Dubbo的SPI机制介绍以及SPI加载class的方法

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1、Dubbo的SPI例子

@SPIpublic 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 extensionLoader = ExtensionLoader.getExtensionLoader(Robot.class);        Robot optimusPrime = extensionLoader.getExtension("optimusPrime");        optimusPrime.sayHello();        Robot bumblebee = extensionLoader.getExtension("bumblebee");        bumblebee.sayHello();    }}

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

2、Dubbo的SPI源码分析

ExtensionLoader extensionLoader = ExtensionLoader.getExtensionLoader(Robot.class);public static  ExtensionLoader getExtensionLoader(Class 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!");    }    // 从缓存中获取与拓展类对应的ExtensionLoader    ExtensionLoader loader = (ExtensionLoader) EXTENSION_LOADERS.get(type);    if (loader == null) {        // 若缓存未命中,则创建一个新的实例,先简单看下new ExtensionLoader(type)        EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));        loader = (ExtensionLoader) EXTENSION_LOADERS.get(type);    }    return loader;}

这里的objectFactory创建可以参考: Dubbo的SPI机制分析3-Dubbo的IOC依赖注入

 private ExtensionLoader(Class type) {    this.type = type;    // 这里的type是Robot.class,所以会执行后面的代码,加载并创建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也是用于持有对象的,用作缓存    Holder