暴露服务的过程中，会涉及到两个Protocol
1. DubboProtocol主要是做网络通信相关初始化
2. RegistryProtocol主要是做zk的注册和订阅相关
在提供一个服务的时候，需要在配置文件里声明如下xml
<dubbo:service....然后Spring会根据对应关系执行对应的BeanDefinitionParser，然后实例化对应的类，提供一个服务的时候会实例化ServiceBean(具体对应关系看DubboNamespaceHandler类；spring解析自定义标签可以看下spring源码关于标签的处理，这里就不说了)
ServiceBean实现了InitializingBean和ApplicationContextAware接口，所以会执行afterPropertiesSet和onApplicationEvent方法，这里就是入口，然后就会执行export方法暴露服务
一路跟下去，都是设置一下属性值，然后到了doExportUrls方法便开始主要的逻辑
private void doExportUrls() {
List<URL> registryURLs = loadRegistries(true);//获取注册中心的url
for (ProtocolConfig protocolConfig : protocols) {
doExportUrlsFor1Protocol(protocolConfig, registryURLs);
}
} 在Zookeeper为注册中心的情况下，registryURLs值如下
[registry://127.0.0.1:2181/com.alibaba.dubbo.registry.RegistryService?application=test&dubbo=2.0.0&owner=william&pid=4444®istry=zookeeper×tamp=1488886235790]
进入doExportUrlsFor1Protocol方法，看下暴露服务的主要逻辑
if (! Constants.SCOPE_NONE.toString().equalsIgnoreCase(scope)) {
//配置不是remote的情况下做本地暴露 (配置为remote，则表示只暴露远程服务)
if (!Constants.SCOPE_REMOTE.toString().equalsIgnoreCase(scope)) {
exportLocal(url);
}
//如果配置不是local则暴露为远程服务.(配置为local，则表示只暴露远程服务)
if (! Constants.SCOPE_LOCAL.toString().equalsIgnoreCase(scope) ){
if (logger.isInfoEnabled()) {
logger.info("Export dubbo service " + interfaceClass.getName() + " to url " + url);
}
if (registryURLs != null && registryURLs.size() > 0
&& url.getParameter("register", true)) {
for (URL registryURL : registryURLs) {
url = url.addParameterIfAbsent("dynamic", registryURL.getParameter("dynamic"));
URL monitorUrl = loadMonitor(registryURL);
if (monitorUrl != null) {
url = url.addParameterAndEncoded(Constants.MONITOR_KEY, monitorUrl.toFullString());
}
if (logger.isInfoEnabled()) {
logger.info("Register dubbo service " + interfaceClass.getName() + " url " + url + " to registry " + registryURL);
}
Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, registryURL.addParameterAndEncoded(Constants.EXPORT_KEY, url.toFullString()));
Exporter<?> exporter = protocol.export(invoker);
exporters.add(exporter);
}
} else {
Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, url);
Exporter<?> exporter = protocol.export(invoker);
exporters.add(exporter);
}
}
}1.本地暴露
会先使用exportLocal暴露本地服务
private void exportLocal(URL url) {
if (!Constants.LOCAL_PROTOCOL.equalsIgnoreCase(url.getProtocol())) {
URL local = URL.valueOf(url.toFullString())
.setProtocol(Constants.LOCAL_PROTOCOL)
.setHost(NetUtils.LOCALHOST)
.setPort(0);
Exporter<?> exporter = protocol.export(
proxyFactory.getInvoker(ref, (Class) interfaceClass, local));
exporters.add(exporter);
logger.info("Export dubbo service " + interfaceClass.getName() +" to local registry");
}
} 看了Dubbo的扩展机制会知道，ProxyFactory默认会使用接口上@Spi注解声明的服务，为了容易理解，我把注解上的@SPI设置成jdk，那么就会使用jdk对应的实现类，即JdkProxyFactory
JdkProxyFactory的getInvoker方法如下：
public <T> Invoker<T> getInvoker(T proxy, Class<T> type, URL url) {
return new AbstractProxyInvoker<T>(proxy, type, url) {
@Override
protected Object doInvoke(T proxy, String methodName,
Class<?>[] parameterTypes,
Object[] arguments) throws Throwable {
Method method = proxy.getClass().getMethod(methodName, parameterTypes);
return method.invoke(proxy, arguments);
}
};
} 返回一个AbstractProxyInvoker类的对象，这个AbstractProxyInvoker主要是接收消费方的请求后，执行本地方法的一个Invoker，其中是使用了反射机制来调用了本地方法
获取到Invoker之后，需要使用Protocol的export来暴露这个服务，在讲Dubbo扩展机制的时候，Protocol外面有两个装饰类，那么export会先调用ProtocolListenerWrapper
public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
if (Constants.REGISTRY_PROTOCOL.equals(invoker.getUrl().getProtocol())) {
return protocol.export(invoker);
}
return new ListenerExporterWrapper<T>(protocol.export(invoker),
Collections.unmodifiableList(ExtensionLoader.getExtensionLoader(ExporterListener.class)
