前言
之前可以先看下Dubbo官方文件對SPI的介紹Dubbo SPI。
我們來看下官方文件是怎麼說的：
SPI 全稱為 Service Provider Interface，是一種服務發現機制。SPI 的本質是將介面實現類的全限定名配置在檔案中，並由服務載入器讀取配置檔案，載入實現類。這樣可以在執行時，動態為介面替換實現類。正因此特性，我們可以很容易的通過 SPI 機制為我們的程式提供擴充功能。
SPI 機制在第三方框架中也有所應用，比如 Dubbo 就是通過 SPI 機制載入所有的元件。不過，Dubbo 並未使用 Java 原生的 SPI 機制，而是對其進行了增強，使其能夠更好的滿足需求。
從官方示例說起
Dubbo SPI 的相關邏輯被封裝在了 ExtensionLoader 類中，通過 ExtensionLoader，我們可以載入指定的實現類。Dubbo SPI 所需的配置檔案需放置在 META-INF/dubbo 路徑下，檔名是擴充套件介面的全限定名。例如com.alibaba.dubbo.rpc.Filter的檔名就是com.alibaba.dubbo.rpc.Filter。
注意：這裡的配置檔案是放在你自己的 jar 包內，不是 dubbo 本身的 jar 包內，Dubbo 會全 ClassPath 掃描所有 jar 包內同名的這個檔案，然後進行合併。
官方例子的配置內容如下。
optimusPrime = org.apache.spi.OptimusPrime
bumblebee = org.apache.spi.Bumblebee
複製程式碼以下是示例程式碼:
public class DubboSPITest {
@Test
public void sayHello() throws Exception {
ExtensionLoader<Robot> extensionLoader =
ExtensionLoader.getExtensionLoader(Robot.class);
Robot optimusPrime = extensionLoader.getExtension("optimusPrime");
optimusPrime.sayHello();
Robot bumblebee = extensionLoader.getExtension("bumblebee");
bumblebee.sayHello();
}
}
複製程式碼實現分析
獲取ExtensionLoad
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type) {
if (type == null) {
throw new IllegalArgumentException("Extension type == null");
}
// 必須是介面
if (!type.isInterface()) {
throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
}
// 必須包含@SPI註釋
if (!withExtensionAnnotation(type)) {
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
}
// 獲得介面對應的擴充套件載入器
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;
}
複製程式碼這裡的EXTENSION_LOADERS是一個靜態變數。
/**
* 擴充套件載入器集合
*/
private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
複製程式碼注意到withExtensionAnnotation方法，每個要擴充套件的介面都必須要包含@SPI。
private static <T> boolean withExtensionAnnotation(Class<T> type) {
return type.isAnnotationPresent(SPI.class);
}
複製程式碼我們看下ExtensionLoader的建構函式。
private ExtensionLoader(Class<?> type) {
this.type = type;
objectFactory = (type == ExtensionFactory.class ? null : ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getAdaptiveExtension());
}
複製程式碼此時這裡的type是上面例子傳入的Robot.class。
獲取擴充物件
public T getExtension(String name) {
if (name == null || name.length() == 0) {
throw new IllegalArgumentException("Extension name == null");
}
// 查詢 預設的 擴充套件物件
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;
}
複製程式碼可以看到如果如果name是true,則返回預設的擴充套件物件。
檢查快取是否命中，沒有的話就呼叫createExtension方法建立擴充套件物件。
查詢預設的擴充套件物件
public T getDefaultExtension() {
getExtensionClasses();
if (null == cachedDefaultName || cachedDefaultName.length() == 0
|| "true".equals(cachedDefaultName)) {
return null;
}
return getExtension(cachedDefaultName);
}
複製程式碼先呼叫getExtensionClasses方法獲取所有的擴充套件實現類，再獲取cachedDefaultName對應的擴充套件物件。那cachedDefaultName的值又是哪裡來的呢？其實就是SPI註解的value。在loadExtensionClasses方法中有這段程式碼。
private Map<String, Class<?>> loadExtensionClasses() {
// 通過@SPI註解,獲得預設的擴充套件實現類名
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if (defaultAnnotation != null) {
String value = defaultAnnotation.value();
if ((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];
}
}
}
// 省略方法程式碼
return extensionClasses;
}
複製程式碼建立擴充套件物件
private T createExtension(String name) {
// 獲得副檔名對應的擴充套件實現類
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
// 從快取中，獲取擴充套件物件
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
// 當快取不存在時，建立擴充套件物件，並新增到快取中
EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
// 注入依賴的屬性
injectExtension(instance);
// 建立Wrapper擴充套件物件
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
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);
}
}
複製程式碼createExtension方法包括以下的幾個步驟:
- 通過 getExtensionClasses 獲取所有的擴充類
