学习手写Dubbo中的SPI
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Dubbo的SPI是什么
SPI的全称是Service Provider Interface，是Java提供的一套用来被第三方实现或者扩展的API，SPI的作用就是为这些被扩展的API寻找服务实现。
Dubbo核心特点
- SPI机制(按需加载)
- 自适应扩展点
- IOC和AOP
- Dubbo怎么和Spring集成
Dubbo配置文件约定
- META-INF/services/ 目录：该目录下的 SPI 配置文件是为了用来兼容 Java SPI 。
- META-INF/dubbo/ 目录：该目录存放用户自定义的 SPI 配置文件。
- META-INF/dubbo/internal/ 目录：该目录存放 Dubbo 内部使用的 SPI 配置文件。
Dubbo SPI源码实例
SPI注解
package com.spi.annotation;
import java.lang.annotation.*;
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface SPI
/**
* 扩展点名。
*/
String value() default "";
加载扩展
package com.spi.loader;
import com.spi.annotation.SPI;
import com.spi.util.Holder;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.Arrays;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.regex.Pattern;
public class ExtensionLoader<T>
/**
* 定义SPI文件的扫描路径,dubbo源码中设置了多个,我们这里只设置一个
*/
private static final String DIRECTORY = "META-INF/dubbo/";
/**
* 分割SPI上默认拓展点字符串用的
*/
private static final Pattern NAME_SEPARATOR = Pattern.compile("\\\\s*[,]+\\\\s*");
/**
* 拓展点加载器的缓存
*/
private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS =
new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
/**
* 拓展点的缓存
*/
private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>();
/**
* 接口的class
*/
private final Class<?> type;
/**
* 接口SPI默认的实现名(就是把接口上填的默认值)
*/
private String cachedDefaultName;
/**
* 异常记录
*/
private Map<String, IllegalStateException> exceptions = new ConcurrentHashMap<String, IllegalStateException>();
//这两个缓存文字比较难描述,debug或者搜索一下调用就知道是缓存什么的了
private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String, Class<?>>>();
private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>();
private static <T> boolean withExtensionAnnotation(Class<T> type)
return type.isAnnotationPresent(SPI.class);
private ExtensionLoader(Class<T> type)
this.type = type;
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type)
if (type == null)//1.拓展点类型非空判断
throw new IllegalArgumentException("Extension type == null");
if (!type.isInterface()) //2.拓展点类型只能是接口
throw new IllegalArgumentException("Extension type(" + type + ") is not interface!");
if (!withExtensionAnnotation(type)) //3.需要添加spi注解,否则抛异常
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
//从缓存EXTENSION_LOADERS中获取,如果不存在则新建后加入缓存
//对于每一个拓展,都会有且只有一个ExtensionLoader与其对应
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;
/**
* 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;
//根据获取到的拓展点class实例化成对象返回
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());//反射生成对象
instance = (T) EXTENSION_INSTANCES.get(clazz);
return instance;
catch (Throwable t)
throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
type + ") could not be instantiated: " + t.getMessage(), t);
//获取拓展点class,并缓存
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;
//1.设置接口默认的实现类名 2.加载文件
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,DIRECTORY);
return extensionClasses;
//加载解析spi配置文件,然后加入缓存
public void loadFile(Map<String, Class<?>> extensionClasses, String dir)
String fileName = dir + type.getName();
try
Enumeration<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.");
extensionClasses.put(name, clazz);//加入缓存
//源码中还有其他的判断,这个版本暂不实现
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 e)
//logger.error("Exception when load extension class(interface: " + type + ", description file: " + fileName + ").", e);
//获取类加载器
private static ClassLoader findClassLoader()
return ExtensionLoader.class.getClassLoader();
//获取默认拓展点
public T getDefaultExtension()
getExtensionClasses();
if (null == cachedDefaultName || cachedDefaultName.length() == 0
|| "true".equals(cachedDefaultName))
return null;
return getExtension(cachedDefaultName);
//异常提示
private IllegalStateException findException(String name)
for (Map.Entry<String, IllegalStateException> entry : exceptions.entrySet())
if (entry.getKey().toLowerCase().以上是关于学习手写Dubbo中的SPI的主要内容，如果未能解决你的问题，请参考以下文章