.getActivateExtension(invoker.getUrl(), Constants.EXPORTER_LISTENER_KEY)));
} 这时的url为
injvm://127.0.0.1/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=test&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&loadbalance=roundrobin&methods=sayHello&owner=william&pid=4444&side=provider×tamp=1488887324869
对象为
那么if条件不满足，将调用ProtocolFilterWrapper的export方法
public <T> Invoker<T> refer(Class<T> type, URL url) throws RpcException {
if (Constants.REGISTRY_PROTOCOL.equals(url.getProtocol())) {
return protocol.refer(type, url);
}
return buildInvokerChain(protocol.refer(type, url), Constants.REFERENCE_FILTER_KEY, Constants.CONSUMER);
} If条件还是不满足，执行下面的代码
注意：这里的protocol就是InjvmProtocol了
然后看下buildInvokerChain方法，这个方法建立了一个个的filter，使用了责任链模式，一个普通的Invoker调用也会经历这些filter，每个filter都有自己特殊的功能
private static <T> Invoker<T> buildInvokerChain(final Invoker<T> invoker, String key, String group) {
Invoker<T> last = invoker;
List<Filter> filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);
if (filters.size() > 0) {
for (int i = filters.size() - 1; i >= 0; i --) {
final Filter filter = filters.get(i);
final Invoker<T> next = last;
last = new Invoker<T>() {
public Class<T> getInterface() {
return invoker.getInterface();
}
public URL getUrl() {
return invoker.getUrl();
}
public boolean isAvailable() {
return invoker.isAvailable();
}
public Result invoke(Invocation invocation) throws RpcException {
return filter.invoke(next, invocation);
}
public void destroy() {
invoker.destroy();
}
@Override
public String toString() {
return invoker.toString();
}
};
}
}
return last;
} 这里的group是provider，key是service.filter
看下getActivateExtension方法实现
public List<T> getActivateExtension(URL url, String key, String group) {
String value = url.getParameter(key);
return getActivateExtension(url, value == null || value.length() == 0 ? null : Constants.COMMA_SPLIT_PATTERN.split(value), group);
}
public List<T> getActivateExtension(URL url, String[] values, String group) {
List<T> exts = new ArrayList<T>();
List<String> names = values == null ? new ArrayList<String>(0) : Arrays.asList(values);
if (! names.contains(Constants.REMOVE_VALUE_PREFIX + Constants.DEFAULT_KEY)) {
getExtensionClasses();
for (Map.Entry<String, Activate> entry : cachedActivates.entrySet()) {
String name = entry.getKey();
Activate activate = entry.getValue();
if (isMatchGroup(group, activate.group())) {
T ext = getExtension(name);
if (! names.contains(name)
&& ! names.contains(Constants.REMOVE_VALUE_PREFIX + name)
&& isActive(activate, url)) {
exts.add(ext);
}
}
}
Collections.sort(exts, ActivateComparator.COMPARATOR);
}
List<T> usrs = new ArrayList<T>();
for (int i = 0; i < names.size(); i ++) {
String name = names.get(i);
if (! name.startsWith(Constants.REMOVE_VALUE_PREFIX)
&& ! names.contains(Constants.REMOVE_VALUE_PREFIX + name)) {
if (Constants.DEFAULT_KEY.equals(name)) {
if (usrs.size() > 0) {
exts.addAll(0, usrs);
usrs.clear();
}
} else {
T ext = getExtension(name);
usrs.add(ext);
}
}
}
if (usrs.size() > 0) {
exts.addAll(usrs);
}
return exts;
} 由于url中没有service.filter的key，所以values为[]
cachedActivates记录了接口实现中带有Activate注解的类，需要筛选出group为provider的实现类，最后进行排序
过滤器调用的顺序和上图的顺序一样
最后会调用InjvmProtocol的export方法，将invoker封装成InjvmExporter返回，得到Exporter之后，放到List里
public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
return new InjvmExporter<T>(invoker, invoker.getUrl().getServiceKey(), exporterMap);
}2.远程暴露
回到doExportUrlsFor1Protocol方法，暴露本地服务之后，根据注册中心的地址暴露远程服务
for (URL registryURL : registryURLs) {
url = url.addParameterIfAbsent("dynamic", registryURL.getParameter("dynamic"));
URL monitorUrl = loadMonitor(registryURL);
if (monitorUrl != null) {
url = url.addParameterAndEncoded(Constants.MONITOR_KEY, monitorUrl.toFullString());