- 通過反射建立擴充物件
- 向擴充物件中注入依賴
- 將擴充物件包裹在相應的 Wrapper 物件中
獲取所有的擴充套件類
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;
}
複製程式碼對cachedClasses進行雙重檢查，若快取的擴充套件實現類集合為空，則呼叫loadExtensionClasses方法載入擴充套件實現類集合。
private Map<String, Class<?>> loadExtensionClasses() {
// 通過@SPI註解,獲得預設的擴充套件實現類名
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if (defaultAnnotation != null) {
String value = defaultAnnotation.value();
if ((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<?>>();
loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName());
loadDirectory(extensionClasses, DUBBO_INTERNAL_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName());
loadDirectory(extensionClasses, DUBBO_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName());
loadDirectory(extensionClasses, SERVICES_DIRECTORY, type.getName().replace("org.apache", "com.alibaba"));
return extensionClasses;
}
複製程式碼載入的配置檔案路徑見下面。
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/";
複製程式碼從配置檔案中，載入擴充套件實現類陣列。
private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir, String type) {
String fileName = dir + type;
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 resourceURL = urls.nextElement();
loadResource(extensionClasses, classLoader, resourceURL);
}
}
} catch (Throwable t) {
logger.error("Exception when load extension class(interface: " +
type + ", description file: " + fileName + ").", t);
}
}
複製程式碼private void loadResource(Map<String, Class<?>> extensionClasses, ClassLoader classLoader, java.net.URL resourceURL) {
try {
BufferedReader reader = new BufferedReader(new InputStreamReader(resourceURL.openStream(), "utf-8"));
try {
String line;
while ((line = reader.readLine()) != null) {
// 跳過當前被註釋調的情況，例如 #spring=xxxxxxxx
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) {
loadClass(extensionClasses, resourceURL, Class.forName(line, true, classLoader), name);
}
} catch (Throwable t) {
IllegalStateException e = new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + resourceURL + ", cause: " + t.getMessage(), t);
exceptions.put(line, e);
}
}
}
} finally {
reader.close();
}
} catch (Throwable t) {
logger.error("Exception when load extension class(interface: " +
type + ", class file: " + resourceURL + ") in " + resourceURL, t);
}
}
複製程式碼private void loadClass(Map<String, Class<?>> extensionClasses, java.net.URL resourceURL, Class<?> clazz, String name) throws NoSuchMethodException {
// 判斷擴充套件實現，是否實現擴充套件介面
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.");
}
// 快取自適應擴充套件物件的類別 `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());
}
// 快取擴充Wrapper實現類到`cachedWrapperClasses`
} else if (isWrapperClass(clazz)) {
Set<Class<?>> wrappers = cachedWrapperClasses;
if (wrappers == null) {
cachedWrapperClasses = new ConcurrentHashSet<Class<?>>();
wrappers = cachedWrapperClasses;
}
wrappers.add(clazz);
} else {
// 快取擴充套件實現類`extensionClasses`
clazz.getConstructor();
if (name == null || name.length() == 0) {
name = findAnnotationName(clazz);
if (name.length() == 0) {
throw new IllegalStateException("No such extension name for the class " + clazz.getName() + " in the config " + resourceURL);
}
}
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);
} else {
// support com.alibaba.dubbo.common.extension.Activate
com.alibaba.dubbo.common.extension.Activate oldActivate = clazz.getAnnotation(com.alibaba.dubbo.common.extension.Activate.class);
if (oldActivate != null) {
cachedActivates.put(names[0], oldActivate);
}
}
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());
}
}
}
}
}
複製程式碼向擴充套件物件中注入依賴
