5Dubbo的SPI机制分析4Dubbo通过Wrapper实现AOP

1、Dubbo的通过Wrapper实现AOP例子

阅读此篇文章之前,建议先学习该系列前面几篇文章,会有一个更好的理解.
参考: Dubbo的SPI机制分析2-Adaptive详解、Dubbo的SPI机制分析3-Dubbo的IOC依赖注入

// 在原有接口、实现类的基础上再加一个Wrapper实现类
public class AdaptiveExtWrapper implements AdaptiveExt {

    private AdaptiveExt adaptiveExt;

    // 实现一个Wrapper类的关键就是提供一个带类似参数的构造函数,后面代码会分析
    public AdaptiveExtWrapper(AdaptiveExt adaptiveExt) {
        this.adaptiveExt = adaptiveExt;
    }

    @Override
    public String echo(String msg, URL url) {
        // do something,实现了AOP
        System.out.println("before");

        adaptiveExt.echo(msg, url);

        System.out.println("after");
        // do something
        return "wrapper";
    }
}

public class DubboAdaptiveExt implements AdaptiveExt {

    private AdaptiveExt adaptiveExt;

    // 此处DubboAdaptiveExt依赖注入一个AdaptiveExt 类型的实例,此处测试用例注入的是ThriftAdaptiveExt
    public void setAdaptiveExt(AdaptiveExt adaptiveExt) {
        this.adaptiveExt = adaptiveExt;
    }

    @Override
    public String echo(String msg, URL url) {
        System.out.println(this.adaptiveExt.echo(msg, url));
        return "dubbo";
    }
}

@Test
public void test1(){
    ExtensionLoader<AdaptiveExt> loader = ExtensionLoader.getExtensionLoader(AdaptiveExt.class);
    AdaptiveExt adaptiveExtension = loader.getExtension("dubbo");
    URL url = URL.valueOf("test://localhost/test");
    adaptiveExtension.echo("d", url);
}

同时记得dubbo的配置文件中要新增一行,否则dubbo框架无法扫描到Wrapper

dubbo=com.alibaba.dubbo.demo.provider.adaptive.impl.DubboAdaptiveExt
cloud=com.alibaba.dubbo.demo.provider.adaptive.impl.SpringCloudAdaptiveExt
thrift=com.alibaba.dubbo.demo.provider.adaptive.impl.ThriftAdaptiveExt
com.alibaba.dubbo.demo.provider.adaptive.impl.AdaptiveExtWrapper

运行代码,输出:
before
thrift
after

2、Dubbo的通过Wrapper实现AOP源码分析

可以发现,上述代码先后输出before、after,中间输出thrift,这其实可以说明adaptiveExtension = loader.getExtension(“dubbo”)返回的是AdaptiveExtWrapper类型的实例,有点像静态代理,下面分析源代码.

// 核心代码,删去一些不重要代码
private T createExtension(String name) {
    // 这里也是通过dubbo的SPI机制去扫描dubbo的默认目录下的文件,去加载实现了AdaptiveExt的实现类,这里会
    // 加载4个,包括AdaptiveExtWrapper,它就是下面的wrapperClasses中仅有的元素,下面先分析这个加载过程
    Class<?> clazz = getExtensionClasses().get(name);
   
    try {
        
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) {
            EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
            instance = (T) EXTENSION_INSTANCES.get(clazz);
        }
        // 依赖注入,
        injectExtension(instance);
        
        // cachedWrapperClasses,
        Set<Class<?>> wrapperClasses = cachedWrapperClasses;
        if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
            for (Class<?> wrapperClass : wrapperClasses) {
                instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
            }
        }
        return instance;
    } 
}

private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException {
    // clazz必须是type类型的,否则抛异常
    if (!type.isAssignableFrom(clazz)) {
        throw new IllegalStateException("Error when load extension class(interface: " +
                type + ", class line: " + clazz.getName() + "), class "
                + clazz.getName() + "is not subtype of interface.");
    }
    // 判断clazz是否为标注了@Adaptive注解,后面分析
    if (clazz.isAnnotationPresent(Adaptive.class)) {
        if (cachedAdaptiveClass == null) {
            cachedAdaptiveClass = clazz;
        } else if (!cachedAdaptiveClass.equals(clazz)) {
            throw new IllegalStateException("More than 1 adaptive class found: "
                    + cachedAdaptiveClass.getClass().getName()
                    + ", " + clazz.getClass().getName());
        }
    }
    // 判断是否是Wrapper类型,后面分析
    else if (isWrapperClass(clazz)) {
        Set<Class<?>> wrappers = cachedWrapperClasses;
        if (wrappers == null) {
            cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
            wrappers = cachedWrapperClasses;
        }
        wrappers.add(clazz);
    }
    // 程序进入此分支,表明clazz是一个普通的拓展类,Robot就是一个普通的拓展类
    else {
        // 检测clazz是否有默认的构造方法,如果没有,则抛出异常
        clazz.getConstructor();
        if (name == null || name.length() == 0) {
            name = findAnnotationName(clazz);
            if (name.length() == 0) {
                throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
            }
        }
        String[] names = NAME_SEPARATOR.split(name);
        if (names != null && names.length > 0) {
            Activate activate = clazz.getAnnotation(Activate.class);
            if (activate != null) {
                cachedActivates.put(names[0], activate);
            }
            for (String n : names) {
                if (!cachedNames.containsKey(clazz)) {
                    cachedNames.put(clazz, n);
                }
                Class<?> c = extensionClasses.get(n);
                if (c == null) {
                    // 存储名称到class的映射关系,这样就解析好了一行
                    extensionClasses.put(n, clazz);
                } else if (c != clazz) {
                    throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
                }
            }
        }
    }
}