}
if (logger.isInfoEnabled()) {
logger.info("Register dubbo service " + interfaceClass.getName() + " url " + url + " to registry " + registryURL);
}
Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, registryURL.addParameterAndEncoded(Constants.EXPORT_KEY, url.toFullString()));
Exporter<?> exporter = protocol.export(invoker);
exporters.add(exporter);
}获取Invoker的方式和之前一样
而实际调用的是哪个Protocol对象呢？根据Invoker中protocol属性的值(值为registry)和Dubbo扩展机制可以知道调用的是RegistryProtocol
public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
//export invoker
final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker);
//registry provider
final Registry registry = getRegistry(originInvoker);
final URL registedProviderUrl = getRegistedProviderUrl(originInvoker);
registry.register(registedProviderUrl);
// 订阅override数据
// FIXME 提供者订阅时，会影响同一JVM即暴露服务，又引用同一服务的的场景，因为subscribed以服务名为缓存的key，导致订阅信息覆盖。
final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registedProviderUrl);
final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl);
overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);
//保证每次export都返回一个新的exporter实例
return new Exporter<T>() {
public Invoker<T> getInvoker() {
return exporter.getInvoker();
}
public void unexport() {
try {
exporter.unexport();
} catch (Throwable t) {
logger.warn(t.getMessage(), t);
}
try {
registry.unregister(registedProviderUrl);
} catch (Throwable t) {
logger.warn(t.getMessage(), t);
}
try {
overrideListeners.remove(overrideSubscribeUrl);
registry.unsubscribe(overrideSubscribeUrl, overrideSubscribeListener);
} catch (Throwable t) {
logger.warn(t.getMessage(), t);
}
}
};
}2.1 doLocalExport
先看下doLocalExport方法
private <T> ExporterChangeableWrapper<T> doLocalExport(final Invoker<T> originInvoker){
String key = getCacheKey(originInvoker);
ExporterChangeableWrapper<T> exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
if (exporter == null) {
synchronized (bounds) {
exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
if (exporter == null) {
final Invoker<?> invokerDelegete = new InvokerDelegete<T>(originInvoker, getProviderUrl(originInvoker));
exporter = new ExporterChangeableWrapper<T>((Exporter<T>)protocol.export(invokerDelegete), originInvoker);
bounds.put(key, exporter);
}
}
}
return (ExporterChangeableWrapper<T>) exporter;
}originInvoker的url值为：
registry://127.0.0.1:2181/com.alibaba.dubbo.registry.RegistryService?application=test&dubbo=2.0.0&export=dubbo%3A%2F%2F10.1.87.36%3A20888%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Dtest%26dubbo%3D2.0.0%26generic%3Dfalse%26interface%3Dcom.alibaba.dubbo.demo.DemoService%26loadbalance%3Droundrobin%26methods%3DsayHello%26owner%3Dwilliam%26pid%3D14204%26side%3Dprovider%26threads%3D1%26timestamp%3D1518580683573&owner=william&pid=14204®istry=zookeeper×tamp=1518580683551
url是代表注册中心相关信息，getCacheKey是获取url中的provider属性，即provider的url：
dubbo://10.1.87.36:20888/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=test&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&loadbalance=roundrobin&methods=sayHello&owner=william&pid=14204&side=provider&threads=1×tamp=1518580683573
第一次进来的时候，bounds中该key对应的值为空，所以根据provider的url和originInvoker封装成新的Invoker，此时Invoker的url就是provider的url，其中protocol的值为dubbo，那么将调用DubboProtocol的export方法，一开始已经介绍，DubboProtocol主要是做网络通信相关初始化
DubboProtocol的export方法如下：
public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
URL url = invoker.getUrl();
String key = serviceKey(url);
DubboExporter<T> exporter = new DubboExporter<T>(invoker, key, exporterMap);
exporterMap.put(key, exporter);
Boolean isStubSupportEvent = url.getParameter(Constants.STUB_EVENT_KEY,Constants.DEFAULT_STUB_EVENT);
Boolean isCallbackservice = url.getParameter(Constants.IS_CALLBACK_SERVICE, false);
