SPI英文全稱爲Service Provider Interface。它的作用就是將接口實現類的全限定名寫在指定目錄的配置文件中,使框架讀取配置文件,從而加載實現類。這樣我們就可以動態的爲接口替換實現類,使得框架拓展性更高。Java其實也有原生的SPI機制,但是Dubbo並未使用它。學習Dubbo源碼的前提就是得弄懂Dubbo SPI機制。
0.Java SPI示例
public interface Hello{
void sayHello();
}
public class testA implements Hello{
@Override
public void sayHello(){
System.out.println("Hello,I am A");
}
}
public class testB implements Hello{
@Override
public void sayHello(){
System.out.println("Hello,I am B");
}
}
寫好實現類和接口後,我們需要在META-INF/services目錄下新建一個文件,名稱爲接口Hello的全限定名。然後在文件裏面寫上所有實現類的全限定名,例如:
com.yelow.spi.testA
com.yelow.spi.testB
測試
public class JavaSPITest{
@Test
public void sayHello() throws Exception{
ServiceLoader<Hello> serviceLoader = ServiceLoader.load(Hello.class);
serviceLoader.forEach(Hello::sayHello);
//分別輸出:
//Hello,I am A
//Hello,I am B
}
}
1.Dubbo SPI示例
Dubbo SPI的使用上,和Java SPI類似的。先定義接口和實現類,接口前加上@SPI註解,代表一個拓展點。再創建一個配置文件。但是這個文件的路徑應該在META-INF/dubbo/路徑下。配置文件內容應該變成鍵值對形式,例如:
testA = com.yelow.spi.testA
testB = com.yelow.spi.testB
最後測試方法爲:
public class JavaSPITest{
@Test
public void sayHello() throws Exception{
ExtensionLoader<Hello> loader=ExtensionLoader.getExtensionLoader(Hello.class);
//按需加載,參數爲配置文件中的key值
Hello testA=loader.getExtension("testA");
testA.sayHello();
//輸出Hello,I am A
}
}
2.Dubbo SPI源碼分析
上面簡單演示了Dubbo的使用方法。先通過ExtensionLoader.getExtensionLoader獲取ExtensionLoader對象。在通過這個對象的getExtension方法獲取實現類對象。先看一下getExtensionLoader方法:
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!");
}
//嘗試從本地緩存中獲取ExtensionLoader對象
ExtensionLoader<T> loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
//如果緩存沒有就新建一個
if (loader == null) {
EXTENSION_LOADERS.putIfAbsent(type, new ExtensionLoader<T>(type));
loader = (ExtensionLoader<T>) EXTENSION_LOADERS.get(type);
}
return loader;
}
getExtensionLoader方法比較簡單,我們接着看一下getExtension:
public T getExtension(String name) {
if (name == null || name.length() == 0)
throw new IllegalArgumentException("Extension name == null");
if ("true".equals(name)) {
//獲取默認的實現類
return getDefaultExtension();
}
//這個類用於持有目標對象,先從緩存中獲取
Holder<Object> holder = cachedInstances.get(name);
if (holder == null) {
cachedInstances.putIfAbsent(name, new Holder<Object>());
holder = cachedInstances.get(name);
}
//獲取實例對象
Object instance = holder.get();
//如果沒有,就新建實例對象。這裏是個雙重檢查。意義可參考單例模式
if (instance == null) {
synchronized (holder) {
instance = holder.get();
if (instance == null) {
//創建實現類實例對象
instance = createExtension(name);
//賦值到holder中
holder.set(instance);
}
}
}
return (T) instance;
}
同樣的,也是先檢查緩存,沒有緩存再新建。下面我們看一下是怎麼新建實例對象的,進入到createExtension方法:
private T createExtension(String name) {
//從配置文件中獲取所有實現類
Class<?> clazz = getExtensionClasses().get(name);
if (clazz == null) {
throw findException(name);
}
try {
T instance = (T) EXTENSION_INSTANCES.get(clazz);
if (instance == null) {
//通過反射創建實例
EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());
instance = (T) EXTENSION_INSTANCES.get(clazz);
}
//向實例對象中注入依賴
injectExtension(instance);
