Dubbo之SPI機制詳解
阿新 • • 發佈：2019-01-06
spi機制的思想提供一種更加靈活的，可插拔式的機制。本文分別對比了java和dubbo的spi的實現的區別，重點討論dubbo的實現原理。
java spi機制
SPI，Service Provider Interface，主要是被框架的開發人員使用，比如java.sql.Driver介面，其他不同廠商可以針對同一介面做出不同的實現，mysql和postgresql都有不同的實現提供給使用者，而Java的SPI機制可以為某個介面尋找服務實現。
當服務的提供者提供了一種介面的實現之後，需要在classpath下的META-INF/services/目錄裡建立一個以服務介面命名的檔案，這個檔案裡的內容就是這個介面的具體的實現類。當其他的程式需要這個服務的時候，就可以通過查詢這個jar包（一般都是以jar包做依賴）的META-INF/services/中的配置檔案，配置檔案中有介面的具體實現類名，可以根據這個類名進行載入例項化，就可以使用該服務了。JDK中查詢服務的實現的工具類是：java.util.ServiceLoader
因為本文的主體內容是dubbo，所以這邊就不對ServiceLoader的原始碼進行深入的解析。這邊寫了一個例子。
package com.shang.spi;
/**
* @author shang
* @date 2019/1/5
*/
public interface DubboService {
void sayHello();
}
DubboService的實現類AppleService
package com.shang.spi; /** * @author shang * @date 2019/1/5 */ public class AppleService implements DubboService { @Override public void sayHello() { System.out.println("apple"); } } package com.shang.spi; import java.util.Iterator; import java.util.ServiceLoader;
/** * @author shang * @date 2019/1/5 */ public class ServiceMain { public static void main(String[] args) { ServiceLoader<DubboService> spiLoader = ServiceLoader.load(DubboService.class); Iterator<DubboService> iteratorSpi = spiLoader.iterator(); while (iteratorSpi.hasNext()) { DubboService dubboService = iteratorSpi.next(); dubboService.sayHello(); } } }
dubbo spi機制
dubbo的spi實現原理和java spi相似，只不過增強了一些功能和優化。java spi的是把所有的spi都載入到記憶體，但對於dubbo來說可能只需要載入使用者指定的實現方式，而不需要全部載入進來，全部載入也會有效能問題，所以dubbo實現的是在有用到的時候去載入這些擴充套件元件。
spi機制有幾個重要的註解：
1、@SPI註解，被此註解標記的介面，就表示是一個可擴充套件的介面，並標註預設值。
2、@Adaptive註解，有兩種註解方式：一種是註解在類上，一種是註解在方法上。
3、@Activate註解，此註解需要註解在類上或者方法上，並註明被啟用的條件，以及所有的被啟用實現類中的排序資訊
本文重點分析1和2的解析過程，3其實是在loadFile的時候可能被啟用。
什麼時候載入擴充套件類？
public class ReferenceConfig<T> extends AbstractReferenceConfig {
private static final long serialVersionUID = -5864351140409987595L;
private static final Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
private static final Cluster cluster = ExtensionLoader.getExtensionLoader(Cluster.class).getAdaptiveExtension();
private static final ProxyFactory proxyFactory = ExtensionLoader.getExtensionLoader(ProxyFactory.class).getAdaptiveExtension();
...
