深入探讨Java SPI机制及其应用场景

package com.hj.test.file.oss;

public interface IFileUpload {
    void upload(String fileName);
}

package com.hj.test.file.oss;
import com.hj.test.file.IFileUpload;
public class FileUploadOss implements IFileUpload {
    @Override
    public void upload(String fileName) {
        System.out.println("上传到阿里云OSS..." + fileName);
    }
}

package com.hj.test.file.s3;
import com.hj.test.file.IFileUpload;
public class FileUploadS3 implements IFileUpload {
    @Override
    public void upload(String fileName) {
        System.out.println("上传到亚马逊s3..." + fileName);
    }
}

<dependency>
    <groupId>com.hj</groupId>
    <artifactId>spi-file-upload</artifactId>
    <version>0.0.1-SNAPSHOT</version>
</dependency>
<dependency>
    <groupId>com.hj</groupId>
    <artifactId>spi-file-upload-oss</artifactId>
    <version>0.0.1-SNAPSHOT</version>
</dependency>
<dependency>
    <groupId>com.hj</groupId>
    <artifactId>spi-file-upload-s3</artifactId>
    <version>0.0.1-SNAPSHOT</version>
</dependency>

package com.hj.test.file.test;
import com.hj.test.file.IFileUpload;
import java.util.Iterator;
import java.util.ServiceLoader;
public class FileTest{
    public static void main(String[] args) {
        ServiceLoader<IFileUpload> loader = ServiceLoader.load(IFileUpload.class);
        for(Iterator<IFileUpload> it = loader.iterator(); it.hasNext();){
            IFileUpload file = it.next();
            file.upload("测试文件上传");
        }
    }
}

public final class ServiceLoader<S>
    implements Iterable<S>
{
    private static final String PREFIX = "META-INF/services/";
    // The class or interface representing the service being loaded
    private final Class<S> service;
    // The class loader used to locate, load, and instantiate providers
    private final ClassLoader loader;
    // The access control context taken when the ServiceLoader is created
    private final AccessControlContext acc;
    // Cached providers, in instantiation order
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
    // The current lazy-lookup iterator
    private LazyIterator lookupIterator;

public static <S> ServiceLoader<S> load(Class<S> service,
                                        ClassLoader loader)
{
    return new ServiceLoader<>(service, loader);
}

public void reload() {
    providers.clear();
    lookupIterator = new LazyIterator(service, loader);
}
private ServiceLoader(Class<S> svc, ClassLoader cl) {
    service = Objects.requireNonNull(svc, "Service interface cannot be null");
    loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
    acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
    reload();
}

public Iterator<S> iterator() {
    return new Iterator<S>() {
        Iterator<Map.Entry<String,S>> knownProviders
            = providers.entrySet().iterator();
        public boolean hasNext() {
            if (knownProviders.hasNext())
                return true;
            return lookupIterator.hasNext();
        }
        public S next() {
            if (knownProviders.hasNext())
                return knownProviders.next().getValue();
            return lookupIterator.next();
        }
        public void remove() {
            throw new UnsupportedOperationException();
        }
    };
}

private class LazyIterator
        implements Iterator<S>
    {
    Class<S> service;
    ClassLoader loader;
    Enumeration<URL> configs = null;
    Iterator<String> pending = null;
    String nextName = null;
    private LazyIterator(Class<S> service, ClassLoader loader) {
        this.service = service;
        this.loader = loader;
    }
    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 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
    }
    public boolean hasNext() {
        if (acc == null) {
            return hasNextService();
        } else {
            PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
                public Boolean run() { return hasNextService(); }
            };
            return AccessController.doPrivileged(action, acc);
        }
    }
    public S next() {
        if (acc == null) {
            return nextService();
        } else {
            PrivilegedAction<S> action = new PrivilegedAction<S>() {
                public S run() { return nextService(); }
            };
            return AccessController.doPrivileged(action, acc);
        }
    }
    public void remove() {
        throw new UnsupportedOperationException();
    }
}