if (isStubSupportEvent && !isCallbackservice){
String stubServiceMethods = url.getParameter(Constants.STUB_EVENT_METHODS_KEY);
if (stubServiceMethods == null || stubServiceMethods.length() == 0 ){
if (logger.isWarnEnabled()){
logger.warn(new IllegalStateException("consumer [" +url.getParameter(Constants.INTERFACE_KEY) +
"], has set stubproxy support event ,but no stub methods founded."));
}
} else {
stubServiceMethodsMap.put(url.getServiceKey(), stubServiceMethods);
}
}
openServer(url);
return exporter;
}key=全类名+dubbo监听端口号
然后将Invoker封装成Export，然后放到map中
所以看下来，Export和invoker是一对一的关系
exporterMap主要是将服务名和export进行关联，看到这里其实已经差不多结束了，Dubbo接收到consumer的请求时，会在exporterMap中找到对应的exporter，然后找到对应的Invoker，有了invoker就可以调用本地的服务
2.2 getRegistry
再回到RegistryProtocol的exprot方法中，接下来就是执行getRegistry(originInvoker)这句代码了，主要是做注册中心的初始化
/**
* 根据invoker的地址获取registry实例
* @param originInvoker
* @return
*/
private Registry getRegistry(final Invoker<?> originInvoker){
URL registryUrl = originInvoker.getUrl();
if (Constants.REGISTRY_PROTOCOL.equals(registryUrl.getProtocol())) {
String protocol = registryUrl.getParameter(Constants.REGISTRY_KEY, Constants.DEFAULT_DIRECTORY);
registryUrl = registryUrl.setProtocol(protocol).removeParameter(Constants.REGISTRY_KEY);
}
return registryFactory.getRegistry(registryUrl);
}在上面我们看到invoker的url中，registry属性是zookeeper，根据spi机制，该RegistryFactory为ZookeeperRegistryFactory，getRegistry会调用到父类AbstractRegistryFactory的方法
public Registry getRegistry(URL url) {
url = url.setPath(RegistryService.class.getName())
.addParameter(Constants.INTERFACE_KEY, RegistryService.class.getName())
.removeParameters(Constants.EXPORT_KEY, Constants.REFER_KEY);
String key = url.toServiceString();
// 锁定注册中心获取过程，保证注册中心单一实例
LOCK.lock();
try {
Registry registry = REGISTRIES.get(key);
if (registry != null) {
return registry;
}
registry = createRegistry(url);
if (registry == null) {
throw new IllegalStateException("Can not create registry " + url);
}
REGISTRIES.put(key, registry);
return registry;
} finally {
// 释放锁
LOCK.unlock();
}
}createRegistry在父类中没有实现，调用到ZookeeperRegistryFactory中
public Registry createRegistry(URL url) {
return new ZookeeperRegistry(url, zookeeperTransporter);
}先看下Registry的继承结构
可以看到ZookeeperRegistry有两个父类，所以ZookeeperRegistry会先调用父类的构造方法，这个过程主要做了几件事：
1. 加载服务缓存文件(AbstractRegistry)
2. 异步(默认)更新缓存文件(AbstractRegistry)
3. 定时重试失败的动作:注册失败，取消注册失败，订阅失败，取消订阅失败，通知失败(FailbackRegistry)
4. 初始化zk通信相关(ZookeeperRegistry)
加载服务缓存文件：
public AbstractRegistry(URL url) {
setUrl(url);
// 启动文件保存定时器
syncSaveFile = url.getParameter(Constants.REGISTRY_FILESAVE_SYNC_KEY, false);
String filename = url.getParameter(Constants.FILE_KEY, System.getProperty("user.home") + "/.dubbo/dubbo-registry-" + url.getHost() + ".cache");
File file = null;
if (ConfigUtils.isNotEmpty(filename)) {
file = new File(filename);
if(! file.exists() && file.getParentFile() != null && ! file.getParentFile().exists()){
if(! file.getParentFile().mkdirs()){
throw new IllegalArgumentException("Invalid registry store file " + file + ", cause: Failed to create directory " + file.getParentFile() + "!");
}
}
}
this.file = file;
loadProperties();
notify(url.getBackupUrls());
}loadProperties主要是将dubbo的服务缓存文件加载进来，转换成Properties对象
Key=服务名 Value=url
更新缓存文件：
AbstractRegistry类中还有一个doSaveProperties方法，主要用来更新缓存文件，在更新前会先把本地文件的内容先更新到properties对象中，然后再进行更新操作
public void doSaveProperties(long version) {
if(version < lastCacheChanged.get()){
return;
}
if (file == null) {
return;
}
Properties newProperties = new Properties();
// 保存之前先读取一遍，防止多个注册中心之间冲突
InputStream in = null;
try {
if (file.exists()) {
in = new FileInputStream(file);
newProperties.load(in);
}
} catch (Throwable e) {
} finally {
....
