Dubbo SPI机制ExtensionLoader浅析-蒲公英云

Dubbo SPI机制ExtensionLoader浅析

1.getExtensionLoader

返回SPI接口对应的ExtensionLoader

// 根据类型查找对应的扩展加载器，如果没有，则创建一个并且加入EXTENSION_LOADERS
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) { 
    if (type == null) { 
        throw new IllegalArgumentException("Extension type == null");
    }
    if (!type.isInterface()) { 
        throw new IllegalArgumentException("Extension type (" + type + ") is not an interface!");
    }
    if (!withExtensionAnnotation(type)) { 
        throw new IllegalArgumentException("Extension type (" + type +
                                           ") is not an extension, because it is NOT annotated with @" + SPI.class.getSimpleName() + "!");
    }
    // private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<>(64);
    ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    if (loader == null) { 
        // private final Class<?> type;
        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    }
    return loader;
}

2.getExtension

根据扩展点名称获取对应的实现类实例

public T getExtension(String name) { 
    if (StringUtils.isEmpty(name)) { 
        throw new IllegalArgumentException("Extension name == null");
    }
    if ("true".equals(name)) { 
        return getDefaultExtension();
    }
    // private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>();
    // 根据扩展名查找对应的Holder,如果没有则创建一个并放入cachedInstances
    // 如果有的话直接返回缓存的实例
    final Holder<Object> holder = getOrCreateHolder(name);
    Object instance = holder.get();
    if (instance == null) { 
        synchronized (holder) { 
            instance = holder.get();
            if (instance == null) { 
                // 创建扩展点实现
                instance = createExtension(name);
                holder.set(instance);
            }
        }
    }
    return (T) instance;
}
private T createExtension(String name) { 
    // 如果cachedClasses为空，使用三种LoadingStrategy从三个目录中获取name对应的CLass
    Class<?> clazz = getExtensionClasses().get(name);
    if (clazz == null) { 
        throw findException(name);
    }
    try { 
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) { 
            // 如果缓存中没有，则通过newInstance方法创建实例并放入EXTENSION_INSTANCES中
            EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
            instance = (T) EXTENSION_INSTANCES.get(clazz);
        }
        injectExtension(instance);
        Set<Class<?>> wrapperClasses = cachedWrapperClasses;
        if (CollectionUtils.isNotEmpty(wrapperClasses)) { 
            for (Class<?> wrapperClass : wrapperClasses) { 
                instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
            }
        }
        initExtension(instance);
        return instance;
    } catch (Throwable t) { 
        throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
                                        type + ") couldn't be instantiated: " + t.getMessage(), t);
    }
}