Dubbo的扩展点(Extension)在JDK的SPI思想的基础上做了一些改进:
ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
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getExtensionLoader方法中传入的了一个Protocol,我们看下Protocol长啥样
@SPI("dubbo")
public interface Protocol {
//获取缺省端口,当用户没有配置端口时使用。
int getDefaultPort();
// 暴露远程服务:
@Adaptive
Exporter export(Invoker invoker) throws RpcException;
//引用远程服务:
@Adaptive
Invoker refer(Class type, URL url) throws RpcException;
//释放协议:
void destroy();
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这是一个协议接口,在类上有@SPI注解,有个默认值dubbo,在方法上有@Adaptive注解,这两个注解是什么作用呢?我们继续往下看getExtensionLoader方法:
public static ExtensionLoader getExtensionLoader(Class type) {
·····
//先从缓存中取值,为null,则去new一个ExtensionLoader
ExtensionLoader loader = (ExtensionLoader) EXTENSION_LOADERS.get(type);
if (loader == null) {
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));
loader = (ExtensionLoader) EXTENSION_LOADERS.get(type);
}
return loader;
}
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继续进入new ExtensionLoader(type)方法:
private ExtensionLoader(Class> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
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给type变量赋值,objectFactory赋值,此时传入的type是Protcol,继续执行ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension()方法,这里我们分为两个步骤:
步骤一:ExtensionLoader.getExtensionLoader(ExtensionFactory.class),此时type为ExtensionFactory.class,这段代码我们得到一个ExtensionLoader实例,这个ExtensionLoader实例中,objetFactory为null, 步骤二:getAdaptiveExtension()方法,为cachedAdaptiveInstance赋值,我们来看这个方法:
public T getAdaptiveExtension() {
Object instance = cachedAdaptiveInstance.get();
if (instance == null) {
if (createAdaptiveInstanceError == null) {
synchronized (cachedAdaptiveInstance) {
instance = cachedAdaptiveInstance.get();
if (instance == null) {
try {
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);
}
}
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双重检查锁判断缓存,如果没有则进入createAdaptiveExtension()方法,这个方法有两部分,一个是injectExtension,一个是getAdaptiveExtensionClass().newInstance()
private T createAdaptiveExtension() {
try {
return injectExtension((T) getAdaptiveExtensionClass().newInstance());
} catch (Exception e) {
throw new IllegalStateException("Can not create adaptive extenstion " + type + ", cause: " + e.getMessage(), e);
}
}
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先看getAdaptiveExtensionClass(),获取一个适配器扩展点的类
private Class> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
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这里我们进入getExtensionClasses()方法,双重检查锁判断,如果没有,继续
private Map> getExtensionClasses() {
Map> classes = cachedClasses.get();
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
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进入loadExtensionClasses()方法
// 此方法已经getExtensionClasses方法同步过。
private Map> 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> extensionClasses = new HashMap>();
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
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这里有个type.getAnnotation(SPI.class),这个type就是刚刚再初始化ExtensionLoader的时候传入的,我们先看type=ExtensionFactory.class的情况,ExtensionFactory接口类上有@SPI注解,但是value为空,然后三次调用loadFile方法,分别对应Dubbo扩展点的三个配置文件路径,在源码中我们可以找到ExtensionFactory对应的文件,
通过loadFile方法,最终extensionClasses返回SpringExtensionFactory和SpiExtensionFactory 缓存到cachedClasses中,为什么只返回了2个类呢,AdaptiveExtensionFactory为什么没有返回呢,因为在loadFile中AdaptiveExtensionFactory因为类上有@Adaptive注解,所以直接缓存到cachedAdaptiveClass中(此时,我们要思考,@Adaptive注解放在类上和放在方法上有什么区别),我们看下loadFile中的关键代码
private void loadFile(Map> extensionClasses, String dir) {
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// 1.判断当前class类上面有没有Adaptive注解,如果有,则直接赋值给cachedAdaptiveClass