}
// 保存
try {
newProperties.putAll(properties);
File lockfile = new File(file.getAbsolutePath() + ".lock");
if (!lockfile.exists()) {
lockfile.createNewFile();
}
RandomAccessFile raf = new RandomAccessFile(lockfile, "rw");
try {
FileChannel channel = raf.getChannel();
try {
FileLock lock = channel.tryLock();
if (lock == null) {
throw new IOException("Can not lock the registry cache file " + file.getAbsolutePath() + ", ignore and retry later, maybe multi java process use the file, please config: dubbo.registry.file=xxx.properties");
}
// 保存
try {
if (! file.exists()) {
file.createNewFile();
}
FileOutputStream outputFile = new FileOutputStream(file);
try {
newProperties.store(outputFile, "Dubbo Registry Cache");
} finally {
outputFile.close();
}
} finally {
lock.release();
}
} finally {
channel.close();
}
} finally {，
}
} catch (Throwable e) {
if (version < lastCacheChanged.get()) {
return;
} else {
registryCacheExecutor.execute(new SaveProperties(lastCacheChanged.incrementAndGet()));
}
}
}更新的时候会对.lock文件进行加锁，这样其他线程/进程就无法修改缓存文件，如果tryLock返回空，证明该文件正在被修改，那么增加版本号进行重试
doSaveProperties方法开头的时候有句代码：
if(version < lastCacheChanged.get()){return;}可以看到保存的时候有版本号的判断，lastCacheChanged这个是在saveProperties方法中增加的，如果当条件满足，那么证明在这个线程执行saveProperties方法之后还没执行doSaveProperties操作的时候，又有线程执行了saveProperties操作，把version+1了，那么当前线程直接return，让后续线程继续
定时重试失败的动作
构造方法里起了一个定时器，定时进行重试操作，代码简化如下：
// 重试失败的动作
protected void retry() {
if (! failedRegistered.isEmpty()) {
Set<URL> failed = new HashSet<URL>(failedRegistered);
for (URL url : failed) {
doRegister(url);
failedRegistered.remove(url);
}
}
if (! failedUnregistered.isEmpty()) {
Set<URL> failed = new HashSet<URL>(failedUnregistered);
if (logger.isInfoEnabled()) {
logger.info("Retry unregister " + failed);
}
for (URL url : failed) {
doUnregister(url);
failedUnregistered.remove(url);
}
}
if (! failedSubscribed.isEmpty()) {
Map<URL, Set<NotifyListener>> failed = new HashMap<URL, Set<NotifyListener>>(failedSubscribed);
for (Map.Entry<URL, Set<NotifyListener>> entry : failed.entrySet()) {
URL url = entry.getKey();
Set<NotifyListener> listeners = entry.getValue();
for (NotifyListener listener : listeners) {
doSubscribe(url, listener);
listeners.remove(listener);
}
}
}
if (! failedUnsubscribed.isEmpty()) {
Map<URL, Set<NotifyListener>> failed = new HashMap<URL, Set<NotifyListener>>(failedUnsubscribed);
for (Map.Entry<URL, Set<NotifyListener>> entry : failed.entrySet()) {
URL url = entry.getKey();
Set<NotifyListener> listeners = entry.getValue();
for (NotifyListener listener : listeners) {
try {
doUnsubscribe(url, listener);
listeners.remove(listener);
} catch (Throwable t) { // 忽略所有异常，等待下次重试
logger.warn("Failed to retry unsubscribe " + failed + ", waiting for again, cause: " + t.getMessage(), t);
}
}
}
}
if (! failedNotified.isEmpty()) {
Map<URL, Map<NotifyListener, List<URL>>> failed = new HashMap<URL, Map<NotifyListener, List<URL>>>(failedNotified);
for (Map<NotifyListener, List<URL>> values : failed.values()) {
for (Map.Entry<NotifyListener, List<URL>> entry : values.entrySet()) {
NotifyListener listener = entry.getKey();
List<URL> urls = entry.getValue();
listener.notify(urls);
values.remove(listener);
}
}
}
}从对面失败的集合从取数据执行相应的重试
