Java SPI机制学习

news2024/10/6 7:54:00

最近阅读源码时看到了SPI加载类的方式这里学习一下

SPI Service Provider Interface

服务提供接口

API和SPI区别

API是接口和实现类均有服务提供方负责服务调用方更多是一种主动调用只是调用SDK来实现某个功能；接口的归属权在于服务提供方

SPI是接口在调用方侧定义，但是具体实现类由服务提供方现接口归属权在于服务调用方

SPI实践

定义一个类和三个实现

public interface ISpiService {
    
    public void diBiz();
}

public class ASpiService implements ISpiService {
    
    @Override
    public void diBiz() {
        System.out.println("ASpiService diBiz");
    }
}

public class BSpiService implements ISpiService {
    
    @Override
    public void diBiz() {
        System.out.println("BSpiService diBiz");
    }
}

public class CSpiService implements ISpiService {
    
    @Override
    public void diBiz() {
        System.out.println("CSpiService diBiz");
    }
}

定义实现类的目录文件

/META-INF/services 目录下

文件名为接口的全限定名

文件类型为实现类的全限定名

com.java.spi.ASpiService
com.java.spi.BSpiService
com.java.spi.CSpiService

使用ServiceLoad来加载实现类

public static void main(String[] args) {
        ServiceLoader<ISpiService> spiServices = ServiceLoader.load(ISpiService.class);
        Iterator<ISpiService> iterator = spiServices.iterator();
        while (iterator.hasNext()) {
            iterator.next().diBiz();
        }
    }

运行结果

ASpiService diBiz
BSpiService diBiz
CSpiService diBiz

可知可以调用所有类实现类的方法

ServiceLoad大致原理

    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

类加载器使用线程上下文加载器

最终load方法初始化lazyIterator对象

 public void reload() {
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

hasNext方法

        private boolean hasNextService() {
            if (nextName != null) {
                return true;
            }
            if (configs == null) {
                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);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

private static final String PREFIX = "META-INF/services/";

从约定目录下找实现类

next方法

        private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                c = Class.forName(cn, false, loader);
            } catch (ClassNotFoundException x) {
                fail(service,
                     "Provider " + cn + " not found");
            }
            if (!service.isAssignableFrom(c)) {
                fail(service,
                     "Provider " + cn  + " not a subtype");
            }
            try {
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider " + cn + " could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen
        }

使用反射根据类名来初始化对象