SPI機制，即Service Provider Interface，服務提供接口。此機制的作用是更加方便、靈活的獲取到接口實現方，而不是硬編碼到代碼中。比如在dubbo中通過Protocol的refer接口獲取到最終執行的invoker，但是Protocol有多種實現，有DubboProtocol，也有RegistryProtocol，那麼我該使用哪一種呢？這時候Dubbo的SPI機制要解決的問題。
一 、Dubbo中的擴展點與Adaptive：
1、擴展點：
可以認爲是需要被擴展的接口，比如Protocol、RegistryFactory等等。
2、Adaptive：
能動態的獲取到上述擴展點的實現類，此方法能根據特定的條件來獲取到實現類，方法是通過調用方法時傳入的URL來找到最終實現類的key，再從配置文件中根據key找到最終的實現類。
具體如何找到我們會在接下來的源碼解析中講到其原理。
二、解析：
下面以ReferenceConfig根據protocol獲取到invoker舉例。
ReferenceConfig中protocol的Adaptive是這樣來獲取到的：
private static final Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
可以看到是通過ExtensionLoader來獲取到最終的Adaptive。我們一步一步來看。
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!");
}
// 擴展點接口必須有 @SPI 註解，否則會報錯
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
// 下面是一個單例模式，可以看到一個擴展點只能有一個對應的ExtensionLoader
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;
}
下面看一下終點，獲取到Adaptive實現類 ，可以看到Adaptive實現類也是單例。
public T getAdaptiveExtension() {
// 得到緩存的Adaptive實現類
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
// 如果爲空，創建一個
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;
}
創建Adaptive的方法爲直接生成擴展點 Adaptive實現的代碼：
private T createAdaptiveExtension() {
try {
// 1、先生成對應的Adapter類
// 2、再爲生成的adapter類中的set方法，且set的類是可擴展的，再爲其加上adapter
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extension " + type + ", cause: " + e.getMessage(), e);
}
}
private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
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);
}
// 直接動態生成實現接口的adapter代碼
private String createAdaptiveExtensionClassCode() {
StringBuilder codeBuilder = new StringBuilder();
Method[] methods = type.getMethods();
boolean hasAdaptiveAnnotation = false;
for (Method m : methods) {
if (m.isAnnotationPresent(Adaptive.class)) {
hasAdaptiveAnnotation = true;
break;
}
}
// no need to generate adaptive class since there's no adaptive method found.
if (!hasAdaptiveAnnotation)
throw new IllegalStateException("No adaptive method on extension " + type.getName() + ", refuse to create the adaptive class!");
codeBuilder.append("package ").append(type.getPackage().getName()).append(";");
codeBuilder.append("\nimport ").append(ExtensionLoader.class.getName()).append(";");
// 注意實現了接口 implements ").append(type.getCanonicalName())
codeBuilder.append("\npublic class ").append(type.getSimpleName()).append("$Adaptive").append(" implements ").append(type.getCanonicalName()).append(" {");
// 實現了接口中的每個方法，每個方法在執行的時候都會先根據url得到真正執行的類。
for (Method method : methods) {
Class<?> rt = method.getReturnType();
Class<?>[] pts = method.getParameterTypes();
Class<?>[] ets = method.getExceptionTypes();
// 要動態擴展的方法必須在接口方法上有 @Adaptive 註解，如果沒有，則這個方法的實現爲拋異常，則不能動態調用，如果調用就拋錯
Adaptive adaptiveAnnotation = method.getAnnotation(Adaptive.class);
StringBuilder code = new StringBuilder(512);
if (adaptiveAnnotation == null) {
code.append("throw new UnsupportedOperationException(\"method ")
.append(method.toString()).append(" of interface ")
.append(type.getName()).append(" is not adaptive method!\");");
} else {
// Adapter是總線設計，也即是根據參數中的URL來獲取到最終執行的類，比如protocol，那麼url.get("protocol") 來得到register，那麼從配置文件中也就能得到
// registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
// 也就得到了最終的執行類 RegistryProtocol
// 下面的代碼即爲獲取到參數中的url
int urlTypeIndex = -1;
for (int i = 0; i < pts.length; ++i) {
if (pts[i].equals(URL.class)) {
urlTypeIndex = i;
break;
}
}
// found parameter in URL type
if (urlTypeIndex != -1) {
// Null Point check
String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"url == null\");",
urlTypeIndex);
code.append(s);
s = String.format("\n%s url = arg%d;", URL.class.getName(), urlTypeIndex);
code.append(s);
}
// 如果方法參數中沒有URL這個類怎麼辦？那麼這個方法的參數中必須有get且返回參數爲URL的方法，否則會報錯
// did not find parameter in URL type
else {
String attribMethod = null;
// find URL getter method
LBL_PTS:
for (int i = 0; i < pts.length; ++i) {
Method[] ms = pts[i].getMethods();
for (Method m : ms) {
String name = m.getName();
if ((name.startsWith("get") || name.length() > 3)
&& Modifier.isPublic(m.getModifiers())
&& !Modifier.isStatic(m.getModifiers())
&& m.getParameterTypes().length == 0
&& m.getReturnType() == URL.class) {
urlTypeIndex = i;
attribMethod = name;
break LBL_PTS;
}
}
}
if (attribMethod == null) {
throw new IllegalStateException("fail to create adaptive class for interface " + type.getName()
+ ": not found url parameter or url attribute in parameters of method " + method.getName());
}
// Null point check