初始化zk通信相关
这里会连接zk，然后增加监听器
2.3 register
再回到RegistryProtocol的export方法，registry初始化完成之后，接下来会调用register方法，仍然是先调用父类的方法
AbstractRegistry.register：将url加入到registered集合中
FailbackRegistry.register：
@Override
public void register(URL url) {
super.register(url);
failedRegistered.remove(url);
failedUnregistered.remove(url);
try {
// 向服务器端发送注册请求
doRegister(url);
} catch (Exception e) {
Throwable t = e;
// 如果开启了启动时检测，则直接抛出异常
boolean check = getUrl().getParameter(Constants.CHECK_KEY, true)
&& url.getParameter(Constants.CHECK_KEY, true)
&& ! Constants.CONSUMER_PROTOCOL.equals(url.getProtocol());
boolean skipFailback = t instanceof SkipFailbackWrapperException;
if (check || skipFailback) {
if(skipFailback) {
t = t.getCause();
}
throw new IllegalStateException("Failed to register " + url + " to registry " + getUrl().getAddress() + ", cause: " + t.getMessage(), t);
} else {
logger.error("Failed to register " + url + ", waiting for retry, cause: " + t.getMessage(), t);
}
// 将失败的注册请求记录到失败列表，定时重试
failedRegistered.add(url);
}
}从代码上看到，可以分为几步
1. failedRegistered和failedUnregistered中移除该url
2. 发送注册请求
可以看到注册会失败，如果设置了check为false，那么放到重试队列中重试
doRegister方法是在ZookeeperRegistry中实现的，主要是在zk上创建节点，创建逻辑如下：
public void create(String path, boolean ephemeral) {
int i = path.lastIndexOf('/');
if (i > 0) {
create(path.substring(0, i), false);
}
if (ephemeral) {
createEphemeral(path);
} else {
createPersistent(path);
}
}Path格式为：
/dubbo/服务名/providers(可能是configurators或者router节点)/url
该方法会递归把每个父节点都创建完毕，可以看到除了dubbo这个节点是持久化节点，其他都是临时节点，那么当服务与zk断开连接一段时间后，zk会删除该节点，服务消费方就会得到通知，知道该提供者下线，做相应操作
2.4 subscribe
接下来看订阅方法subscribe，同理会先调用父类的方法
AbstractRegistry.subscribe：将listener保存到subscribed中
FailbackRegistry.subscribe：
@Override
public void subscribe(URL url, NotifyListener listener) {
super.subscribe(url, listener);
removeFailedSubscribed(url, listener);
try {
// 向服务器端发送订阅请求
doSubscribe(url, listener);
} catch (Exception e) {
Throwable t = e;
List<URL> urls = getCacheUrls(url);
if (urls != null && urls.size() > 0) {
notify(url, listener, urls);
logger.error("Failed to subscribe " + url + ", Using cached list: " + urls + " from cache file: " + getUrl().getParameter(Constants.FILE_KEY, System.getProperty("user.home") + "/dubbo-registry-" + url.getHost() + ".cache") + ", cause: " + t.getMessage(), t);
} else {
// 如果开启了启动时检测，则直接抛出异常
boolean check = getUrl().getParameter(Constants.CHECK_KEY, true)
&& url.getParameter(Constants.CHECK_KEY, true);
boolean skipFailback = t instanceof SkipFailbackWrapperException;
if (check || skipFailback) {
if(skipFailback) {
t = t.getCause();
}
throw new IllegalStateException("Failed to subscribe " + url + ", cause: " + t.getMessage(), t);
} else {
logger.error("Failed to subscribe " + url + ", waiting for retry, cause: " + t.getMessage(), t);
}
}
// 将失败的订阅请求记录到失败列表，定时重试
addFailedSubscribed(url, listener);
}
}和注册类似，区别是订阅失败的时候，首先会调用getCacheUrls获取url，这个方法就是从缓存的properties对象里获取服务的url，如果没有数据才执行和注册一样的操作，有则调用notify方法，这个后面会说到
看下核心的ZookeeperRegistry的doSubscribe方法
List<URL> urls = new ArrayList<URL>();
for (String path : toCategoriesPath(url)) {
ConcurrentMap<NotifyListener, ChildListener> listeners = zkListeners.get(url);
if (listeners == null) {
