ExtensionLoader

2019-08-08 00:00:00 ExtensionLoader

ExtensionLoader

《ExtensionLoader》
从上图中看到该类的构造方法被私有化,并且提供了一个静态方法来获取实例对象,
是的,该类使用了单例模式,懒汉模式

ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS...  key:被扩展类,value:扩展器

一、构造方法:

  • 将当前接口类型赋值给 this.type
  • 将AdaptiveExtensionFactory对象赋值给 objectFactory对象
// 构造方法
private ExtensionLoader(Class<?> type) {
    this.type = type;
    objectFactory = (type == ExtensionFactory.class ? null :                                        ExtensionLoader.getExtensionLoader(ExtensionFactory.class)
                          .getAdaptiveExtension());
}

二、获取对象的静态方法

public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
    if (type == null)
        throw new IllegalArgumentException("扩展类必须存在");
    if (!type.isInterface()) {
        throw new IllegalArgumentException("需要扩展的类型必须是接口");
    }
    if (!withExtensionAnnotation(type)) {
        throw new IllegalArgumentException("被扩展的类必须被@SPI修饰");
    }
    // 从缓存中获取
    ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    if (loader == null) {
        // 缓存中没有,创建一个
        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
        loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
    }
    return loader;
}

可以得出结论:

被扩展类必须是一个被@SPI注解修饰的接口
​ dubbo先从缓存中获取扩展类的实例,如果没有则通过单例的懒汉式创建ExtensionLoader对象, 然后放入缓存

三、现在来看看下面这句代码, 创建当前类型的类扩展器,并放入缓存

EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));

然后是构造方法,调用构造方法时,一个新的上下文,注意其中给objectFactory赋值的语句

private ExtensionLoader(Class<?> type) {
    this.type = type;
    objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}

注意:getAdaptiveExtension并没有为ExtensionFactory类型生成动态类, 在getAdaptiveExtensionClass方法中有控制(因为当前接口对应的扩展类文件中有AdaptiveExtensionFactory类,给cachedAdaptiveClass赋值了)

本类中的属性

private static final Logger logger = LoggerFactory.getLogger(ExtensionLoader.class);
// 扩展文件位置一
private static final String SERVICES_DIRECTORY = "META-INF/services/";
// 扩展文件位置二  这两个一般用来自定义扩展
private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";
// 扩展文件位置三
private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";
// 扩展名分割
private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*");
// 扩展接口 与对应的扩展器(存放所有的扩展器)
private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
// 具体扩展类(class)与其实例对象(new 之后的,存放所有的实例)
private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>();

// ==============================
//被扩展的具体接口
private final Class<?> type;
// 当前接口对应的文件内@Adaptive标识的对象(被IOC注入过)  用于获取扩展对象
private final ExtensionFactory objectFactory;
// key:扩展类型 value: 扩展名  用于通过接口字节对象(class对象)获取其扩展名
private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<Class<?>, String>();
// 除了@Adaptive和AOP扩展类之外所有的扩展类
private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String, Class<?>>>();

// 被@Activate注解修饰的扩展类  k:扩展名  v:扩展实例 eg:在获取Filter时有用到
private final Map<String, Activate> cachedActivates = new ConcurrentHashMap<String, Activate>();
// k: 扩展名, value: 包含扩展实例的 Holder对象
private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>();

// 持有动态扩展类的holder对象  调用getAdaptiveExtension方法才会有值
private final Holder<Object> cachedAdaptiveInstance = new Holder<Object>();

//被@Adaptive注解修饰的扩展类
private volatile Class<?> cachedAdaptiveClass = null;

// 被扩展类的@SPI注解中的value数组的第一个value[0]
private String cachedDefaultName;

//创建动态类是的异常对象
private volatile Throwable createAdaptiveInstanceError;
// 扩展类的AOP增强类
private Set<Class<?>> cachedWrapperClasses;

//加载扩展类配置文件出错的信息
private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<String, IllegalStateException>();

getAdaptiveExtension

获取动态代理类

public T getAdaptiveExtension() {
    //从缓存中获取动态扩展类
    Object instance = cachedAdaptiveInstance.get();
    if (instance == null) {
        if (createAdaptiveInstanceError == null) {
            synchronized (cachedAdaptiveInstance) {
                instance = cachedAdaptiveInstance.get();
                if (instance == null) {
                    try {
                        //1、创建  2、放入到
                        instance = createAdaptiveExtension();
                        cachedAdaptiveInstance.set(instance);
                    } catch (Throwable t) {
                        ...
                    }
                }
            }
        } else {
            ...
        }
    }

    return (T) instance;
}

createAdaptiveExtension

这个类用来给动态类注入属性(IOC) ,包含了@Adaptive修饰的类

private T createAdaptiveExtension() {
    try {
        // 给创建的动态类注入属性(IOC)
        return injectExtension((T) getAdaptiveExtensionClass().newInstance());
    } catch (Exception e) {
        ...
    }
}

getAdaptiveExtensionClass 获取动态类

private Class<?> getAdaptiveExtensionClass() {
    //加载动态类
    getExtensionClasses();
    //如果存在@Adaptive注解修饰的扩展类,直接返回(被它修饰的类是个固定的类,不需要生成动态类)
    if (cachedAdaptiveClass != null) {
        return cachedAdaptiveClass;
    }
    // 创建动态类
    return cachedAdaptiveClass = createAdaptiveExtensionClass();
}

