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
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 ！
@Adaptive
String 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 2 impl3=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
@Test
public void test_getAdaptiveExtension_defaultAdaptiveKey() throws Exception {
{
SimpleExt ext = ExtensionLoader.getExtensionLoader(SimpleExt.class).getAdaptiveExtension();
Map map = new HashMap();
//没有指定具体parameters参数，所以选用默认实现，最后返回impl1
URL 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 map = new HashMap();
map.put("simple.ext", "impl2");//手动在参数中配置impl2,参数为simple.ext
URL 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 map = new HashMap();
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
@Test
public void testLoadActivateExtension() throws Exception {
// test default
URL url = URL.valueOf("test://localhost/test");
List 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 value
url = 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 order
url = 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