二、Dubbo的SPI機制分析1-SPI加載class

一、Dubbo的SPI例子

@SPI
public interface Robot {
    void sayHello();
}

public class OptimusPrime implements Robot{
    @Override
    public void sayHello() {
        System.out.println("Hello, I am Optimus Prime.");
    }
}

public class Bumblebee implements Robot{
    @Override
    public void sayHello() {
        System.out.println("Hello, I am Bumblebee.");
    }
}

public class DubboSPITest {

    @Test
    public void sayHelloDubbo() throws Exception {
        ExtensionLoader<Robot> extensionLoader = ExtensionLoader.getExtensionLoader(Robot.class);
        Robot optimusPrime = extensionLoader.getExtension("optimusPrime");
        optimusPrime.sayHello();
        Robot bumblebee = extensionLoader.getExtension("bumblebee");
        bumblebee.sayHello();
    }
}

輸出:
Hello, I am Optimus Prime.
Hello, I am Bumblebee.

二、Dubbo的SPI源碼分析

ExtensionLoader<Robot> extensionLoader = ExtensionLoader.getExtensionLoader(Robot.class);

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 interface!");
    }
    if (!withExtensionAnnotation(type)) {
        throw new IllegalArgumentException("Extension type(" + type +
                ") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
    }

    // 從緩存中獲取與拓展類對應的ExtensionLoader
    ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    if (loader == null) {
        // 若緩存未命中,則建立一個新的實例,先簡單看下new ExtensionLoader<T>(type)
        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    }
    return loader;
}

這裏的objectFactory建立能夠參考: Dubbo的SPI機制分析3-Dubbo的IOC依賴注入

private ExtensionLoader(Class<?> type) {
    this.type = type;
    // 這裏的type是Robot.class,因此會執行後面的代碼,加載並建立SpiExtensionFactory和SpringExtensionFactory,
    objectFactory = (type == ExtensionFactory.class ? null : 
                   ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}

Robot optimusPrime = extensionLoader.getExtension("optimusPrime");

public T getExtension(String name) {
    if ("true".equals(name)) {
        return getDefaultExtension();
    }
    // Holder也是用於持有對象的,用做緩存
    Holder<Object> holder = cachedInstances.get(name);
    if (holder == null) {
        cachedInstances.putIfAbsent(name, new Holder<Object>());
        holder = cachedInstances.get(name);
    }
    Object instance = holder.get();
    if (instance == null) {
        synchronized (holder) {
            instance = holder.get();
            if (instance == null) {
                // 建立拓展實例,下面分析createExtension(name)
                instance = createExtension(name);
                holder.set(instance);
            }
        }
    }
    return (T) instance;
}

這裏依賴注入能夠參考: Dubbo的SPI機制分析3-Dubbo的IOC依賴注入

private T createExtension(String name) {
    // 從配置文件中加載全部的拓展類,可獲得「配置項名稱」到「配置類」的映射關係表
    // 加載完後根據name獲取,獲得形如com.alibaba.dubbo.demo.provider.spi.OptimusPrime
    // 待會重點分析getExtensionClasses(),刪去了一些非關鍵代碼
    Class<?> clazz = getExtensionClasses().get(name);
    try {
        // 也是嘗試先從緩存獲取,獲取不到經過反射建立一個並放到緩存中
        T instance = (T) EXTENSION_INSTANCES.get(clazz);
        if (instance == null) {
            EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
            instance = (T) EXTENSION_INSTANCES.get(clazz);
        }
        // 依賴注入和cachedWrapperClasses,後面分析
        injectExtension(instance);
        Set<Class<?>> wrapperClasses = cachedWrapperClasses;
        if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
           for (Class<?> wrapperClass : wrapperClasses) {
              instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
           }
        }
        return instance;
    } 
}

private Map<String, Class<?>> getExtensionClasses() {
    // 從緩存中獲取映射關係表
    Map<String, Class<?>> classes = cachedClasses.get();
    // 雙重檢查
    if (classes == null) {
        synchronized (cachedClasses) {
            classes = cachedClasses.get();
            if (classes == null) {
                // 若緩存中沒有,去加載映射關係
                classes = loadExtensionClasses();
                cachedClasses.set(classes);
            }
        }
    }
    return classes;
}

private Map<String, Class<?>> loadExtensionClasses() {
    // 獲取SPI註解,這裏的type是在調用getExtensionLoader方法時傳入的
    final SPI defaultAnnotation = type.getAnnotation(SPI.class);
    if (defaultAnnotation != null) {
        String value = defaultAnnotation.value();
        if ((value = value.trim()).length() > 0) {
            String[] names = NAME_SEPARATOR.split(value);
            if (names.length > 1) {
                // 拋異常
            }
            // 獲取@SPI註解中的值,並緩存起來,能夠關注下,後面會用到
            if (names.length == 1) cachedDefaultName = names[0];
        }
    }

    Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
    // 加載指定文件夾下的配置文件,META-INF/services/、META-INF/dubbo/、META-INF/dubbo/internal/
    loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
    // 我是放在這個目錄下的
    loadDirectory(extensionClasses, DUBBO_DIRECTORY);
    loadDirectory(extensionClasses, SERVICES_DIRECTORY);
    return extensionClasses;
}

private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir) {
    // fileName是META-INF/dubbo/com.alibaba.dubbo.demo.provider.spi.Robot
    String fileName = dir + type.getName();
    try {
        Enumeration<java.net.URL> urls;
        ClassLoader classLoader = findClassLoader();
        // 根據文件名加載全部同名文件
        if (classLoader != null) {
            urls = classLoader.getResources(fileName);
        } else {
            urls = ClassLoader.getSystemResources(fileName);
        }
        if (urls != null) {
            while (urls.hasMoreElements()) {
                // 一個resourceURL就是一個文件
                java.net.URL resourceURL = urls.nextElement();
                loadResource(extensionClasses, classLoader, resourceURL);
            }
        }
    } 
}

private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, 
                                                                       java.net.URL resourceURL) {
    try {
        BufferedReader reader = 
                     new BufferedReader(new InputStreamReader(resourceURL.openStream(), "utf-8"));
        try {
            String line;
            // 按行讀取配置內容
            while ((line = reader.readLine()) != null) {
                final int ci = line.indexOf('#');
                // 定位#字符,#以後的爲註釋,跳過
                if (ci >= 0) line = line.substring(0, ci);
                line = line.trim();

if (line.length() > 0) {
                    try {
                        String name = null;
                        // 按等號切割
                        int i = line.indexOf('=');
                        if (i > 0) {
                            name = line.substring(0, i).trim();
                            line = line.substring(i + 1).trim();
                        }
                        if (line.length() > 0) {
                            // 真正的去加載類
                            loadClass(extensionClasses, resourceURL, 
                                                    Class.forName(line, true, classLoader), name);
                        }
                    }
                }
            }
        }
    } 
}

標註1，這裏的能夠參考: Dubbo的SPI機制分析2-Adaptive詳解
標註2，這裏的能夠參考: Dubbo的SPI機制分析4-Dubbo經過Wrapper實現AOP
標註3，這裏的cachedActivates能夠參考: Dubbo的SPI機制分析5-Activate詳解

private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, 
                                      Class<?> clazz, String name) throws NoSuchMethodException {
    // clazz必須是type類型的,不然拋異常
    if (!type.isAssignableFrom(clazz)) {
    }
    // 判斷clazz是否爲標註了@Adaptive註解,標註1
    if (clazz.isAnnotationPresent(Adaptive.class)) {
        if (cachedAdaptiveClass == null) {
            cachedAdaptiveClass = clazz;
        } else if (!cachedAdaptiveClass.equals(clazz)) {
            // 拋異常,不能有多個標註有@Adaptive註解的類
        }
    }
    // 判斷是不是Wrapper類型,標註2
    else if (isWrapperClass(clazz)) {
        Set<Class<?>> wrappers = cachedWrapperClasses;
        if (wrappers == null) {
            cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
            wrappers = cachedWrapperClasses;
        }
        wrappers.add(clazz);
    }

// 程序進入此分支,代表clazz是一個普通的拓展類,Robot就是一個普通的拓展類
    else {
        // 檢測clazz是否有默認的構造方法,若是沒有,則拋出異常
        clazz.getConstructor();
        if (name == null || name.length() == 0) {
            name = findAnnotationName(clazz);
            if (name.length() == 0) {
               // 拋異常
            }
        }

String[] names = NAME_SEPARATOR.split(name);
        if (names != null && names.length > 0) {
            // 用於@Activate根據條件激活,標註3
            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) {
                    // 存儲名稱到class的映射關係,這樣就解析好了一行
                    extensionClasses.put(n, clazz);
                } else if (c != clazz) {
                    // 拋異常
                }
            }
        }
    }
}