Driver被实例化时会注册当前驱动到DriverManager中，然后我们就可以在DriverManager连接数据库，这就是jdk1.6之前我们需要连接数据库必须要做的实例化操作

public class Driver extends NonRegisteringDriver implements java.sql.Driver { public Driver() throws SQLException { }static { try { //new一个Driver对象，并将它注册到DriverManage中 DriverManager.registerDriver(new Driver()); } catch (SQLException var1) { throw new RuntimeException("Can't register driver!"); } } }

2、jdk1.6之后提供了SPI接口类ServiceLoader，同时DriverManager也增加了对此的支持

public class DriverManager {// List of registered JDBC drivers private final static CopyOnWriteArrayList registeredDrivers = new CopyOnWriteArrayList<>(); private static volatile int loginTimeout = 0; private static volatile java.io.PrintWriter logWriter = null; private static volatile java.io.PrintStream logStream = null; // Used in println() to synchronize logWriter private final staticObject logSync = new Object(); /* Prevent the DriverManager class from being instantiated. */ private DriverManager(){}/** * Load the initial JDBC drivers by checking the System property * jdbc.properties and then use the {@code ServiceLoader} mechanism */ static { loadInitialDrivers(); println("JDBC DriverManager initialized"); } ...... }

核心方法loadInitialDrivers，在静态块执行

private static void loadInitialDrivers() { String drivers; try { // 核心代码 1、从环境变量读取数据库驱动类 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 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 java.util.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(driversIterator.hasNext()) { // 核心代码 2、SPI方式读取META-INF/services下面java.sql.Driver文件中的驱动 driversIterator.next(); } } catch(Throwable t) { // Do nothing } 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); // 核心代码 3、实例化环境变量读取的驱动类 Class.forName(aDriver, true, ClassLoader.getSystemClassLoader()); } catch (Exception ex) { println("DriverManager.Initialize: load failed: " + ex); } } }

由上代码可得，jdk1.6之后Driver驱动类注册到DriverManager的方式有以下几种
1、配置环境变量jdbc.drivers指定驱动类路径
2、SPI方式在classpath下的META-INF/services下面创建文件java.sql.Driver中指定驱动类的路径（目前最新版使用，即只需要引入驱动的jar包就可以了）
3、手动调用Class.forName(xxx)
个人理解：
SPI是服务提供者如java提供给第三方用来实现某些特定功能的约定，如Driver接口，提供给不同的数据库厂商实现的默认约定
【java中的SPI介绍及使用】优点：可插拔式，解耦，应用程序可以根据实际业务情况启用框架扩展或替换框架组件
缺点：需要提前去了解SPI的机制