String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"%s argument == null\");",
urlTypeIndex, pts[urlTypeIndex].getName());
code.append(s);
s = String.format("\nif (arg%d.%s() == null) throw new IllegalArgumentException(\"%s argument %s() == null\");",
urlTypeIndex, attribMethod, pts[urlTypeIndex].getName(), attribMethod);
code.append(s);
s = String.format("%s url = arg%d.%s();", URL.class.getName(), urlTypeIndex, attribMethod);
code.append(s);
}
String[] value = adaptiveAnnotation.value();
// value is not set, use the value generated from class name as the key
if (value.length == 0) {
char[] charArray = type.getSimpleName().toCharArray();
StringBuilder sb = new StringBuilder(128);
for (int i = 0; i < charArray.length; i++) {
if (Character.isUpperCase(charArray[i])) {
if (i != 0) {
sb.append(".");
}
sb.append(Character.toLowerCase(charArray[i]));
} else {
sb.append(charArray[i]);
}
}
value = new String[]{sb.toString()};
}
boolean hasInvocation = false;
for (int i = 0; i < pts.length; ++i) {
if (pts[i].getName().equals("com.alibaba.dubbo.rpc.Invocation")) {
// Null Point check
String s = String.format("\nif (arg%d == null) throw new IllegalArgumentException(\"invocation == null\");", i);
code.append(s);
s = String.format("\nString methodName = arg%d.getMethodName();", i);
code.append(s);
hasInvocation = true;
break;
}
}
// 下面即爲獲取到protocol的代碼，比如 url.getProtocol() == null ? \"%s\" : url.getProtocol()
// 來獲取到最終執行的protocol的key。
String defaultExtName = cachedDefaultName;
String getNameCode = null;
for (int i = value.length - 1; i >= 0; --i) {
if (i == value.length - 1) {
if (null != defaultExtName) {
if (!"protocol".equals(value[i]))
if (hasInvocation)
getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
else
getNameCode = String.format("url.getParameter(\"%s\", \"%s\")", value[i], defaultExtName);
else
getNameCode = String.format("( url.getProtocol() == null ? \"%s\" : url.getProtocol() )", defaultExtName);
} else {
if (!"protocol".equals(value[i]))
if (hasInvocation)
getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
else
getNameCode = String.format("url.getParameter(\"%s\")", value[i]);
else
getNameCode = "url.getProtocol()";
}
} else {
if (!"protocol".equals(value[i]))
if (hasInvocation)
getNameCode = String.format("url.getMethodParameter(methodName, \"%s\", \"%s\")", value[i], defaultExtName);
else
getNameCode = String.format("url.getParameter(\"%s\", %s)", value[i], getNameCode);
else
getNameCode = String.format("url.getProtocol() == null ? (%s) : url.getProtocol()", getNameCode);
}
}
// extName即爲最終實現類的key
code.append("\nString extName = ").append(getNameCode).append(";");
// check extName == null?
String s = String.format("\nif(extName == null) " +
"throw new IllegalStateException(\"Fail to get extension(%s) name from url(\" + url.toString() + \") use keys(%s)\");",
type.getName(), Arrays.toString(value));
code.append(s);
// 根據key來獲取到最終的執行類
s = String.format("\n%s extension = (%<s)%s.getExtensionLoader(%s.class).getExtension(extName);",
type.getName(), ExtensionLoader.class.getSimpleName(), type.getName());
code.append(s);
// return statement
if (!rt.equals(void.class)) {
code.append("\nreturn ");
}
// 最終調用執行類來執行方法，並傳入參數
s = String.format("extension.%s(", method.getName());
code.append(s);
for (int i = 0; i < pts.length; i++) {
if (i != 0)
code.append(", ");
code.append("arg").append(i);
}
code.append(");");
}
// 最終拼接成方法
codeBuilder.append("\npublic ").append(rt.getCanonicalName()).append(" ").append(method.getName()).append("(");
for (int i = 0; i < pts.length; i++) {
if (i > 0) {
codeBuilder.append(", ");
}
codeBuilder.append(pts[i].getCanonicalName());
codeBuilder.append(" ");
codeBuilder.append("arg").append(i);
}
codeBuilder.append(")");
if (ets.length > 0) {
codeBuilder.append(" throws ");
for (int i = 0; i < ets.length; i++) {
if (i > 0) {
codeBuilder.append(", ");
}
codeBuilder.append(ets[i].getCanonicalName());
}
}
codeBuilder.append(" {");
codeBuilder.append(code.toString());
codeBuilder.append("\n}");
}
codeBuilder.append("\n}");
if (logger.isDebugEnabled()) {
logger.debug(codeBuilder.toString());
}
return codeBuilder.toString();
}
可以看到，Dubbo的SPI是根據動態生成Adaptive的代碼來做到做到動態實現擴展點的，比如在ReferenceConfig中
invoker = refprotocol.refer(interfaceClass, urls.get(0));
url的protocol爲registry，那麼其最終獲取的invoker則是registry對應的實現類來生成的，找到其配置文件爲
registry=com.alibaba.dubbo.registry.integration.RegistryProtocol
那麼其最終是通過RegistryProtocol來得到最終的invoker。