spi(service providerinterface)，是DUBBO功能强大的保障。核心支持类ExtensionLoader。

具体分析可以参照<Dubbo原理解析-Dubbo内核实现之基于SPI思想Dubbo内核实现>.

1.比较重要的注解

@SPI:扩展点接口的标识 ：作用域在类上；

@Adaptive:为生成Adaptive实例提供参数，作用域在类或方法上；

Adatpive，字面意思是个适配，但其实是个代理，它的意思是适配合适的对象处理请求。类似jdk的动态代理，因为dubbo底层会大量使用反射，出于性能考虑会默认使用javassist字节码编译生成一个adaptive拦截所有请求，然后由它基于策略动态委派合适的provider进行处理。
SPI接口会动态编译出一个adaptive，用于适配provider处理请求。用户可以自己实现一个adaptive，只需要对某个provider打上@adaptive即可，例如Dubbo自身的AdaptiveExtensionFactory类。
对于默认编译生成Adaptive的方案，需要使用@Adaptive声明接口上的哪些方法是adaptive方法，没有被声明的方法如果被请求会抛出异常非adaptive方法的异常

@Activate：可以被框架中自动激活加载扩展，此Annotation用于配置扩展被自动激活加载条件。

Activate，看起来有点不好理解，它的意思是条件激活，用户通过group和value配置激活条件。被activate注解的扩展点在满足某种条件时会被激活，它一般用来配合filter和Invokelistener，声明他们的使用场景。

1.1 测试对象代码

#1.声明SPI 默认为imp1
@SPI("impl1")
public interface SimpleExt {// 没有使用key的@Adaptive ！@AdaptiveString echo(URL url, String s);@Adaptive({"key1", "key2"})String yell(URL url, String s);// 无@Adaptive ！String bang(URL url, int i);
}//实现类1
public class SimpleExtImpl1 implements SimpleExt {public String echo(URL url, String s) {return "Ext1Impl1-echo";}public String yell(URL url, String s) {return "Ext1Impl1-yell";}public String bang(URL url, int i) {return "bang1";}
}
//实现类2
public class SimpleExtImpl2 implements SimpleExt {public String echo(URL url, String s) {return "Ext1Impl2-echo";}public String yell(URL url, String s) {return "Ext1Impl2-yell";}public String bang(URL url, int i) {return "bang2";}}
//实现类3
public class SimpleExtImpl3 implements SimpleExt {public String echo(URL url, String s) {return "Ext1Impl3-echo";}public String yell(URL url, String s) {return "Ext1Impl3-yell";}public String bang(URL url, int i) {return "bang3";}}

1.2 配置文件com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt

位置要放在如下位置

private static final String SERVICES_DIRECTORY = "META-INF/services/";private static final String DUBBO_DIRECTORY = "META-INF/dubbo/";private static final String DUBBO_INTERNAL_DIRECTORY = DUBBO_DIRECTORY + "internal/";

具体内容如下

# Comment 1
impl1=com.alibaba.dubbo.common.extensionloader.ext1.impl.SimpleExtImpl1#Hello World
impl2=com.alibaba.dubbo.common.extensionloader.ext1.impl.SimpleExtImpl2  # Comment 2impl3=com.alibaba.dubbo.common.extensionloader.ext1.impl.SimpleExtImpl3 # with head space

定义3个实现。

1.4 测试

@Test
public void test_getDefaultExtension() throws Exception {SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getDefaultExtension();assertThat(ext, instanceOf(SimpleExtImpl1.class));String name = ExtensionLoader.getExtensionLoader(SimpleExt.class).getDefaultExtensionName();assertEquals("impl1", name);
}

由于@SPI("impl1")，定义了默认实现的名称为imp1.

@Test
public void test_getExtension() throws Exception {assertTrue(ExtensionLoader.getExtensionLoader(SimpleExt.class).getExtension("impl1") instanceof SimpleExtImpl1);assertTrue(ExtensionLoader.getExtensionLoader(SimpleExt.class).getExtension("impl2") instanceof SimpleExtImpl2);
}

getExtensionLoader(Class<T> type):根据类名，返回具体实现类。这些配置信息在META对应文件中配置。当然，也可以使用@Extention注解配置（只不过，这个注解已经废弃了）

