dubbo源码分析-spi机制

用法

1. 定义接口

@SPI
public interface SayHello {

    void sayHello();
}

2. 实现接口

public class Hilary implements SayHello{
    @Override
    public void sayHello() {
        System.out.println("Trump is pig!");
    }
}

public class MarcoRubio implements SayHello{
    @Override
    public void sayHello() {
        System.out.println("the ZTE is doing harm to our country!");
    }
}

3. 在类路径下resources下建立META-INF/dubbo，在以上目录建立以接口全名为文件名，key=value格式定义实现类 文件内容以下

hilary = com.dubbo.learn.java.Hilary
marcoRubio = com.dubbo.learn.java.MarcoRubio

4. 使用

public class SayHelloTest {

    private ServiceLoader<SayHello> serviceLoader = ServiceLoader.load(SayHello.class);

    @Test
    public void sayHello() {
        ExtensionLoader<SayHello> extensionLoader =
                ExtensionLoader.getExtensionLoader(SayHello.class);
        SayHello optimusPrime = extensionLoader.getExtension("hilary");
        optimusPrime.sayHello();
        SayHello bumblebee = extensionLoader.getExtension("marcoRubio");
        bumblebee.sayHello();
    }
}

源码分析

1.getExtensionLoader获取ExtensionLoader对象，并设置ExtensionLoader的ExtensionFactory成员变量。若是获取ExtensionFactory接口的ExtensionLoader， ExtensionFactory成员变量 = null,其余接口的ExtensionLoader对应ExtensionFactory成员变量 = ExtensionFactory的实现类 2.

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
        if (type == null) {
            throw new IllegalArgumentException("Extension type == null");
        } else if (!type.isInterface()) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
        } else if (!withExtensionAnnotation(type)) {
            throw new IllegalArgumentException("Extension type(" + type + ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
        } else {
            ExtensionLoader<T> loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
            if (loader == null) {
                EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));
                loader = (ExtensionLoader)EXTENSION_LOADERS.get(type);
            }

            return loader;
        }
    }
	
ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap() 存放SPI的接口对应的ExtensionLoader实例
Holder<Object> cachedAdaptiveInstance = new Holder()存放SPI的接口对应的adaptiveInstance实例
  private T createAdaptiveExtension() {
        try {
            return this.injectExtension(this.getAdaptiveExtensionClass().newInstance());
        } catch (Exception var2) {
            throw new IllegalStateException("Can not create adaptive extension " + this.type + ", cause: " + var2.getMessage(), var2);
        }
    }
	//加载配置文件，缓存cachedClasses数据
	//获取cachedAdaptiveClass
	 private Class<?> getAdaptiveExtensionClass() {
        this.getExtensionClasses();
        return this.cachedAdaptiveClass != null ? this.cachedAdaptiveClass : (this.cachedAdaptiveClass = this.createAdaptiveExtensionClass());
    }
	
	private Map<String, Class<?>> getExtensionClasses() {
        Map<String, Class<?>> classes = (Map)this.cachedClasses.get();
        if (classes == null) {
            Holder var2 = this.cachedClasses;
            synchronized(this.cachedClasses) {
                classes = (Map)this.cachedClasses.get();
                if (classes == null) {
                    classes = this.loadExtensionClasses();
                    this.cachedClasses.set(classes);
                }
            }
        }

        return classes;
    }

this.getExtensionClasses()中调用方法loadExtensionClasses，
loadExtensionClasses返回key为name，value为实现类Class对象的Map
getExtensionClasses最终把全部解析到的Map存入缓存成员变量cachedClasses,并返回该Map
（Holder<Map<String, Class<?>>> cachedClasses = new Holder()）

private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
        String fileName = dir + this.type.getName();

        try {
            ClassLoader classLoader = findClassLoader();
            Enumeration urls;
            if (classLoader != null) {
                urls = classLoader.getResources(fileName);
            } else {
                urls = ClassLoader.getSystemResources(fileName);
            }

            if (urls != null) {
                label269:
                while(urls.hasMoreElements()) {
                    java.net.URL url = (java.net.URL)urls.nextElement();

                    try {
                        BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));

                        try {
                            String line = null;

                            while(true) {
                                do {
								//读取有效行 若是有空格行 会继续向下读取有效行 当没有新的内容时 直接跳转到finally执行reader.close();
                                    if ((line = reader.readLine()) == null) {
                                        continue label269;
                                    }

                                    int ci = line.indexOf(35);
                                    if (ci >= 0) {
                                        line = line.substring(0, ci);
                                    }

                                    line = line.trim();
                                } while(line.length() <= 0);

                                try {
                                    String name = null;
                                    int i = line.indexOf(61);
                                    if (i > 0) {
                                        name = line.substring(0, i).trim();
                                        line = line.substring(i + 1).trim();
                                    }

                                    if (line.length() > 0) {
                                        Class<?> clazz = Class.forName(line, true, classLoader);
                                        if (!this.type.isAssignableFrom(clazz)) {
                                            throw new IllegalStateException("Error when load extension class(interface: " + this.type + ", class line: " + clazz.getName() + "), class " + clazz.getName() + "is not subtype of interface.");
                                        }