getExtensionClasses

private Map<String, Class<?>> getExtensionClasses() {
    Map<String, Class<?>> classes = cachedClasses.get();
    if (classes == null) {
        synchronized (cachedClasses) {
            classes = cachedClasses.get();
            if (classes == null) {
                classes = loadExtensionClasses();
                cachedClasses.set(classes);
            }
        }
    }
    return classes;
}

loadExtensionClasses

用来加载指定扩展文件下,文件名与当前接口的限定名相同的文件,里面的所有扩展类

private Map<String, Class<?>> loadExtensionClasses() {
    //获取接口
    final SPI defaultAnnotation = type.getAnnotation(SPI.class);
    //这个if操作是将接口@SPI注解上的值赋值给cachedDefaultName
    if (defaultAnnotation != null) {
        String value = defaultAnnotation.value();
        if ((value = value.trim()).length() > 0) {
            String[] names = NAME_SEPARATOR.split(value);
            if (names.length > 1) {
                throw ...
            }
            if (names.length == 1) cachedDefaultName = names[0];
        }
    }
    // 这个map主要用来从三个固定目录下获取 与当前接口的全限定名相同的文件里面的所有扩展类
    // 比如 接口名为per.qiao.service.TestService 那么就会加载下面三个路径下的扩展类
    //META-INF/services/per.qiao.service.TestService
    //META-INF/dubbo/per.qiao.service.TestService
    //META-INF/dubbo/internal/per.qiao.service.TestService
    Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
    loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
    loadDirectory(extensionClasses, DUBBO_DIRECTORY);
    loadDirectory(extensionClasses, SERVICES_DIRECTORY);
    return extensionClasses;
}

loadDirectory

加载一个路径下的扩展类

private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir) {
    String fileName = dir + type.getName();
    try {
        Enumeration<java.net.URL> urls;
        ClassLoader classLoader = findClassLoader();
        if (classLoader != null) {
            urls = classLoader.getResources(fileName);
        } else {
            urls = ClassLoader.getSystemResources(fileName);
        }
        if (urls != null) {
            while (urls.hasMoreElements()) {
                java.net.URL resourceURL = urls.nextElement();
                //加载资源
                loadResource(extensionClasses, classLoader, resourceURL);
            }
        }
    } catch (Throwable t) {
        ...
    }
}

loadClass

加载一个文件内的扩展类, 一个接口在同一个文件里只能有一个扩展类被@Adaptive修饰

private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException {
    if (!type.isAssignableFrom(clazz)) {
        throw ...
    }
    //如果当前文件中的扩展类有@Adaptive修饰,赋值给cachedAdaptiveClass
    if (clazz.isAnnotationPresent(Adaptive.class)) {
        if (cachedAdaptiveClass == null) {
            cachedAdaptiveClass = clazz;
        } else if (!cachedAdaptiveClass.equals(clazz)) {
            //如果同一文件内有多余一个的类被Adaptive修饰,抛异常;
        }
    } else if (isWrapperClass(clazz)) {
        //如果当前扫描的类有个构造方法,并且该构造方法的参数与当前类型相同(type),保存到        cachedWrapperClasses,这里是实现AOP的一个关键点
        Set<Class<?>> wrappers = cachedWrapperClasses;
        if (wrappers == null) {
            cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
            wrappers = cachedWrapperClasses;
        }
        wrappers.add(clazz);
    } else {
        //扩展类要有无参构造
        clazz.getConstructor();
        //校验扩展名, 这里可以发现扩展名可以为空 即配置文件不使用key=value,学JDK直接使用value
        // 那么1.该扩展类上有@Extension注解 或者2.该类的名字的末尾包含当前类的名字 
        // 即: 如果当前类叫 Qiao  那么该类的名字为 AbcQiao 那么这个类的扩展名就是abc
        if (name == null || name.length() == 0) {
            name = findAnnotationName(clazz);
            if (name.length() == 0) {
                throw ...
            }
        }
        String[] names = NAME_SEPARATOR.split(name);
        if (names != null && names.length > 0) {
            Activate activate = clazz.getAnnotation(Activate.class);
            //将被@Activate修饰的类缓存到cachedActivates中
            if (activate != null) {
                cachedActivates.put(names[0], activate);
            }
            for (String n : names) {
               //将@Adaptive, AOP的Wrapper除外的其他本文件下的扩展类缓存到cachedNames
                if (!cachedNames.containsKey(clazz)) {
                    cachedNames.put(clazz, n);
                }
                //将@Adaptive, AOP的Wrapper除外的其他本文件下的扩展类添加到extensionClasses(map)
                Class<?> c = extensionClasses.get(n);
                if (c == null) {
                    extensionClasses.put(n, clazz);
                } else if (c != clazz) {
                    throw new ...
                }
            }
        }
    }
}

createAdaptiveExtensionClass

private Class<?> createAdaptiveExtensionClass() {
    //创建动态类代码
    String code = createAdaptiveExtensionClassCode();
    ClassLoader classLoader = findClassLoader();
    //获取编译器
    com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
    //编译动态代码
    return compiler.compile(code, classLoader);
}

相关文章