if (clazz.isAnnotationPresent(Adaptive.class)) {
if (cachedAdaptiveClass == null) {
cachedAdaptiveClass = clazz;
} else if (!cachedAdaptiveClass.equals(clazz)) {
throw new IllegalStateException("More than 1 adaptive class found: "
+ cachedAdaptiveClass.getClass().getName()
+ ", " + clazz.getClass().getName());
}
} else {
//2.如果没有类注解,那么判断该class中没有参数是type的构造方法,如果有,则把calss放入cachedWrapperClasses中
try {
clazz.getConstructor(type);
Set> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} catch (NoSuchMethodException e) {
//3.判断是否有默认构造方法
clazz.getConstructor();
if (name == null || name.length() == 0) {
name = findAnnotationName(clazz);
if (name == null || name.length() == 0) {
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);
}
}
}
String[] names = NAME_SEPARATOR.split(name);
if (names != null && names.length > 0) {
//4.判断class是否有@Activate注解,如果有,则放入cachedActivates
Activate activate = clazz.getAnnotation(Activate.class);
if (activate != null) {
cachedActivates.put(names[0], activate);
}
for (String n : names) {
//5.缓存calss到cachedNames中
if (!cachedNames.containsKey(clazz)) {
cachedNames.put(clazz, n);
}
Class> c = extensionClasses.get(n);
if (c == null) {
extensionClasses.put(n, clazz);
} else if (c != clazz) {
throw new IllegalStateException("Duplicate extension " + type.getName() + " name " + n + " on " + c.getName() + " and " + clazz.getName());
}
}
}
}
}
}
}
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至此,我们已经拿到了extensionClasses,并缓存到了cachedClasses中,回到getAdaptiveExtensionClass()方法中
private Class> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
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如果cachedAdaptiveClass不为空,那么就返回cachedAdaptiveClass,刚刚我们在loadFile()方法中讲过,@Adaptive注解在类上,那么就会缓存到cachedAdaptiveClass中,这个时候cachedAdaptiveClass有值,为AdaptiveExtensionFactory,所以这里直接返回AdaptiveExtensionFactory,继续返回createAdaptiveExtension()方法,刚刚我们只是走完了createAdaptiveExtension()方法中的一个部分,还有injectExtension方法,这个方法是干什么的,在type=ExtensionFactory.class流程中,这个方法的作用没有体现,先不看injectExtension,我们放在后面的流程去看,然后继续返回到getAdaptiveExtension方法中,把实例AdaptiveExtensionFactory缓存到cachedAdaptiveInstance中,继续返回到ExtensionLoader方法中
private ExtensionLoader(Class> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
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这个时候,objectFactory已经有值了,就是AdaptiveExtensionFactory,继续返回getExtensionLoader方法
public static ExtensionLoader getExtensionLoader(Class type) {
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//EXTENSION_LOADERS判断是否有type,ConcurrentMap, ExtensionLoader>>
ExtensionLoader loader = (ExtensionLoader) EXTENSION_LOADERS.get(type);
if (loader == null) {
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader(type));
loader = (ExtensionLoader) EXTENSION_LOADERS.get(type);
}
return loader;
}
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我们把返回的ExtensionLoader实例缓存到EXTENSION_LOADERS中,此时type=Protocol
ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
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至此,我们已经执行完了ExtensionLoader.getExtensionLoader(Protocol.class),得到了ExtensionLoader实例,继续执行getAdaptiveExtension()方法,这个方法在上面已经分析过了,我们再看下跟type=ExtensionFactory的时候有什么区别,先看下com.alibaba.dubbo.rpc.Protocol文件中有哪些扩展点(这个文件在源码中是分散的,可以在Dubbo的jar包中找,jar包中是合并的)
这个时候再看下当前内存中的数据
private Class> createAdaptiveExtensionClass() {
//生成字节码文件
String code = createAdaptiveExtensionClassCode();
//获得类加载器
ClassLoader classLoader = findClassLoader();
com.alibaba.dubbo.common.compiler.Compiler compiler =
ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.common.compiler.Compiler.class).getAdaptiveExtension();
//动态编译
return compiler.compile(code, classLoader);
}
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执行compiler.compile(code, classLoader),先看下AdaptiveCompiler类