                                        if (clazz.isAnnotationPresent(Adaptive.class)) {
                                            if (this.cachedAdaptiveClass == null) {
                                                this.cachedAdaptiveClass = clazz;
                                            } else if (!this.cachedAdaptiveClass.equals(clazz)) {
                                                throw new IllegalStateException("More than 1 adaptive class found: " + this.cachedAdaptiveClass.getClass().getName() + ", " + clazz.getClass().getName());
                                            }
                                        } else {
                                            try {
                                                clazz.getConstructor(this.type);
                                                Set<Class<?>> wrappers = this.cachedWrapperClasses;
                                                if (wrappers == null) {
                                                    this.cachedWrapperClasses = new ConcurrentHashSet();
                                                    wrappers = this.cachedWrapperClasses;
                                                }

                                                wrappers.add(clazz);
                                            } catch (NoSuchMethodException var27) {
                                                clazz.getConstructor();
                                                if (name == null || name.length() == 0) {
                                                    name = this.findAnnotationName(clazz);
                                                    if (name == null || name.length() == 0) {
                                                        if (clazz.getSimpleName().length() <= this.type.getSimpleName().length() || !clazz.getSimpleName().endsWith(this.type.getSimpleName())) {
                                                            throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
                                                        }

                                                        name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - this.type.getSimpleName().length()).toLowerCase();
                                                    }
                                                }

                                                String[] names = NAME_SEPARATOR.split(name);
                                                if (names != null && names.length > 0) {
                                                    Activate activate = (Activate)clazz.getAnnotation(Activate.class);
                                                    if (activate != null) {
                                                        this.cachedActivates.put(names[0], activate);
                                                    }

                                                    String[] arr$ = names;
                                                    int len$ = names.length;

                                                    for(int i$ = 0; i$ < len$; ++i$) {
                                                        String n = arr$[i$];
                                                        if (!this.cachedNames.containsKey(clazz)) {
                                                            this.cachedNames.put(clazz, n);
                                                        }

                                                        Class<?> c = (Class)extensionClasses.get(n);
                                                        if (c == null) {
                                                            extensionClasses.put(n, clazz);
                                                        } else if (c != clazz) {
                                                            throw new IllegalStateException("Duplicate extension " + this.type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
                                                        }
                                                    }
                                                }
                                            }
                                        }
                                    }
                                } catch (Throwable var28) {
                                    IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + this.type + ", class line: " + line + ") in " + url + ", cause: " + var28.getMessage(), var28);
                                    this.exceptions.put(line, e);
                                }
                            }
                        } finally {
                            reader.close();
                        }
                    } catch (Throwable var30) {
                        logger.error("Exception when load extension class(interface: " + this.type + ", class file: " + url + ") in " + url, var30);
                    }
                }
            }
        } catch (Throwable var31) {
            logger.error("Exception when load extension class(interface: " + this.type + ", description file: " + fileName + ").", var31);
        }

    }

一、若是实现类存在@Adaptive注解
cachedAdaptiveClass = 实现类的Class对象
二、若是实现类不存在@Adaptive注解
经过clazz.getConstructor(this.type)这一行判断实现类是否存在SPI的接口做为参数的构造函数；
若是有,实现类的Class对象放入cachedWrapperClasses（ConcurrentHashSet）
若是没有,经过clazz.getConstructor()这一行判断实现类是否存无参构造函数；
2.1 获取name的值
若是name=null,经过findAnnotationName再次获取name的值
private String findAnnotationName(Class<?> clazz) {
    Extension extension = (Extension)clazz.getAnnotation(Extension.class);
    if (extension == null) {
        String name = clazz.getSimpleName();
        if (name.endsWith(this.type.getSimpleName())) {
            name = name.substring(0, name.length() - this.type.getSimpleName().length());
        }

        return name.toLowerCase();
    } else {
        return extension.value();
    }
}
即若是实现类上存在Extension注解的话，name = 注解的value;
若是不存在,若是实现类的类名以SPI接口名称做为后缀，name= 去掉后缀部分，不然name = 实现类的类名（首字母小写）
通过以上步骤，若是name仍是拿不到（null或者""）,name= 实现类名.substring(0,实现类名长度-SPI接口长度)
2.1 name拆分逗号，返回数组
若是size>0
若是实现类上有Activate注解 把name[0],Activate存入如下缓存
Map<String, Activate> cachedActivates = new ConcurrentHashMap();
遍历name数组
ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap()
若是cachedNames不存在实现类的Class的key，cachedNames.put(实现类的Class，当前遍历的name)
extensionClasses中获取当前name的值
若是不存在，存入（当前name的值，实现类的Class对象）
若是已经存在的话，抛异常（name重复了）

