### 扩展点加载(ExtensionLoader)
每一种类型的扩展点都有一个ExtensionLoader实例
变量说明
public class ExtensionLoader<T> { // dubbo服务扫描目录 private static final String SERVICES_DIRECTORY = "META-INF/services/"; // dubbo扩展点配置扫描目录(自定义扩展时使用此目录) private static final String DUBBO_DIRECTORY = "META-INF/dubbo/"; // dubbo内部扩展点配置扫描目录 private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/"; private static final Pattern NAME_SEPARATOR = Pattern.compile("\\s*[,]+\\s*"); // 缓存ExtensionLoader private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>(); // 缓存扩展点实例 private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>(); // 前面是常量，以下是变量 // ============================== // 当前扩展点的接口类型 private final Class<?> type; // 对象工厂 private final ExtensionFactory objectFactory; private final ConcurrentMap<Class<?>, String> cachedNames = new ConcurrentHashMap<Class<?>, String>(); // 该扩展点类型所有配置的实现类类型 private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String, Class<?>>>(); // 配置中自适应扩展的注解信息 private final Map<String, Activate> cachedActivates = new ConcurrentHashMap<String, Activate>(); // 扩展点实例 private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>(); // 自适应扩展点实例 private final Holder<Object> cachedAdaptiveInstance = new Holder<Object>(); // 自适应扩展点类型 private volatile Class<?> cachedAdaptiveClass = null; // 默认扩展点的名 private String cachedDefaultName; // 包装类类型 private Set<Class<?>> cachedWrapperClasses; private volatile Throwable createAdaptiveInstanceError; private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<String, IllegalStateException>(); // ... }初始化
先从全局缓存里面取，如果取不到则新建
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!");
}
ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
if (loader == null) {
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}
return loader;
}ExtensionLoader构建方法,保存扩展点接口类型和对象工厂
扩展点对象工厂也是从通过ExtensionLoader加载出来的
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}- 获取扩展点实例
先从缓存中取，如果没有则开始创建
Holder对象主要是上同步锁的时候用，锁在Holder级别，保证之后get和set方法原子性
public T getExtension(String name) {
if (name == null || name.length() == 0)
throw new IllegalArgumentException("Extension name == null");
if ("true".equals(name)) {
return getDefaultExtension();
}
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) {
instance = createExtension(name);
holder.set(instance);
}
}
}
return (T) instance;
}createExtension是在同步块中调用的，所以不需要加synchroneized，是线程安全的
private T createExtension(String name) {
// 取出对应类型
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
// 从缓存的实例取出,如果没有则新建
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
// 给实例注入属性
injectExtension(instance);
// 如果有配置包装类，则实例化包装类并注入属性
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && wrapperClasses.size() > 0) {
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 + ") could not be instantiated: " + t.getMessage(), t);
}
}
// 获取所有扩展点类型的map,如果缓存中没有就从配置文件中取出
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;
}private T injectExtension(T instance) {
try {
if (objectFactory != null) {
for (Method method : instance.getClass().getMethods()) {
// 只处理set开头，只有一个参数且是public的方法
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) : "";
// 从对象工厂中获取属性值,对象工厂中会递归注入值
Object object = objectFactory.getExtension(pt, property);
if (object != null) {
method.invoke(instance, object);
}
} catch (Exception e) {
logger.error("fail to inject via method " + method.getName()
+ " of interface " + type.getName() + ": " + e.getMessage(), e);
}
}
}
}
} catch (Exception e) {
logger.error(e.getMessage(), e);
}
return instance;
}默认的对象工厂实现是AdaptiveExtensionFactory，其实就是SpringExtensionFactory和SpiExtensionFactory两个一起用。
主要看SpiExtensionFactory实现
可以看出这里进入了递归，直到相关扩展点全部加载完成
public <T> T getExtension(Class<T> type, String name) {
if (type.isInterface() && type.isAnnotationPresent(SPI.class)) {
ExtensionLoader<T> loader = ExtensionLoader.getExtensionLoader(type);
if (loader.getSupportedExtensions().size() > 0) {
return loader.getAdaptiveExtension();
}
}
return null;
}前面的代码是返回普通扩展点，接下来的是返回自适应扩展点，AdaptiveExtension
自适应扩展点不同的地方在于，不是直接返回扩展点实现，而是通过字节码技术生成一个代理类，
代理类会根据调用时的参数不同，再去选择不同的扩展点实现。也就是调用了获取扩展点的方法getExtension(name)
// 和普通扩展点基本一致
public T getAdaptiveExtension() {
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
instance = createAdaptiveExtension();
cachedAdaptiveInstance.set(instance);
} catch (Throwable t) {
createAdaptiveInstanceError = t;
throw new IllegalStateException("fail to create adaptive instance: " + t.toString(), t);
}
}
}
} else {
throw new IllegalStateException("fail to create adaptive instance: " + createAdaptiveInstanceError.toString(), createAdaptiveInstanceError);
}
}
return (T) instance;
}
// 这里类型不是从getExtensionClasses中取而是getAdaptiveExtensionClass
private T createAdaptiveExtension() {
try {
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
}
}
private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
// 这里使用字节码技术，生成了代理类
private Class<?> createAdaptiveExtensionClass() {
String code = createAdaptiveExtensionClassCode();
ClassLoader classLoader = findClassLoader();
com.alibaba.dubbo.common.compiler.Compiler compiler = ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
return compiler.compile(code, classLoader);
}createAdaptiveExtensionClassCode代码太长就不贴出来了
这是其中一个扩展点生成的源代码，可以看出代码里根据url中的参数选择合适的扩展点实现
这些用反射用动态代理也是可以做的，不过效率肯定没字节码好，这个可以学习下。
package com.alibaba.dubbo.rpc;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adaptive implements com.alibaba.dubbo.rpc.Protocol {
public void destroy() {