}我們來看下消費者生成refer的程式碼。從ReferenceConfig這個類中我們可以看到需要初始化的擴充套件類有Protocol、Cluster和ProxyFactory。那麼其實就是需要的時候再去載入。所有的實現都在ExtensionLoader類中。這個類也不負責，一千行左右的程式碼。
下面我們就跟著Protocol的擴充套件來看下原始碼的實現方式。
Protocol refprotocol = ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
在看實現的程式碼之前我們先看加Protocol這個擴充套件類，為了儘量的簡潔刪除了一些包路徑和註釋。
可以看到Protocol的spi的預設值是dubbo，這個在初始化類的時候很有用，假如你的Protocol實現類有很多，在dubbo中就有：
@SPI("dubbo")
public interface Protocol {
int getDefaultPort();
@Adaptive
<T> Exporter<T> export(Invoker<T> invoker) throws RpcException;
@Adaptive
<T> Invoker<T> refer(Class<T> type, URL url) throws RpcException;
void destroy();
default void destroyServer() {
//空實現
}
}那麼spi的Adaptive就是一種自適應的註解，具體是怎麼實現的我們繼續看程式碼。
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!");
}
if(!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
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;
}getExtensionLoader方法最主要的實現就是把所有的擴充套件放到EXTENSION_LOADERS這個容器中，避免二次載入。
private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
如何實現自適應擴充套件機制的？
下面其實就是為了獲取自適應的擴充套件機制，Adaptive這個註解的自適應。呼叫的路徑大概是這樣的：
getAdaptiveExtension()-->
createAdaptiveExtension()-->
getAdaptiveExtensionClass()-->
getExtensionClasses()-->
loadExtensionClasses()public T getAdaptiveExtension() {
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if(createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
//建立自適應擴充套件
instance = createAdaptiveExtension();
//設定快取
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;
}createAdaptiveExtension通過名字就能可以知道這是建立自適應的擴充套件物件
private T createAdaptiveExtension() {
try {
//獲取自適應擴充套件類，通過反射例項化
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
}
}路徑：getAdaptiveExtensionClass->getExtensionClasses->loadExtensionClasses->loadFile
cachedAdaptiveClass會在loadFile進行獲取
private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
//如果快取中已經找到自適應類的話直接返回，意思也就是這個spi有Adaptive的註解類
//比如：當前獲取的自適應實現類是AdaptiveExtensionFactory或者是AdaptiveCompiler，就直接返回，這兩個類是特殊用處的，不用程式碼生成，而是現成的程式碼
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
//否則需要代理類生成相關代理
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}getExtensionClasses->loadFile直接從檔案載入
private Map<String, Class<?>> getExtensionClasses() {
Map<String, Class<?>> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
// 此方法已經getExtensionClasses方法同步過。
private Map<String, Class<?>> loadExtensionClasses() {
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if(defaultAnnotation != null) {
String value = defaultAnnotation.value();
if(value != null && (value = value.trim()).length() > 0) {
String[] names = NAME_SEPARATOR.split(value);
if(names.length > 1) {
throw new IllegalStateException("more than 1 default extension name on extension " + type.getName()
+ ": " + Arrays.toString(names));
}
if(names.length == 1) cachedDefaultName = names[0];
}
}
Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
//從檔案載入
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}主動載入相關type的spi子類
private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
String fileName = dir + type.getName();
try {
Enumeration<java.net.URL> urls;
ClassLoader classLoader = findClassLoader();
if (classLoader != null) {
urls = classLoader.getResources(fileName);
} else {
urls = ClassLoader.getSystemResources(fileName);
}
if (urls != null) {
while (urls.hasMoreElements()) {
//配置檔案路徑
java.net.URL url = urls.nextElement();
try {
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
try {
String line = null;
//每次處理一行
while ((line = reader.readLine()) != null) {
//#號以後的為註釋
final int ci = line.indexOf('#');
//註釋去掉
if (ci >= 0) line = line.substring(0, ci);
line = line.trim();
if (line.length() > 0) {
try {
String name = null;
//=號之前的為副檔名字，後面的為擴充套件類實現的全限定名
int i = line.indexOf('=');
if (i > 0) {
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
if (line.length() > 0) {
//載入擴充套件類的實現