Set<Class<?>> wrapperClasses = cachedWrapperClasses;
if (wrapperClasses != null && wrapperClasses.size() > 0) {
for (Class<?> wrapperClass : wrapperClasses) {
//將當前實例對象作爲參數傳給Wrapper的構造方法,並通過反射創建Wrapper對象
//再向wrapper實例對象中注入依賴,最後把wrapper賦值給instance
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);
}
}
上面的方法中,給instance賦值的那行代碼稍微有點複雜,其實最終目的只是把實現類對象包裹在Wrapper對象中。從上面的註釋看,createExtension方法的目的有四個。重點關注getExtensionClasses和injectExtension方法:
private Map<String, Class<?>> getExtensionClasses() {
//從緩存獲取已經加載的實現類
Map<String, Class<?>> classes = cachedClasses.get();
//雙重檢查
if (classes == null) {
synchronized (cachedClasses) {
classes = cachedClasses.get();
if (classes == null) {
//加載實現類
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
依舊是先檢查緩存,沒有緩存再新建。我們進入到loadExtensionClasses方法:
private Map<String, Class<?>> loadExtensionClasses() {
//獲取SPI註解
final SPI defaultAnnotation = type.getAnnotation(SPI.class);
if (defaultAnnotation != null) {
String value = defaultAnnotation.value();
if (value != null && (value = value.trim()).length() > 0) {
//對SPI註解的值進行拆分
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));
}
//設置默認名稱
if (names.length == 1) cachedDefaultName = names[0];
}
}
Map<String, Class<?>> extensionClasses = new HashMap<String, Class<?>>();
//加載指定文件夾下的配置文件
loadFile(extensionClasses, DUBBO_INTERNAL_DIRECTORY);
loadFile(extensionClasses, DUBBO_DIRECTORY);
loadFile(extensionClasses, SERVICES_DIRECTORY);
return extensionClasses;
}
我們看一下loadFile方法指定的目錄常量分別是啥:
可以看到,前面示例說到的目錄就是在這規定的。而META-INF/services/是爲了兼容Java SPI,internal/是Dubbo內部自己的拓展類配置文件。最後我們分析一下loadFile方法:
private void loadFile(Map<String, Class<?>> extensionClasses, String dir) {
//文件夾路徑+接口全限定名=配置文件具體路徑
String fileName = dir + type.getName();
try {
Enumeration<java.net.URL> urls;
ClassLoader classLoader = findClassLoader();
//根據文件名加載所有同名文件
if (classLoader != null) {
urls = classLoader.getResources(fileName);
} else {
urls = ClassLoader.getSystemResources(fileName);
}
//獲取到文件後進行遍歷讀取配置文件
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;
//按行讀取
while ((line = reader.readLine()) != null) {
//解析配置文件
//通過反射加載實現類
//操作緩存
//略。。。,最好自己debug調試一下,最清楚
} // 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);
}
}
獲取實現類的源碼分析的差不多了,現在回到createExtension方法,接着看看injectExtension,也就是Dubbo的依賴注入功能。Dubbo IOC是通過setter方法注入依賴。它會通過反射獲取實例的方法列表,再遍歷方法是否具備setter方法的特徵,若有就通過反射調用這個setter方法將依賴設置到目標對象中。代碼分析如下:
private T injectExtension(T instance) {
try {
if (objectFactory != null) {
//遍歷方法
for (Method method : instance.getClass().getMethods()) {
//判斷方法是否以set開頭,且只有一個參數,且方法是public
if (method.getName().startsWith("set")
&& method.getParameterTypes().length == 1
&& Modifier.isPublic(method.getModifiers())) {
//獲取setter方法參數類型
Class<?> pt = method.getParameterTypes()[0];
try {
//獲取熟悉名,比如setName,其對應的屬性應該是name
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) {
//通過反射調用setter方法完成依賴注入
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;
}