比较jdk spi和dubbo spi(1)

ServiceLoader

首先讲下常见用法：

//spi接口
public interface IHello {
    void sayHello();
}

//接口实现
public class HelloImpl1 implements IHello {
    @Override
    public void sayHello() {
        System.out.println("hello world1");
    }
}

public class HelloImpl2 implements IHello {
    @Override
    public void sayHello() {
        System.out.println("hello world2");
    }
}

public static void main(String[] args){
    ServiceLoader s = ServiceLoader.load(IHello.class);
    Iterator iHelloIterator = s.iterator();
    while (iHelloIterator.hasNext()) {
        IHello iHello = iHelloIterator.next();
        iHello.sayHello();
    }
}

源码分析

构造函数

private ServiceLoader(Class 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 //是否启动了SecurityManager，这里会决定后面是否用特权doPrivileged方法
    reload();
}

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

ServiceLoader的构造函数很简单，唯一可能有疑问的点是这个acc。本文主要介绍spi，所以简单提一下这个acc，这个是java的安全管理机制，当启动了SecurityManager的时候，会去检查权限（默认不启动，拥有所有权限）。

接下来是他的iterator

 public Iterator iterator() {
    return new Iterator() {
        
        Iterator> 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();
            }

        };
    }

knownProviders用来遍历providers的迭代器，而providers缓存了之前生成的实例，防止多次遍历是重复生成对象。

// Cached providers, in instantiation order
private LinkedHashMap providers = new LinkedHashMap<>();

如果knownProviders.hasNext()为true，表示缓存中存在，直接取出即可。缓存不存在时用了lookupIterator.hasNext()

// The current lazy-lookup iterator
    private LazyIterator lookupIterator;

这个LazyIterator是个inner class

private class LazyIterator implements Iterator{

        Class service;
        ClassLoader loader;
        Enumeration configs = null;
        Iterator pending = null;
        String nextName = null;

        private LazyIterator(Class service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        private boolean hasNextService() {
           ......
        }

        private S nextService() {
           ......
        }

        public boolean hasNext() {
            if (acc == null) {
                return hasNextService();
            } else {
                PrivilegedAction action = new PrivilegedAction() {
                    public Boolean run() { return hasNextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

        public S next() {
            if (acc == null) {
                return nextService();
            } else {
                PrivilegedAction action = new PrivilegedAction() {
                    public S run() { return nextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }

        public void remove() {
            throw new UnsupportedOperationException();
        }

    }

hasNext和next方法很简单，根据是否启动了SecurityManager来决定是否要用AccessController.doPrivileged来调用hasNextService和nextService。重点来看一下hasNextService和nextService方法。

hasNextService

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;
} 

这一步解析META-INF/services/路径下，以spi名称命名的文件中的实现类的完整类名。

nextService

    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
        }

这一步根据之前得到的类名，加载类并反射生成对象，然后将生成好的对象放到providers的缓存中。这样完成了spi接口实现类的实例化，下一篇文章我们来看看dubbo中的spi是怎么做的。