下面解析getAdaptiveExtensionClass方法中createAdaptiveExtensionClass
生成动态代码
1.遍历SPI接口全部方法
2.若是没有Adaptive注解，抛异常；
不然构造class文件
具体步骤：
2.1 StringBuilder codeBuidler = new StringBuilder();
2.1 拼接package ，import, class
2.3 构造class源代码
遍历SPI接口全部方法，若是没有Adaptive注解，抛异常

injectExtension方法解析

injectExtension做用是检查SPI实现类中的set方法 实现set注入

private T injectExtension(T instance) {
        try {
            if (this.objectFactory != null) {
                Method[] arr$ = instance.getClass().getMethods();
                int len$ = arr$.length;

                for(int i$ = 0; i$ < len$; ++i$) {
                    Method method = arr$[i$];
                    if (method.getName().startsWith("set") && method.getParameterTypes().length == 1 && Modifier.isPublic(method.getModifiers())) {
                        Class pt = method.getParameterTypes()[0];

                        try {
                            String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
                            Object object = this.objectFactory.getExtension(pt, property);
                            if (object != null) {
                                method.invoke(instance, object);
                            }
                        } catch (Exception var9) {
                            logger.error("fail to inject via method " + method.getName() + " of interface " + this.type.getName() + ": " + var9.getMessage(), var9);
                        }
                    }
                }
            }
        } catch (Exception var10) {
            logger.error(var10.getMessage(), var10);
        }

        return instance;
    }

injectExtension遍历实现类的全部set方法, object = this.objectFactory.getExtension(pt, property);这句话调用AdaptiveExtensionFactory的getExtension方法，分别使用SpiExtensionFactory获取Adaptive类型的扩展 以及使用SpringExtensionFactory获取Spring方式实现的扩展

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.alibaba.dubbo.common.extension.factory;

import com.alibaba.dubbo.common.extension.Adaptive;
import com.alibaba.dubbo.common.extension.ExtensionFactory;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

@Adaptive
public class AdaptiveExtensionFactory implements ExtensionFactory {
    private final List<ExtensionFactory> factories;

    public AdaptiveExtensionFactory() {
        ExtensionLoader<ExtensionFactory> loader = ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
        List<ExtensionFactory> list = new ArrayList();
        Iterator i$ = loader.getSupportedExtensions().iterator();

        while(i$.hasNext()) {
            String name = (String)i$.next();
            list.add(loader.getExtension(name));
        }

        this.factories = Collections.unmodifiableList(list);
    }

    public <T> T getExtension(Class<T> type, String name) {
        Iterator i$ = this.factories.iterator();

        Object extension;
        do {
            if (!i$.hasNext()) {
                return null;
            }

            ExtensionFactory factory = (ExtensionFactory)i$.next();
            extension = factory.getExtension(type, name);
        } while(extension == null);

        return extension;
    }
}

也就是说，若是用spi实现依赖的话，只有加上@Adaptive注解，才能得到依赖属性的扩展实现，实现set注入； 显然injectExtension方法，内部使用递归方式实现set注入

注意点

1. 对于有@Adaptive注解的实现类getExtensionClasses不会把name和实现类Class存入缓存成员变量cachedClasses,只设置cacedAdaptiveClass属性。

@Adaptive加到MarcoRubio上

@Test
    public void fac() {
        ExtensionLoader<ExtensionFactory> extensionLoader =
                ExtensionLoader.getExtensionLoader(ExtensionFactory.class);
        ExtensionFactory adaptiveExtension = extensionLoader.getAdaptiveExtension();
        //获取adaptiva extension ,若是实现类都没有@Adaptive 则调用失败
        SayHello hilary = adaptiveExtension.getExtension(SayHello.class, "hilary");
        hilary.sayHello();
    }

    @Test
    public void sayHello() {
        ExtensionLoader<SayHello> extensionLoader =
                ExtensionLoader.getExtensionLoader(SayHello.class);
        SayHello optimusPrime = extensionLoader.getExtension("hilary");
        optimusPrime.sayHello();
		SayHello adaptiveExtension = extensionLoader.getAdaptiveExtension();
        adaptiveExtension.sayHello();
        SayHello bumblebee = extensionLoader.getExtension("marcoRubio");
        bumblebee.sayHello();
    }

以上测试类中fac（）调用失败，由于hilary上并无@Adaptive注解，全部不是自适配的实现类。 sayHello（）方法中optimusPrime.sayHello()成功 adaptiveExtension.sayHello()成功 bumblebee.sayHello()失败

2. AdaptiveExtensionFactory实现类中factorys属性 初始化后包含SpringExtensionFactory和SpiExtensionFactory实例，由于只有 AdaptiveExtensionFactory实现类上包含@Adaptive注解

3. ExtensionFactory的实现类有SpiExtensionFactory，SpringExtensionFactory，AdaptiveExtensionFactory，而AdaptiveExtensionFactory 的实例属性factories包含了SpiExtensionFactory和SpringExtensionFactory,全部调用AdaptiveExtensionFactory得到扩展会默认调用其余两种方式得到扩展实现。

4. createAdaptiveExtensionClassCode方法用于cachedAdaptiveClass为null时，建立AdaptiveExtensionClass扩展。 此时SPI接口方法必须包含@Adaptive注解，参考LoadBalance接口实现。