一、扩展点配置
详见我在《Java的SPI机制分析》文章中关于Dubbo的SPI机制的介绍，在此不再赘述。
二、扩展点流程分析之SPI
下面以Container加载的过程为例，来说明SPI扩展的实现流程：
所有加上@SPI注解的扩展点可以有不同的扩展，Container代码如下：
package com.alibaba.dubbo.container;
import com.alibaba.dubbo.common.extension.SPI;
/**
* Container. (SPI, Singleton, ThreadSafe)
*
* @author william.liangf
*/
@SPI("spring")
public interface Container {
/**
* start.
*/
void start();
/**
* stop.
*/
void stop();
}
Container这个接口有很多的实现版本，如下：
但是具体哪个种类的Container使用哪个具体实现类还需要通过spi中的配置来指定。例如：
dubbo/dubbo-container/dubbo-container-spring/src/main/resources/META-INF/dubbo/internal/com.alibaba.dubbo.container.Container文件中指定：
spring=com.alibaba.dubbo.container.spring.SpringContainer
则当需要获取spring的Extension的时候，最终会匹配到SpringContainer
下面说明详细的流程：测试类：dubbo/dubbo-demo/dubbo-demo-provider/src/test/java/com/alibaba/dubbo/demo/provider/DemoProvider.java代码如下：
package com.alibaba.dubbo.demo.provider;
public class DemoProvider {
public static void main(String[] args) {
com.alibaba.dubbo.container.Main.main(args);
}
}
会调起：dubbo/dubbo-container/dubbo-container-api/src/main/java/com/alibaba/dubbo/container/Main.java这个类：
代码如下：
package com.alibaba.dubbo.container;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.List;
import com.alibaba.dubbo.common.Constants;
import com.alibaba.dubbo.common.extension.ExtensionLoader;
import com.alibaba.dubbo.common.logger.Logger;
import com.alibaba.dubbo.common.logger.LoggerFactory;
import com.alibaba.dubbo.common.utils.ConfigUtils;
/**
* Main. (API, Static, ThreadSafe)
*
* @author william.liangf
*/
public class Main {
public static final String CONTAINER_KEY = "dubbo.container";
public static final String SHUTDOWN_HOOK_KEY = "dubbo.shutdown.hook";
private static final Logger logger = LoggerFactory.getLogger(Main.class);
/**
* Add by Jason
* ExtensionLoader是一个单例工厂类,它对外暴露getExtensionLoader静态方法返回一个ExtensionLoader实体,
* 这个方法的入参是一个Class类型,这个方法的意思是返回某个接口的ExtensionLoader,对于一个接口,只会有一个
* ExtensionLoader实体。
* ExtensionLoader实体对外暴露一些接口来获取扩展实现,这些接口分为几类,分别是
* Activate Extension,Adaptive Extension,get extension by name ,supported extension
*/
private static final ExtensionLoader<Container> loader = ExtensionLoader.getExtensionLoader(Container.class);
private static volatile boolean running = true;
public static void main(String[] args) {
try {
if (args == null || args.length == 0) {
String config = ConfigUtils.getProperty(CONTAINER_KEY, loader.getDefaultExtensionName());
args = Constants.COMMA_SPLIT_PATTERN.split(config);
}
final List<Container> containers = new ArrayList<Container>();
for (int i = 0; i < args.length; i ++) {
containers.add(loader.getExtension(args[i]));
}
logger.info("Use container type(" + Arrays.toString(args) + ") to run dubbo serivce.");
if ("true".equals(System.getProperty(SHUTDOWN_HOOK_KEY))) {
Runtime.getRuntime().addShutdownHook(new Thread() {
public void run() {
for (Container container : containers) {
try {
container.stop();
logger.info("Dubbo " + container.getClass().getSimpleName() + " stopped!");
} catch (Throwable t) {
logger.error(t.getMessage(), t);
}
synchronized (Main.class) {
running = false;
Main.class.notify();
}
}
}
});
}
for (Container container : containers) {
container.start();
logger.info("Dubbo " + container.getClass().getSimpleName() + " started!");
}
System.out.println(new SimpleDateFormat("[yyyy-MM-dd HH:mm:ss]").format(new Date()) + " Dubbo service server started!");
} catch (RuntimeException e) {
e.printStackTrace();
logger.error(e.getMessage(), e);
System.exit(1);
}
synchronized (Main.class) {
while (running) {
try {
Main.class.wait();
} catch (Throwable e) {
}
}
}
}
}
String config = ConfigUtils.getProperty(CONTAINER_KEY, loader.getDefaultExtensionName());
会读取相关的配置去去获取Container的具体实现：
CONTAINER_KEY = dubbo.container
@SuppressWarnings({ "unchecked", "rawtypes" })
public static String getProperty(String key, String defaultValue) {
String value = System.getProperty(key);
if (value != null && value.length() > 0) {
return value;
}
Properties properties = getProperties();
return replaceProperty(properties.getProperty(key, defaultValue), (Map)properties);
}
先从系统配置中读取是否有dubbo.container这个配置，没有则继续搜索：
public static Properties getProperties() {
if (PROPERTIES == null) {
synchronized (ConfigUtils.class) {
if (PROPERTIES == null) {
String path = System.getProperty(Constants.DUBBO_PROPERTIES_KEY);
if (path == null || path.length() == 0) {
path = System.getenv(Constants.DUBBO_PROPERTIES_KEY);
if (path == null || path.length() == 0) {
path = Constants.DEFAULT_DUBBO_PROPERTIES;
}
}
PROPERTIES = ConfigUtils.loadProperties(path, false, true);
}
}
}
return PROPERTIES;
}
如果都没有则会去一个默认的路径去获取配置文件，如下：
public static final String DEFAULT_DUBBO_PROPERTIES = "dubbo.properties";
该路径在classpath下面，路径为：dubbo/dubbo-demo/dubbo-demo-provider/src/test/resources/dubbo.properties
具体内容为：
## dubbo.container=log4j,spring dubbo.application.name=demo-provider dubbo.application.owner=william #dubbo.registry.address=multicast://224.5.6.7:1234 dubbo.registry.address=zookeeper://127.0.0.1:2181 #dubbo.registry.address=redis://127.0.0.1:6379 #dubbo.registry.address=dubbo://127.0.0.1:9090 #dubbo.monitor.protocol=registry dubbo.protocol.name=dubbo dubbo.protocol.port=20880 dubbo.service.loadbalance=roundrobin #dubbo.log4j.file=logs/dubbo-demo-consumer.log #dubbo.log4j.level=WARN
从中可以看到dubbo.cantainer配置了log4j和spring两个key的容器实现。
接下来会将获取到的配置log4j和spring 放入一个待启动Container列表:
final List<Container> containers = new ArrayList<Container>();
for (int i = 0; i < args.length; i ++) {
containers.add(loader.getExtension(args[i]));
}
通过loader.getExtension()方法获取具体的key对应的实例。我们以args[1](即key为spring)为例：
/**
* 返回指定名字的扩展。如果指定名字的扩展不存在，则抛异常 {@link IllegalStateException}.
