2、Dubbo的SPI机制分析1-SPI加载class

1、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 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