1. Basic concepts

SPI (Service Provider Interface), yes Java A set of functions provided to be implemented or extended by a third party API, It can be used to enable framework extensions and replace components

Java SPI It's actually “ Interface based programming ＋ The strategy pattern ＋ The configuration file ” Dynamic loading mechanism of composite implementation

2. Use scenarios

• Database driven
• journal SLF4J
• Spring Use SPI
• Dubbo Use SPI, But for the original SPI Do the packaging , Allow users to extend the implementation Filter Interface

3. Use requirement

To use SPI, The following conventions need to be followed

• SPI There must be a parameterless constructor in the implementation class of
• When the service provider provides the implementation of the interface , stay jar Bag META-INF Create a directory to “ The interface is fully qualified ” Name the file , The content is the fully qualified name of the implementation class

• JDK Middle path time META-INF/services/
• Spring The configuration in is META-INF/spring.factories

• Interface implementation class jar The package is in the main program classpath in
• adopt Loader Class will META-INF The configuration file under the directory is loaded , Resolve the fully qualified class name in the file , Load the class into JVM

• JDK It is used in ServiceLoader load META-INF/services/ The files under the
• Spring Is used in SpringFactoriesLoader load META-INF/spring.factories

4. JDK SPI Realization

Analysis of database driven source code

MySQL The driver is om.mysql.cj.jdbc It's a bag Driver Implement the interface JDK Of java.sql.Driver, And in META-INF/services The file name created in the directory is java.sql.Driver The file of , Fill in the full path information of the implementation class in the file com.mysql.cj.jdbc.Driver

//  Realization  JDK  Of  java.sql.Driver  class 
public class Driver extends NonRegisteringDriver implements java.sql.Driver {
    
    // SPI  There must be a parameterless constructor in the implementation class of 
    public Driver() throws SQLException {
    
    }

    static {
    
        try {
    
            DriverManager.registerDriver(new Driver());
        } catch (SQLException var1) {
    
            throw new RuntimeException("Can't register driver!");
        }
    }
}

see java.util.ServiceLoader Source code

public final class ServiceLoader<S> implements Iterable<S> {
    
    private static final String PREFIX = "META-INF/services/";

    //  Represents the loaded class or interface 
    private final Class<S> service;

    //  Used to locate , Load and instantiate providers Class loader for 
    private final ClassLoader loader;

    //  establish ServiceLoader The access control context used when 
    private final AccessControlContext acc;

    //  cache providers, In the order of instantiation 
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

    //  Lazy search iterator 
    private LazyIterator lookupIterator;
    ......
}

ServiceLoader Can span jar Package for META-INF/services The configuration file under the directory

try {
    
    String fullName = PREFIX + service.getName();
    if (loader == null)
        configs = ClassLoader.getSystemResources(fullName);
    else
        configs = loader.getResources(fullName);
} catch (IOException x) {
    
    fail(service, "Error locating configuration files", x);
}

After obtaining the full class name of the implementation class , By reflection Class.forName() Load class objects

5. Spring SPI Realization

Spring Of SPI Implementation is made by org.springframework.core.io.support Under bag SpringFactoriesLoader Realized , The configuration file path is META-INF/spring.factories And JDK There are differences

public final class SpringFactoriesLoader {
    
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

	public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    
        ...
        //  scanning  jar  It's a bag  META-INF/spring.factories  file 
        Enumeration<URL> urls = (classLoader != null ?
					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        ...
    }
}

stay Spring Of spring-data-redis Chinese vs JDK and Spring All done. SPI Realization