*
* @param name
* @return
*/
@SuppressWarnings("unchecked")
public T getExtension(String name) {
if (name == null || name.length() == 0)
throw new IllegalArgumentException("Extension name == null");
if ("true".equals(name)) {
return getDefaultExtension();
}
Holder<Object> holder = cachedInstances.get(name);
if (holder == null) {
cachedInstances.putIfAbsent(name, new Holder<Object>());
holder = cachedInstances.get(name);
}
Object instance = holder.get();
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
instance = createExtension(name);
holder.set(instance);
}
}
}
return (T) instance;
}
继续createExtension(),如下：
private T createExtension(String name) {
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
injectExtension(instance);
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && wrapperClasses.size() > 0) {
for (Class<?> wrapperClass : wrapperClasses) {
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);
}
}
会调用：
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;
}
最终会调用：
// 此方法已经getExtensionClasses方法同步过。
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<?>>();
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
这个方法会从：
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/";
这几个路径中去加载对应的配置文件：
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()) {
java.net.URL url = urls.nextElement();
try {
BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream(), "utf-8"));
try {
String line = null;
while ((line = reader.readLine()) != null) {
final int ci = line.indexOf('#');
if (ci >= 0) line = line.substring(0, ci);
line = line.trim();
if (line.length() > 0) {
try {
String name = null;
int i = line.indexOf('=');
if (i > 0) {
name = line.substring(0, i).trim();
line = line.substring(i + 1).trim();
}
if (line.length() > 0) {
Class<?> clazz = Class.forName(line, true, classLoader);
if (! type.isAssignableFrom(clazz)) {
throw new IllegalStateException("Error when load extension class(interface: " +
type + ", class line: " + clazz.getName() + "), class "
+ clazz.getName() + "is not subtype of interface.");
}
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 {
try {
clazz.getConstructor(type);
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} catch (NoSuchMethodException e) {
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) {
Activate activate = clazz.getAnnotation(Activate.class);
if (activate != null) {
cachedActivates.put(names[0], activate);
}
for (String n : names) {
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());
}
}
}
}
}
}
} catch (Throwable t) {
IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + url + ", cause: " + t.getMessage(), t);
exceptions.put(line, e);
}
}
} // end of while read lines
} finally {
reader.close();
}
} catch (Throwable t) {
logger.error("Exception when load extension class(interface: " +
type + ", class file: " + url + ") in " + url, t);
}
} // end of while urls
}
} catch (Throwable t) {
logger.error("Exception when load extension class(interface: " +
type + ", description file: " + fileName + ").", t);
}
}
对于不同类型的类，会做不同的处理，大致包括@Adaptive Wrapper等。这里只说Container会走到的分支，后续会继续详细分析@Adaptive等相关的实现。
在搜索的路径dubbo/dubbo-container/dubbo-container-spring/src/main/resources/META-INF/dubbo/internal/com.alibaba.dubbo.container.Container 中这个配置文件的内容为：
spring=com.alibaba.dubbo.container.spring.SpringContainer
会把从配置文件中读取的相关信息读取到缓存extensionClasses中：
for (String n : names) {
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());
}
}
最终，又回到这里：
@SuppressWarnings("unchecked")
private T createExtension(String name) {
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
injectExtension(instance);
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && wrapperClasses.size() > 0) {
for (Class<?> wrapperClass : wrapperClasses) {
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);
}
}
Class<?> clazz = getExtensionClasses().get(name);
得到具体的实现类为：com.alibaba.dubbo.container.spring.SpringContainer
接下来通过反射将该类实例化出来并返回，至此，一个完整的搜索与实例创建完成了。zongjie
总结
本篇以一个Container的加载流程讲解了一个没有Adaptive 和Wrapper的扩展，《Dubbo扩展点机制分析（二）》https://my.oschina.net/u/3729778/blog/1626971 会详细介绍ExtensionLoader及Adaptive的相关扩展实现。
参考文献：
1、参考：http://blog.csdn.net/jdluojing/article/details/44947221
http://www.tuicool.com/articles/FR7NnyQ Dubbo源码学习之ExtentionLoader
http://blog.csdn.net/jdluojing/article/details/44947221