throw new UnsupportedOperationException("method public abstract void com.alibaba.dubbo.rpc.Protocol.destroy() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
}
public int getDefaultPort() {
throw new UnsupportedOperationException("method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
}
public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.RpcException {
if (arg0 == null)
throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument == null");
if (arg0.getUrl() == null)
throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");com.alibaba.dubbo.common.URL url = arg0.getUrl();
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null)
throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])");
com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.export(arg0);
}
public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) throws com.alibaba.dubbo.rpc.RpcException {
if (arg1 == null)
throw new IllegalArgumentException("url == null");
com.alibaba.dubbo.common.URL url = arg1;
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null)
throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])");
com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.refer(arg0, arg1);
}
}为了方便理解下面上个小例子
新建一个maven项目，结构如下：
|--pom.xml
|--src
|--main
|--java
|--com
|--serviceloader
|--service
|--Book.java
|--Car.java
|--English.java
|--Honda.java
|--Human.java
|--Man.java
|--ServiceLoader.java
|--SPI.java
|--resources
|--config.propertiesSPI注解，用来指定实现者
// SPI.java
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
public @interface SPI {
String value() default "";
}建3个接口，并加上注解，设置默认实现者
@SPI("english")
public interface Book {
String read();
}
@SPI("honda")
public interface Car {
void driver(String name);
}
// 也可以是woman
@SPI("man")
public interface Human {
String sayHello();
}以及实现者
public class Man implements Human {
private Car car;
private Book book;
@Override
public String sayHello() {
return "hello man";
}
public Car getCar() {
return car;
}
public void setCar(Car car) {
this.car = car;
}
public Book getBook() {
return book;
}
public void setBook(Book book) {
this.book = book;
}
}
public class Woman implements Human {
@Override
public String sayHello() {
return "hello man";
}
}
public class Honda implements Car {
private Book book;
@Override
public void driver(String name) {
System.out.println("i am " + name);
}
public Book getBook() {
return book;
}
public void setBook(Book book) {
this.book = book;
}
}
public class English implements Book {
@Override
public String read() {
return "hello my name is denis";
}
}配置文件，用来配置实现者的类型
man=com.serviceloader.service.Man
woman=com.serviceloader.service.Woman
english=com.serviceloader.service.English
honda=com.serviceloader.service.Honda最后是服务加载器
public class ServiceLoader {
private static ConcurrentMap<Class<?>, Object> SERVICE_INSTANCES = new ConcurrentHashMap<>();
private static ConcurrentMap<String, Class<?>> SERVICE_CLASS;
@SuppressWarnings("unchecked")
public static <T> T get(Class<T> clazz) {
if (SERVICE_CLASS == null) {
SERVICE_CLASS = getServiceClass();
}
SPI spi = clazz.getAnnotation(SPI.class);
if (spi == null) {
throw new RuntimeException("不是SPI接口");
}
Class<?> targetClass = SERVICE_CLASS.get(spi.value()); // 这里可以根据其它配置更换实现者
if (targetClass == null) {
throw new RuntimeException("没有配置实现类型");
}
try {
T instance = (T) SERVICE_INSTANCES.get(clazz);
if (instance == null) {
SERVICE_INSTANCES.putIfAbsent(clazz, targetClass.newInstance());
instance = (T) SERVICE_INSTANCES.get(clazz);
}
injectExtension(instance);
return instance;
} catch (InstantiationException | IllegalAccessException e) {
throw new RuntimeException(e.getMessage());
}
}
/**
* 注入属性
*
* @param instance
* @param <T>
*/
private static <T> void injectExtension(T instance) {
Method[] methods = instance.getClass().getMethods();
for (Method method : methods) {
if (method.getName().startsWith("set") && method.getName().length() > 3
&& method.getParameterTypes().length == 1
&& Modifier.isPublic(method.getModifiers())) {
try {
Class<?> pt = method.getParameterTypes()[0];
Object object = get(pt); // 递归
if (object != null) {
method.invoke(instance, object);
}
} catch (IllegalAccessException | InvocationTargetException e) {
e.printStackTrace();
}
}
}
}
/**
* 从配置文件中取出实现者名称与类型对应map
*
* @return
*/
private static ConcurrentMap<String,Class<?>> getServiceClass() {
try {
if (SERVICE_CLASS == null) {
synchronized (ServiceLoader.class) {
if (SERVICE_CLASS == null) {
SERVICE_CLASS = new ConcurrentHashMap<>();
InputStream is = ServiceLoader.class.getClassLoader().getResourceAsStream("config.properties");
Properties p = new Properties();
p.load(is);
Set<String> keys = p.stringPropertyNames();
for (String key : keys) {
Class<?> clazz = Class.forName(String.valueOf(p.get(key)));
SERVICE_CLASS.putIfAbsent(key, clazz);
}
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
return SERVICE_CLASS;
}
public static void main(String[] args) {
Human human = ServiceLoader.get(Human.class);
System.out.println("class : " + human.getClass().getName());
System.out.println(human.sayHello());
Car car = ServiceLoader.get(Car.class);
System.out.println("class : " + car.getClass().getName());
car.driver("大卡车");
Book book = ServiceLoader.get(Book.class);
System.out.println("class : " + book.getClass().getName());
System.out.println(book.read());
}
}运行后输出：
class : com.serviceloader.service.Man
hello man
class : com.serviceloader.service.Honda
i am 大卡车
class : com.serviceloader.service.English
hello my name is denis