之前
maven 的runtime作用范围
其中JDBC驱动通常使用Class.forName("com.mysql.jdbc.Driver");来引入所需要的驱动。在编译期间不用引入具体jdbc的具体实现类（不管mysql还是oracle等）。所以JDBC包的scope应该设置为runtime。
复制代码 mysql mysql-connector-java ${mysql.driver.version} runtime
JDBC4之后使用SPI技术 也不用写Class.forName了
Service Provider Interface
Java API提供的使用ServiceLoader来实现的控制反转工具类。用于从classpath中找到接口的实现类。
先定义一个接口
package ch.frankel.blog.serviceloader;public interface Foo {}复制代码 再定义两个实现类（实现类可能是其他工程中定义的）
package ch.frankel.blog.serviceloader;public class FooImpl1 implements Foo {}public class FooImpl2 implements Foo {}复制代码 然后在Resources/META-INF下的service中新建一个与接口名对应的文件ch.frankel.blog.serviceloader.Foo
ch.frankel.blog.serviceloader.FooImpl1ch.frankel.blog.serviceloader.FooImpl2复制代码
serviceLoader会从该文件中找到要加载的类。
public class JavaServiceLoaderTest { @Test public void java_service_loader_should_load_correct_implementations() { //Service.load会从ch.frankel.blog.serviceloader.Foo中找到要加载的类，然后加载。 ServiceLoader loader = ServiceLoader.load(Foo.class); List foos = new ArrayList<>(); loader.iterator().forEachRemaining(foos::add); assertThat(foos) .isNotNull() .isNotEmpty() .hasSize(2); }}复制代码 Spring 集成 ServiceLoader
首先需要配置对应的factoryBean
@Configurationpublic class ServiceConfiguration { @Bean public ServiceListFactoryBean serviceListFactoryBean() { ServiceListFactoryBean serviceListFactoryBean = new ServiceListFactoryBean(); serviceListFactoryBean.setServiceType(Foo.class); return serviceListFactoryBean; }}复制代码 然后通过ServiceListFactoryBean就可以找到接口对应的实现类。
@ContextConfiguration(classes = ServiceConfiguration.class)public class ServiceLoaderWithSpringTest extends AbstractTestNGSpringContextTests { @Autowired private ServiceListFactoryBean serviceListFactoryBean; @Test public void spring_service_loader_integration_should_load_correct_implementations() throws Exception { Object object = serviceListFactoryBean.getObject(); Assertions.assertThat(object) .isNotNull() .asList() .isNotEmpty() .hasSize(2); }}复制代码 Spring Facotories Loader
spring 自己也提供了类似的工具类。使用起来更方便。
首先在META-INF下建立一个spring.factories文件。
ch.frankel.blog.serviceloader.Foo=ch.frankel.blog.serviceloader.FooImpl1,ch.frankel.blog.serviceloader.FooImpl2复制代码
直接指定接口对应的实现类。
然后通过Spring提供的静态方法SpringFactoriesLoader就可以直接使用了。
public class SpringFactoriesTest { @Test public void spring_factories_should_load_correct_implementations() { List foos = SpringFactoriesLoader.loadFactories(Foo.class, null); assertThat(foos) .isNotNull() .isNotEmpty() .hasSize(2); }}复制代码 Service Provider Interface的应用
JDBC SPI
查看jdbc的代码
//java.sql.DriverManagerpublic class 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"); } private static void loadInitialDrivers() { String drivers; try { drivers = AccessController.doPrivileged(new PrivilegedAction () { public String run() { //从系统变量中获取jdbc.drivers //System.getProperty 可以在vm arguments中设置。java -Djdbc.drivers=xxxx 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()) { driversIterator.next(); } } catch(Throwable t) { // Do nothing } return null; } }); println("DriverManager.initialize: jdbc.drivers = " + drivers); if (drivers == null || drivers.equals("")) { return; } //多个drivers？ String[] driversList = drivers.split(":"); println("number of Drivers:" + driversList.length); for (String aDriver : driversList) { try { println("DriverManager.Initialize: loading " + aDriver); //还是使用Class.forName来加载驱动 Class.forName(aDriver, true, ClassLoader.getSystemClassLoader()); } catch (Exception ex) { println("DriverManager.Initialize: load failed: " + ex); } } }｝复制代码 在jdbc的实现包中META-INF下有java.sql.Driver文件。
com.mysql.jdbc.Drivercom.mysql.fabric.jdbc.FabricMySQLDriver复制代码
文件中有两个驱动名。
其实jdbc可以同时管理多个驱动，（）
Commons-Logging
//org.apache.commons.logging.LogFactory#getFactorypublic static LogFactory getFactory() throws LogConfigurationException { ... // Second, try to find a service by using the JDK1.3 class // discovery mechanism, which involves putting a file with the name // of an interface class in the META-INF/services directory, where the // contents of the file is a single line specifying a concrete class // that implements the desired interface. if (factory == null) { if (isDiagnosticsEnabled()) { logDiagnostic( "[LOOKUP] Looking for a resource file of name [" + SERVICE_ID + "] to define the LogFactory subclass to use..."); } try { InputStream is = getResourceAsStream(contextClassLoader, SERVICE_ID); if( is != null ) { // This code is needed by EBCDIC and other strange systems. // It's a fix for bugs reported in xerces BufferedReader rd; try { rd = new BufferedReader(new InputStreamReader(is, "UTF-8")); } catch (java.io.UnsupportedEncodingException e) { rd = new BufferedReader(new InputStreamReader(is)); } String factoryClassName = rd.readLine(); rd.close(); if (factoryClassName != null && ! "".equals(factoryClassName)) { if (isDiagnosticsEnabled()) { logDiagnostic( "[LOOKUP] Creating an instance of LogFactory class " + factoryClassName + " as specified by file '" + SERVICE_ID + "' which was present in the path of the context" + " classloader."); } factory = newFactory(factoryClassName, baseClassLoader, contextClassLoader ); } } else { // is == null if (isDiagnosticsEnabled()) { logDiagnostic( "[LOOKUP] No resource file with name '" + SERVICE_ID + "' found."); } } } catch( Exception ex ) { // note: if the specified LogFactory class wasn't compatible with LogFactory // for some reason, a ClassCastException will be caught here, and attempts will // continue to find a compatible class. if (isDiagnosticsEnabled()) { logDiagnostic( "[LOOKUP] A security exception occurred while trying to create an" + " instance of the custom factory class" + ": [" + ex.getMessage().trim() + "]. Trying alternative implementations..."); } ; // ignore } } ...}复制代码 其中也使用到SPI。从 META-INF/services/org.apache.commons.logging.LogFactory中找要加载的实现类（）。
slf4j
而slf4j不是通过SPI来找实现类的。slf4j 1.7是通过找一个固定包下的固定类StaticLoggerBinder类（而SPI是找固定文件下的内容）。这个类定义在各个实现包中。
貌似slf4j 1.8 开始使用SPI了，如下图。