Class<?> clazz = Class.forName(line, true, classLoader);
//檢視型別是否匹配
//type是Protocol介面
//clazz就是Protocol的各個實現類
if (! type.isAssignableFrom(clazz)) {
throw new IllegalStateException();
}
//如果實現類是@Adaptive型別的，會賦值給cachedAdaptiveClass，這個用來存放被@Adaptive註解的實現類
if (clazz.isAnnotationPresent(Adaptive.class)) {
if(cachedAdaptiveClass == null) {
cachedAdaptiveClass = clazz;
} else if (! cachedAdaptiveClass.equals(clazz)) {
throw new IllegalStateException();
}
} else {//不是@Adaptice型別的類，就是沒有註解@Adaptive的實現類
try {//判斷是否是wrapper型別
//如果得到的實現類的構造方法中的引數是擴充套件點型別的，就是一個Wrapper類
//比如ProtocolFilterWrapper，實現了Protocol類，
//而它的構造方法是這樣public ProtocolFilterWrapper(Protocol protocol)
//就說明這個類是一個包裝類
clazz.getConstructor(type);
//cachedWrapperClasses用來存放當前擴充套件點實現類中的包裝類
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} catch (NoSuchMethodException e) {
//沒有上面提到的構造器，則說明不是wrapper型別
//獲取無參構造
clazz.getConstructor();
//沒有名字，就是配置檔案中沒有xxx=xxxx.com.xxx這種
if (name == null || name.length() == 0) {
//去找@Extension註解中配置的值
name = findAnnotationName(clazz);
//如果還沒找到名字，從類名中獲取
if (name == null || name.length() == 0) {
//比如clazz是DubboProtocol，type是Protocol
//這裡得到的name就是dubbo
if (clazz.getSimpleName().length() > type.getSimpleName().length()
&& clazz.getSimpleName().endsWith(type.getSimpleName())) {
name = clazz.getSimpleName().substring(0, clazz.getSimpleName().length() - type.getSimpleName().length()).toLowerCase();
} else {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + url);
}
}
}
//有可能配置了多個名字
String[] names = NAME_SEPARATOR.split(name);
if (names != null && names.length > 0) {
//是否是Active型別的類
Activate activate = clazz.getAnnotation(Activate.class);
if (activate != null) {
//第一個名字作為鍵，放進cachedActivates這個map中快取
cachedActivates.put(names[0], activate);
}
for (String n : names) {
if (! cachedNames.containsKey(clazz)) {
//放入Extension實現類與名稱對映的快取中去，每個class只對應第一個名稱有效
cachedNames.put(clazz, n);
}
Class<?> c = extensionClasses.get(n);
if (c == null) {
//放入到extensionClasses快取中去，多個name可能對應一份extensionClasses
extensionClasses.put(n, clazz);
} else if (c != clazz) {
throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
}
}
}
}
}
}
} catch (Throwable t) { }
}
} // end of while read lines
} finally {
reader.close();
}
} catch (Throwable t) { }
} // end of while urls
}
} catch (Throwable t) { }
}
好了，還是要回到getAdaptiveExtensionClass這個方法，上面的loadFile是按規則載入了所有的type型別的spi類，那麼如果生成自適應類呢？
我們繼續看createAdaptiveExtensionClass這個方法的createAdaptiveExtensionClassCode
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);
}編寫自適應代理類
createAdaptiveExtensionClassCode的程式碼有點長，具體就是生成代理類，我們先看下程式碼：
private String createAdaptiveExtensionClassCode() {
StringBuilder codeBuidler = new StringBuilder();
Method[] methods = type.getMethods();
boolean hasAdaptiveAnnotation = false;
for(Method m : methods) {
if(m.isAnnotationPresent(Adaptive.class)) {
hasAdaptiveAnnotation = true;
break;
}
}
// 完全沒有Adaptive方法，則不需要生成Adaptive類
if(! hasAdaptiveAnnotation)
throw new IllegalStateException("No adaptive method on extension " + type.getName() + ", refuse to create the adaptive class!");
codeBuidler.append("package " + type.getPackage().getName() + ";");
codeBuidler.append("\nimport " + ExtensionLoader.class.getName() + ";");
codeBuidler.append("\npublic class " + type.getSimpleName() + "$Adpative" + " implements " + type.getCanonicalName() + " {");
for (Method method : methods) {
Class<?> rt = method.getReturnType();
Class<?>[] pts = method.getParameterTypes();
Class<?>[] ets = method.getExceptionTypes();
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 {
int urlTypeIndex = -1;
for (int i = 0; i < pts.length; ++i) {
if (pts[i].equals(URL.class)) {
urlTypeIndex = i;
break;
}
}
// 有型別為URL的引數
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);
}
// 引數沒有URL型別
else {
String attribMethod = null;
// 找到引數的URL屬性