zkListeners.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, ChildListener>());
listeners = zkListeners.get(url);
}
ChildListener zkListener = listeners.get(listener);
if (zkListener == null) {
listeners.putIfAbsent(listener, new ChildListener() {
public void childChanged(String parentPath, List<String> currentChilds) {
ZookeeperRegistry.this.notify(url, listener, toUrlsWithEmpty(url, parentPath, currentChilds));
}
});
zkListener = listeners.get(listener);
}
zkClient.create(path, false);
List<String> children = zkClient.addChildListener(path, zkListener);
if (children != null) {
urls.addAll(toUrlsWithEmpty(url, path, children));
}
}
notify(url, listener, urls);toCategoriesPath方法是从url中获取category属性的值，转换成zk上路径的形式，由于provider只有一个configurators，所以path如下
/dubbo/服务名/configurators，接下来就是为该节点添加监听回调，然后返回子节点
如果子节点为空，那么toUrlsWithEmpty方法会返回empty://….格式的url，即protocol为empty，这个协议后面会用到
接下来notify一路会调用到AbstractRegistry的notify方法
protected void notify(URL url, NotifyListener listener, List<URL> urls) {
....
Map<String, List<URL>> result = new HashMap<String, List<URL>>();
for (URL u : urls) {
if (UrlUtils.isMatch(url, u)) {
String category = u.getParameter(Constants.CATEGORY_KEY, Constants.DEFAULT_CATEGORY);
List<URL> categoryList = result.get(category);
if (categoryList == null) {
categoryList = new ArrayList<URL>();
result.put(category, categoryList);
}
categoryList.add(u);
}
}
if (result.size() == 0) {
return;
}
Map<String, List<URL>> categoryNotified = notified.get(url);
if (categoryNotified == null) {
notified.putIfAbsent(url, new ConcurrentHashMap<String, List<URL>>());
categoryNotified = notified.get(url);
}
for (Map.Entry<String, List<URL>> entry : result.entrySet()) {
String category = entry.getKey();
List<URL> categoryList = entry.getValue();
categoryNotified.put(category, categoryList);
saveProperties(url);
listener.notify(categoryList);
}
}主要看下最下面的循环，即遍历每个类目下的url，执行一遍saveProperties方法(把服务信息保存到文件)，调用监听器的notify方法，listener最初初始化为OverrideListener
public void notify(List<URL> urls) {
List<URL> result = null;
for (URL url : urls) {
URL overrideUrl = url;
if (url.getParameter(Constants.CATEGORY_KEY) == null
&& Constants.OVERRIDE_PROTOCOL.equals(url.getProtocol())) {
// 兼容旧版本
overrideUrl = url.addParameter(Constants.CATEGORY_KEY, Constants.CONFIGURATORS_CATEGORY);
}
if (! UrlUtils.isMatch(subscribeUrl, overrideUrl)) {
if (result == null) {
result = new ArrayList<URL>(urls);
}
result.remove(url);
}
}
if (result != null) {
urls = result;
}
this.configurators = RegistryDirectory.toConfigurators(urls);
List<ExporterChangeableWrapper<?>> exporters = new ArrayList<ExporterChangeableWrapper<?>>(bounds.values());
for (ExporterChangeableWrapper<?> exporter : exporters){
Invoker<?> invoker = exporter.getOriginInvoker();
final Invoker<?> originInvoker ;
if (invoker instanceof InvokerDelegete){
originInvoker = ((InvokerDelegete<?>)invoker).getInvoker();
}else {
originInvoker = invoker;
}
URL originUrl = RegistryProtocol.this.getProviderUrl(originInvoker);
URL newUrl = getNewInvokerUrl(originUrl, urls);
if (! originUrl.equals(newUrl)){
RegistryProtocol.this.doChangeLocalExport(originInvoker, newUrl);
}
}
}主要看下最下面的循环，会比对invoker的url是新的url是否是一样，如果不一样，那么更新invoker的url