Java Service Provider Interface

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。

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>${mysql.driver.version}</version>
    <scope>runtime</scope>
</dependency>

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 {
}

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 {
}

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.FooImpl1
ch.frankel.blog.serviceloader.FooImpl2

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<Foo> loader = ServiceLoader.load(Foo.class);
        List<Foo> foos = new ArrayList<>();
        loader.iterator().forEachRemaining(foos::add);
        assertThat(foos)
                .isNotNull()
                .isNotEmpty()
                .hasSize(2);
    }
}

Spring 集成 ServiceLoader

首先需要配置对应的factoryBean

@Configuration
public class ServiceConfiguration {
    @Bean
    public ServiceListFactoryBean serviceListFactoryBean() {
        ServiceListFactoryBean serviceListFactoryBean = new ServiceListFactoryBean();
        serviceListFactoryBean.setServiceType(Foo.class);
        return serviceListFactoryBean;
    }
}

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);
    }
}

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

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<Foo> foos = SpringFactoriesLoader.loadFactories(Foo.class, null);
        assertThat(foos)
                .isNotNull()
                .isNotEmpty()
                .hasSize(2);
    }
}

Service Provider Interface的应用

JDBC SPI

查看jdbc的代码

//java.sql.DriverManager
public 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<String>() {
                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<Void>() {
            public Void run() {

                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                Iterator<Driver> 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);
            }
        }
    }

｝

在jdbc的实现包中META-INF下有java.sql.Driver文件。

com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver

文件中有两个驱动名。

其实jdbc可以同时管理多个驱动，（jdbc详解：2、DriverManager管理多个数据库驱动）

Commons-Logging

//org.apache.commons.logging.LogFactory#getFactory
public 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
            }
        }
    
    ...
}

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（Apache Commons Logging 是如何决定使用哪个日志实现类的）。

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。