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 adative 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();
// 沒有設定Key，則使用“擴充套件點介面名的點分隔 作為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;
}
}
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);
}
}
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);
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 ");
}
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(");");
}
codeBuidler.append("\npublic " + rt.getCanonicalName() + " " + method.getName() + "(");
for (int i = 0; i < pts.length; i ++) {
if (i > 0) {
codeBuidler.append(", ");
}
codeBuidler.append(pts[i].getCanonicalName());
codeBuidler.append(" ");
codeBuidler.append("arg" + i);
}
codeBuidler.append(")");
if (ets.length > 0) {
codeBuidler.append(" throws ");
for (int i = 0; i < ets.length; i ++) {
if (i > 0) {
codeBuidler.append(", ");
}
codeBuidler.append(pts[i].getCanonicalName());
}
}
codeBuidler.append(" {");
codeBuidler.append(code.toString());
codeBuidler.append("\n}");
}
codeBuidler.append("\n}");
if (logger.isDebugEnabled()) {
logger.debug(codeBuidler.toString());
}
return codeBuidler.toString();
}那麼他生成的代理類是怎麼樣的呢？我們還是以com.alibaba.dubbo.rpc.Protocol 為例，他生成的代理類就是Protocol$Adpative，中間多了一個$。
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) throws java.lang.Class {
if (arg1 == null) throw new IllegalArgumentException("url == null");
com.alibaba.dubbo.common.URL url = arg1;
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])");
com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.refer(arg0, arg1);
}
public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.Invoker {
if (arg0 == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument == null");
if (arg0.getUrl() == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");com.alibaba.dubbo.common.URL url = arg0.getUrl();
String extName = ( url.getProtocol() == null ? "dubbo" : url.getProtocol() );
if(extName == null) throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])");
com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol)ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
return extension.export(arg0);
}
public void destroy() {
throw new UnsupportedOperationException("method public abstract void com.alibaba.dubbo.rpc.Protocol.destroy() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
}
public int getDefaultPort() {
throw new UnsupportedOperationException("method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
}
}其他的型別的代理程式碼生成是類似的，上面的程式碼是硬編碼，需要把他編譯並載入到記憶體中，具體還是要回到createAdaptiveExtensionClass這個方法，程式碼在上面，這裡就不再貼了，需要通過Compiler這個類找到對應的自適應實現，這裡得到的就是AdaptiveCompiler，最後呼叫compiler.compile(code, classLoader);來編譯上面生成的類並返回，先進入AdaptiveCompiler的compile方法：
public Class<?> compile(String code, ClassLoader classLoader) {
Compiler compiler;
ExtensionLoader<Compiler> loader = ExtensionLoader.getExtensionLoader(Compiler.class);
//預設的Compiler名字
String name = DEFAULT_COMPILER; // copy reference
//有指定了Compiler名字，就使用指定的名字來找到Compiler實現類
if (name != null && name.length() > 0) {
compiler = loader.getExtension(name);
} else {//沒有指定Compiler名字，就查詢預設的Compiler的實現類
compiler = loader.getDefaultExtension();
}
return compiler.compile(code, classLoader);
}
/*
* Copyright 1999-2011 Alibaba Group.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.alibaba.dubbo.common.compiler;
import com.alibaba.dubbo.common.extension.SPI;
/**
* Compiler. (SPI, Singleton, ThreadSafe)
*
* @author william.liangf
*/
@SPI("javassist")
public interface Compiler {
/**
* Compile java source code.
*
* @param code Java source code
* @param classLoader TODO
* @return Compiled class
*/
Class<?> compile(String code, ClassLoader classLoader);
}
至此整個自適應的代理方式都已經解析完了。但是我覺得實現的有點過於複雜了，是適應類的存在是很有必要的嗎？我覺得也未必。
感覺在spi的實現中插入了Adaptive等的實現是把簡單的spi機制搞得複雜化了，繞了一大圈去解決一個自適應代理類等方式是否可以簡單化。