Dubbo擴展點SPI簡單實例

　　dubbo采用微內核+插件機制方便框架使用者自行擴展，這個插件機制的實現就是JDK的SPI（參見Java的SPI簡單實例）。dubbo擴展了JDK的SPI，加入了注解和Spring容器的支持，給配置文件中的全限定實現類添加了自定義名稱映射，支持按不同的映射參數加載不同的實現類等。按dubbo官方說法，進化后的SPI叫擴展點SPI，ServiceLoader變成了ExtensionLoader。接下來看一個例子，使用擴展點SPI來自定義一個LoadBalance。

　　先看dubbo源碼中的LoadBalance接口：

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 * this work for additional information regarding copyright ownership.
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package org.apache.dubbo.rpc.cluster;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.Adaptive;
import org.apache.dubbo.common.extension.SPI;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.cluster.loadbalance.RandomLoadBalance;

import java.util.List;

/**
 * LoadBalance. (SPI, Singleton, ThreadSafe)
 * <p>
 * <a href="http://en.wikipedia.org/wiki/Load_balancing_(computing)">Load-Balancing</a>
 *
 * @see org.apache.dubbo.rpc.cluster.Cluster#join(Directory)
 */ @SPI(RandomLoadBalance.NAME)
public interface LoadBalance {

    /**
     * select one invoker in list.
     *
     * @param invokers   invokers.
     * @param url        refer url
     * @param invocation invocation.
     * @return selected invoker.
     */ @Adaptive("loadbalance")
    <T> Invoker<T> select(List<Invoker<T>> invokers, URL url, Invocation invocation) throws RpcException;

}

　　我們看到該接口使用了@SPI和@Adaptive注解，而且它們都有參數。@SPI用來標記擴展接口，@Adaptive使ExtesionLoader知道應該加載哪個擴展實現類。同ServiceLoader一樣，ExtesionLoader也是主要的實現類：

    /**
     * Find the extension with the given name. If the specified name is not found, then {@link IllegalStateException}
     * will be thrown.
     */
    @SuppressWarnings("unchecked")
    public T getExtension(String name) {
        if (StringUtils.isEmpty(name)) {
            throw new IllegalArgumentException("Extension name == null");
        }
        if ("true".equals(name)) {
            return getDefaultExtension();
        }
        Holder<Object> holder = getOrCreateHolder(name);
        Object instance = holder.get();
        if (instance == null) {
            synchronized (holder) {
                instance = holder.get();
                if (instance == null) {
                    instance = createExtension(name);
                    holder.set(instance);
                }
            }
        }
        return (T) instance;
    }

    @SuppressWarnings("unchecked")
    private T createExtension(String name) {
        Class<?> clazz = getExtensionClasses().get(name);
        if (clazz == null) {
            throw findException(name);
        }
        try {
            T instance = (T) EXTENSION_INSTANCES.get(clazz);
            if (instance == null) {
                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));
                }
            }
            return instance;
        } catch (Throwable t) {
            throw new IllegalStateException("Extension instance (name: " + name + ", class: " +
                    type + ") couldn't be instantiated: " + t.getMessage(), t);
        }
    }

 

　　下面自定義一個LoadBalance實現，擴展點實現起來非常簡單。項目原代碼參見一個spring boot集成dubbo的小例子，只需修改消費者，服務提供者不用動。下面重點把修改點列出來：

　　1、消費者項目新增配置文件：resources/META-INF/dubbo/org.apache.dubbo.rpc.cluster.LoadBalance

demo=com.example.dubbo.democonsumer.loadbalance.DemoLoadBalance

 

 

　　2、消費者項目新增自定義LoadBalance實現：

package com.example.dubbo.democonsumer.loadbalance;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.cluster.LoadBalance;

import java.util.List;

public class DemoLoadBalance implements LoadBalance {
    @Override
    public <T> Invoker<T> select(List<Invoker<T>> list, URL url, Invocation invocation) throws RpcException {
        System.out.printf("DemoLoadBalance coming, url %s", url.toFullString());
        return list.get(0);
    }
}

　　3、消費者項目的Controller類新增LoadBalance注解：

package com.example.dubbo.democonsumer.controller;

import com.example.dubbo.demo.domain.DemoBean;
import com.example.dubbo.demo.service.DemoService;
import org.apache.dubbo.config.annotation.Reference;
import org.springframework.web.bind.annotation.*;

/**
 * 服務消費者
 */
@RestController
public class ConsumerController {

    // 引入API
    @Reference(check = false, loadbalance = "demo")
    DemoService demoService;

    @ResponseBody
    @RequestMapping("/hello")
    public String sayHelo(@RequestParam(value = "msg") String msg) {
        return demoService.sayHelo(msg);
    }

    @ResponseBody
    @RequestMapping(value = "/login", method = RequestMethod.POST)
    public String login(DemoBean demoBean) {
        return demoService.login(demoBean);
    }
}

 

　　完事了。啟動消費者和兩個服務提供者（IDEA啟動多實例參見IDEA同一項目啟動多個實例），注意兩個provider配置的rpc端口不能相同：

 

 

　　瀏覽器輸入消費者hello接口跑一次：

 

　　

　　從消費者日志可以看到進入我們自定義的LoadBalance：