什么是Spring SPI？

Spring SPI是Spring框架提供的一种服务发现和扩展机制，它允许开发者通过配置文件的方式来扩展Spring的功能，而无需修改Spring的源代码。这种机制使得Spring框架具有极高的可扩展性，也是Spring生态系统如此丰富的重要原因之一。

1. 定义服务接口

package com.example.spi;

public interface MessageService {
    String getMessage();
}

2. 提供实现类

package com.example.spi.impl;

import com.example.spi.MessageService;

public class HelloMessageService implements MessageService {
    @Override
    public String getMessage() {
        return "Hello, World!";
    }
}

3. 创建配置类

package com.example.spi.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class BasicFeatureConfiguration {
    @Bean
    public FeatureService featureService() {
        return new BasicFeatureService();
    }
    
    public static class BasicFeatureService implements FeatureService {
        @Override
        public void execute() {
            System.out.println("Executing basic feature");
        }
    }
    
    public interface FeatureService {
        void execute();
    }
}

package com.example.spi.config;

import com.example.spi.config.BasicFeatureConfiguration.FeatureService;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AdvancedFeatureConfiguration {
    @Bean
    public FeatureService featureService() {
        return new AdvancedFeatureService();
    }
    
    public static class AdvancedFeatureService implements FeatureService {
        @Override
        public void execute() {
            System.out.println("Executing advanced feature");
        }
    }
}

4. 使用示例

package com.example;

import com.example.spi.annotation.EnableCustomFeature;
import com.example.spi.config.BasicFeatureConfiguration.FeatureService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
@EnableCustomFeature("advanced")
public class Application implements CommandLineRunner {
    
    @Autowired
    private FeatureService featureService;
    
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
    
    @Override
    public void run(String... args) throws Exception {
        featureService.execute();
    }
}

1. 创建条件化配置类

package com.example.spi.config;

import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class ConditionalConfiguration {
    
    @Bean
    @ConditionalOnClass(name = "com.example.ExternalClass")
    public ExternalService externalService() {
        return new ExternalService();
    }
    
    @Bean
    @ConditionalOnProperty(prefix = "feature", name = "enabled", havingValue = "true")
    public FeatureService featureService() {
        return new FeatureService();
    }
    
    @Bean
    @ConditionalOnProperty(prefix = "feature", name = "mode", havingValue = "advanced")
    public AdvancedFeatureService advancedFeatureService() {
        return new AdvancedFeatureService();
    }
    
    public static class ExternalService {
        public void doSomething() {
            System.out.println("External service is doing something");
        }
    }
    
    public static class FeatureService {
        public void execute() {
            System.out.println("Feature service is executing");
        }
    }
    
    public static class AdvancedFeatureService {
        public void execute() {
            System.out.println("Advanced feature service is executing");
        }
    }
}

2. 配置属性

在application.properties或application.yml中添加：

feature.enabled=true
feature.mode=advanced

3. 使用示例

package com.example;

import com.example.spi.config.ConditionalConfiguration.AdvancedFeatureService;
import com.example.spi.config.ConditionalConfiguration.FeatureService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application implements CommandLineRunner {
    
    @Autowired
    private FeatureService featureService;
    
    @Autowired
    private AdvancedFeatureService advancedFeatureService;
    
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
    
    @Override
    public void run(String... args) throws Exception {
        featureService.execute();
        advancedFeatureService.execute();
    }
}

SpringFactoriesLoader的工作流程

1. 扫描类路径下所有的META-INF/spring.factories文件
2. 解析这些文件，将其中的配置加载到内存中
3. 根据指定的接口或类，返回对应的实现类列表

1. 定义接口

package com.example.spi;

public interface DataProcessor {
    void process(String data);
}

2. 提供实现类

package com.example.spi.impl;

import com.example.spi.DataProcessor;
import org.springframework.core.annotation.Order;
import org.springframework.stereotype.Component;

@Component
@Order(1)
public class LoggingDataProcessor implements DataProcessor {
    @Override
    public void process(String data) {
        System.out.println("Logging data: " + data);
    }
}

3.3 Spring SPI的类型

Spring SPI支持多种类型的扩展点，主要包括：

@Configuration
@ConditionalOnClass(name = "com.example.SomeClass")
public class CustomAutoConfiguration {
    // 配置代码
}

public class CustomApplicationListener implements ApplicationListener<ApplicationEvent> {
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        // 处理事件
    }
}

public class CustomEnvironmentPostProcessor implements EnvironmentPostProcessor {
    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        // 处理环境
    }
}

public class CustomFailureAnalyzer extends AbstractFailureAnalyzer<Exception> {
    @Override
    protected FailureAnalysis analyze(Throwable rootFailure, Exception cause) {
        return new FailureAnalysis(
            "失败描述",
            "失败原因",
            "解决方案"
        );
    }
}

3.4 Spring SPI的配置方式

Spring SPI支持多种配置方式，主要包括：

1. spring.factories文件配置

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.example.CustomAutoConfiguration

org.springframework.context.ApplicationListener=\
com.example.CustomApplicationListener

1. 注解配置

@Configuration
@Import({CustomConfiguration.class})
public class AppConfig {
    // 配置代码
}

1. Java API配置

public class CustomImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        return new String[] {
            "com.example.CustomConfiguration"
        };
    }
}

这些配置方式可以根据具体需求灵活选择。通常情况下：

• 对于自动配置类，推荐使用spring.factories文件配置
• 对于应用内的扩展，推荐使用注解配置
• 对于需要动态决定是否启用的配置，推荐使用Java API配置