@Adaptive
public class AdaptiveCompiler implements Compiler {
private static volatile String DEFAULT_COMPILER;
public static void setDefaultCompiler(String compiler) {
DEFAULT_COMPILER = compiler;
}
public Class> compile(String code, ClassLoader classLoader) {
Compiler compiler;
ExtensionLoader loader = ExtensionLoader.getExtensionLoader(Compiler.class);
String name = DEFAULT_COMPILER; // copy reference
if (name != null && name.length() > 0) {
compiler = loader.getExtension(name);
} else {
compiler = loader.getDefaultExtension();
}
return compiler.compile(code, classLoader);
}
}
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这里的DEFAULT_COMPILER值为JavassistCompiler,执行loader.getExtension(name),这个方法这里暂时不展开,结果是得到JavassistCompiler实例,这里是一个装饰模式的设计,最终调用JavassistCompiler.compile()方法得到Protocol$Adpative,
回到我们最初的代码的入口
ExtensionLoader.getExtensionLoader(Protocol.class).getAdaptiveExtension();
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这句代码就最终的返回结果就是Protocol$Adpative,我们把这个代理类拿出来看一下
package com.alibaba.dubbo.rpc;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
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!");
}
public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.RpcException {
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 com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) throws com.alibaba.dubbo.rpc.RpcException {
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);
}
}
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这个时候如果执行Protocol$Adpative.export方法,我们看下这个适配器代理类里面的export()方法,通过url来获取extName,所以Dubbo是基于URL来驱动的, 看到Protocol extension = (Protocol)ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(extName)这个方法,这个方法是不是又又又很熟悉,接下来我们来分析getExtension(String name)方法,假设此时extName=dubbo
public T getExtension(String name) {
if (name == null || name.length() == 0)
throw new IllegalArgumentException("Extension name == null");
if ("true".equals(name)) {
return getDefaultExtension();
}
Holder
进入createExtension()方法
private T createExtension(String name) {
//1.通过name获取ExtensionClasses,此时为DubboProtocol
Class> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
//2.获取DubboProtocol实例
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
//3.dubbo的IOC反转控制,就是从spi和spring里面提取对象赋值。
injectExtension(instance);
Set> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && wrapperClasses.size() > 0) {
for (Class> wrapperClass : wrapperClasses) {
//4.如果是包装类
instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
}
}
return instance;
} catch (Throwable t) {
throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
type + ") could not be instantiated: " + t.getMessage(), t);
}
}
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第三步injectExtension(instance),看一下代码:
private T injectExtension(T instance) {
try {
if (objectFactory != null) {
//1.拿到所有的方法
for (Method method : instance.getClass().getMethods()) {
//判断是否是set方法
if (method.getName().startsWith("set")
&& method.getParameterTypes().length == 1
&& Modifier.isPublic(method.getModifiers())) {
Class> pt = method.getParameterTypes()[0];
try {
String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
//从objectFactory中获取所需要注入的实例
Object object = objectFactory.getExtension(pt, property);
if (object != null) {
method.invoke(instance, object);
}
} catch (Exception e) {
logger.error("fail to inject via method " + method.getName()
+ " of interface " + type.getName() + ": " + e.getMessage(), e);
}
}
}
}
} catch (Exception e) {
logger.error(e.getMessage(), e);
}
return instance;
}
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这个方法就是Dubbo完成依赖注入的地方,到这里关于Dubbo的扩展点机制的代码就分析完成了。