private T injectExtension(T instance) {
try {
if (objectFactory != null) {
for (Method method : instance.getClass().getMethods()) {
if (method.getName().startsWith("set")
&& method.getParameterTypes().length == 1
&& Modifier.isPublic(method.getModifiers())) {
/**
* Check {@link DisableInject} to see if we need auto injection for this property
*/
if (method.getAnnotation(DisableInject.class) != null) {
continue;
}
Class<?> pt = method.getParameterTypes()[0];
try {
String property = method.getName().length() > 3 ? method.getName().substring(3, 4).toLowerCase() + method.getName().substring(4) : "";
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;
}
複製程式碼遍歷查詢set方法，反射注入依賴。
擴充套件點自適應
參考官方文件的自適應擴充套件機制。
有關Adaptive註解還可參考Dubbo SPI之Adaptive詳解。
在類上加Adaptive註解優先順序最高。
private Class<?> getAdaptiveExtensionClass() {
getExtensionClasses();
if (cachedAdaptiveClass != null) {
return cachedAdaptiveClass;
}
return cachedAdaptiveClass = createAdaptiveExtensionClass();
}
複製程式碼在loadClass方法中可以看到帶有Adaptive註解的類物件會被賦值給cachedAdaptiveClass。
// 快取自適應擴充套件物件的類別 `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());
}
// 快取擴充Wrapper實現類到`cachedWrapperClasses`
}
複製程式碼SPI註解的value值是是預設值，在上面我們知道被賦值給了cachedDefaultName，如果通過url獲取不到取哪個類作為Adaptive類的話就用這個預設值。
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);
}
}
}
複製程式碼擴充套件點自動啟用
對於一些集合類擴充套件點，比如：Filter，可以同時載入多個實現，此時可以用自動啟用來簡化配置。
import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
@Activate // 無條件自動啟用
public class XxxFilter implements Filter {
// ...
}
複製程式碼或者
import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
@Activate("xxx") // 當配置了xxx引數，並且引數為有效值時啟用，比如配了cache="lru"，自動啟用CacheFilter。
public class XxxFilter implements Filter {
// ...
}
複製程式碼或者
import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
@Activate(group = "provider", value = "xxx") // 只對提供方啟用，group可選"provider"或"consumer"
public class XxxFilter implements Filter {
// ...
}
複製程式碼或者
import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.rpc.Filter;
@Activate(group = "provider", value = "xxx", order=1) // order表示優先順序，低的排序優先順序高
public class XxxFilter implements Filter {
// ...
}
複製程式碼我們來看下實現
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);
}
複製程式碼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);
// 處理自動啟用的擴充套件物件們
// 判斷不存在配置 "-name"
if (!names.contains(Constants.REMOVE_VALUE_PREFIX + Constants.DEFAULT_KEY)) {
// 獲得擴充套件實現類陣列
getExtensionClasses();
for (Map.Entry<String, Object> entry : cachedActivates.entrySet()) {
String name = entry.getKey();
Object activate = entry.getValue();
String[] activateGroup, activateValue;
if (activate instanceof Activate) {
activateGroup = ((Activate) activate).group();
activateValue = ((Activate) activate).value();
} else if (activate instanceof com.alibaba.dubbo.common.extension.Activate) {
activateGroup = ((com.alibaba.dubbo.common.extension.Activate) activate).group();
activateValue = ((com.alibaba.dubbo.common.extension.Activate) activate).value();
} else {
continue;
}
if (isMatchGroup(group, activateGroup)) {
T ext = getExtension(name);
if (!names.contains(name)
&& !names.contains(Constants.REMOVE_VALUE_PREFIX + name)
&& isActive(activateValue, url)) {
exts.add(ext);
}
}
}
Collections.sort(exts, ActivateComparator.COMPARATOR);
}
// 處理自定義配置的擴充套件物件們
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)) {
// 將配置的自定義在自動啟用的擴充套件物件們前面
if (Constants.DEFAULT_KEY.equals(name)) {
if (!usrs.isEmpty()) {
exts.addAll(0, usrs);
usrs.clear();
}
} else {
T ext = getExtension(name);
usrs.add(ext);
}
}
}
if (!usrs.isEmpty()) {
exts.addAll(usrs);
}
return exts;
}
複製程式碼