@Test
public void test_getAdaptiveExtension_defaultAdaptiveKey() throws Exception {{SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();Map<String, String> map = new HashMap<String, String>();//没有指定具体parameters参数，所以选用默认实现，最后返回impl1URL url = new URL("p1", "1.2.3.4", 1010, "path1", map);//如果不设置默认的SPI实现类，则报异常//java.lang.IllegalStateException: Fail to get extension(com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt) name from url(p1://1.2.3.4:1010/path1) use keys([simple.ext])String echo = ext.echo(url, "haha");assertEquals("Ext1Impl1-echo", echo);}{SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();Map<String, String> map = new HashMap<String, String>();map.put("simple.ext", "impl2");//手动在参数中配置impl2,参数为simple.extURL url = new URL("p1", "1.2.3.4", 1010, "path1", map);String echo = ext.echo(url, "haha");assertEquals("Ext1Impl2-echo", echo);}
}

@Adaptive 测试

由于 yell方法声明了，@Adaptive({"key1", "key2"})

@Test
public void test_getAdaptiveExtension_customizeAdaptiveKey() throws Exception {SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();Map<String, String> map = new HashMap<String, String>();map.put("key2", "impl2");URL url = new URL("p1", "1.2.3.4", 1010, "path1", map);String echo = ext.yell(url, "haha");assertEquals("Ext1Impl2-yell", echo);url = url.addParameter("key1", "impl3"); // 注意： URL是值类型echo = ext.yell(url, "haha");assertEquals("Ext1Impl3-yell", echo);
}

如果参数不是key1,key2,即使参数值输入impl1,impl2也是无意义的。

由于bang方法，没有被@Adaptive 修饰，所以以下代码，会报异常

ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension().bang(..);

of interface com.alibaba.dubbo.common.extensionloader.ext1.SimpleExt is not adaptive method!

2.@Activate注解

配置文件

group=com.alibaba.dubbo.common.extensionloader.activate.impl.GroupActivateExtImpl
value=com.alibaba.dubbo.common.extensionloader.activate.impl.ValueActivateExtImpl
order1=com.alibaba.dubbo.common.extensionloader.activate.impl.OrderActivateExtImpl1
order2=com.alibaba.dubbo.common.extensionloader.activate.impl.OrderActivateExtImpl2
com.alibaba.dubbo.common.extensionloader.activate.impl.ActivateExt1Impl1

@SPI("impl1")
public interface ActivateExt1 {String echo(String msg);
}
@Activate(group = {"default_group"})
public class ActivateExt1Impl1 implements ActivateExt1 {public String echo(String msg) {return msg;}
}
@Activate(group = {"group1", "group2"})
public class GroupActivateExtImpl implements ActivateExt1 {public String echo(String msg) {return msg;}
}
@Activate(order = 1, group = {"order"})
public class OrderActivateExtImpl1 implements ActivateExt1 {public String echo(String msg) {return msg;}
}
@Activate(order = 2, group = {"order"})
public class OrderActivateExtImpl2 implements ActivateExt1 {public String echo(String msg) {return msg;}
}
@Activate(value = {"value"}, group = {"value"})
public class ValueActivateExtImpl implements ActivateExt1 {public String echo(String msg) {return msg;}
}

2.2测试代码

   @Testpublic void testLoadActivateExtension() throws Exception {// test defaultURL url = URL.valueOf("test://localhost/test");List<ActivateExt1> list = ExtensionLoader.getExtensionLoader(ActivateExt1.class).getActivateExtension(url, new String[]{}, "default_group");Assert.assertEquals(1, list.size());Assert.assertTrue(list.get(0).getClass() == ActivateExt1Impl1.class);// test group
//        url = url.addParameter(Constants.GROUP_KEY, "group1");list = ExtensionLoader.getExtensionLoader(ActivateExt1.class).getActivateExtension(url, new String[]{}, "group1");Assert.assertEquals(1, list.size());Assert.assertTrue(list.get(0).getClass() == GroupActivateExtImpl.class);// test valueurl = url.removeParameter(Constants.GROUP_KEY);
//        url = url.addParameter(Constants.GROUP_KEY, "value");url = url.addParameter("value", "value");list = ExtensionLoader.getExtensionLoader(ActivateExt1.class).getActivateExtension(url, new String[]{}, "value");Assert.assertEquals(1, list.size());Assert.assertTrue(list.get(0).getClass() == ValueActivateExtImpl.class);// test orderurl = URL.valueOf("test://localhost/test");
//        url = url.addParameter(Constants.GROUP_KEY, "order");list = ExtensionLoader.getExtensionLoader(ActivateExt1.class).getActivateExtension(url, new String[]{}, "order");Assert.assertEquals(2, list.size());Assert.assertTrue(list.get(0).getClass() == OrderActivateExtImpl1.class);Assert.assertTrue(list.get(1).getClass() == OrderActivateExtImpl2.class);}

以上内容，是通过代码演练的方式，讲解了dubbo SPI机制的威力。
如果对底层实现感兴趣，可参看博客。

参照：Dubbo原理解析-Dubbo内核实现之基于SPI思想Dubbo内核实现