改写自：
Dubbo 源码
实现效果为两种调用方式:
//1. 指定实现类
Message msg = ExtensionLoader.getExtensionLoader(Serializer.class).getExtension(SerializationTypeEnum.getName(serializeType)).deserialize(clazz, bytes);
//2. 注解上指定接口实现类
Message msg = ExtensionLoader.getExtensionLoader(Serializer.class).getExtension().deserialize(clazz, bytes);ExtensionLoader:
package com.fastrpc.extension;
import com.sun.org.apache.bcel.internal.generic.IF_ACMPEQ;
import lombok.extern.slf4j.Slf4j;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/**
* 效果: 通过getExtenison可以接口的实现类 接口上必须有SPI注解
* 指定接口实现类有两种方法
* 1.getExtension方法中传过参获取实现类
* 2.如果getExtension没有传参,则从SPI获取默认实现类
* 下面对应了两个重载的放啊
* @author: @zyz
*/
@Slf4j
public class ExtensionLoader<T> {
private static final String SERVICES_DIRECTORY = "META-INF/services/";
//为每一个接口保存一个加载类
private static final ConcurrentMap<Class<?>, ExtensionLoader<?>> EXTENSION_LOADERS = new ConcurrentHashMap<Class<?>, ExtensionLoader<?>>();
private static final ConcurrentMap<Class<?>, Object> EXTENSION_INSTANCES = new ConcurrentHashMap<Class<?>, Object>();
private final Class<?> type;
//缓存接口的实现类的Class
private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<Map<String, Class<?>>>();
// 保存Class实例化过的类
private final ConcurrentMap<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<String, Holder<Object>>();
private volatile String cachedDefaultName;
private ExtensionLoader(Class<?> type) {
this.type = type;
}
/**
* 每一个接口维持一个类构造器
* @param type
* @param <T>
* @return
*/
public static <T> ExtensionLoader<T> getExtensionLoader(Class<T> type)
{
if (type==null)
throw new IllegalArgumentException("Extension type == null");
if (!type.isInterface())
throw new IllegalArgumentException("Extenison type ("+type+") is not interface!");
if (!withExtensionAnnotation(type))
throw new IllegalArgumentException("Extension type(" + type +
") is not extension, because WITHOUT @" + SPI.class.getSimpleName() + " Annotation!");
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;
}
/**
* 判断接口是否是SPI注解
* @param type
* @return
*/
public static <T> boolean withExtensionAnnotation(Class<T> type)
{
return (type.isAnnotationPresent(SPI.class));
}
/**
* SPI指定默认实现类接口
*/
public T getExtension()
{
return getExtension("default");
}
/**
* 根据给的名字获取具体的实现类
* @param name
* @return
*/
public T getExtension(String name)
{
if (name==null||name.length()==0)
throw new IllegalArgumentException("Extension name == null");
//没有指定具体实现类则从SPI中获取指定的实现类
if("default".equalsIgnoreCase(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();
// double check
if (instance == null)
{
synchronized (holder)
{
instance=holder.get();
if (instance==null)
{
instance=createExtension(name);
holder.set(instance);
}
}
}
return (T) instance;
}
private T createExtension(String name) {
Class<?> clazz=getExtensionClasses().get(name);
if (clazz==null)
{
throw new IllegalArgumentException("No such extension "+name);
}
// 注入
T instance= (T) EXTENSION_INSTANCES.get(clazz);
if (instance==null){
try {
EXTENSION_INSTANCES.putIfAbsent(clazz,clazz.newInstance());
instance= (T) EXTENSION_INSTANCES.get(clazz);
} catch (Exception e) {
log.error(e.getMessage());
}
}
return instance;
}
private Map<String, Class<?>> getExtensionClasses() {
Map<String,Class<?>> classes=cachedClasses.get();
if (classes==null)
{
synchronized (cachedClasses)
{
classes=cachedClasses.get();
if (classes==null)
{
// 加载该接口配置的所有扩展类
classes = loadExtensionClasses();
cachedClasses.set(classes);
}
}
}
return classes;
}
/**
* 从 .extensions文件中加载实现类
* @return
*/
private Map<String, Class<?>> loadExtensionClasses() {
Map<String,Class<?>> extensionClasses=new HashMap<>();
loadDirectory(extensionClasses,SERVICES_DIRECTORY);
return extensionClasses;
}
private void loadDirectory(Map<String, Class<?>> extensionClasses, String dir) {
String fileName= dir+ type.getName();
Enumeration<URL> urls;
ClassLoader classLoader=findClassLoader();
try {
if (classLoader!=null)
{
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 (IOException e) {
log.error("Exception when load extension class(interface: " +
type + ", description file: " + fileName + ").", e);
}
}
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) {
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&&line.length()>0) {
loadClass(extensionClasses, classLoader.loadClass(line), name);
}
} catch (Throwable t) {
throw new IllegalStateException("Failed to load extension class(interface: " + type + ", class line: " + line + ") in " + resourceURL + ", cause: " + t.getMessage(), t);
}
}
}
} finally {
reader.close();
}
} catch (Throwable t) {
log.error("Exception when load extension class(interface: " +
type + ", class file: " + resourceURL + ") in " + resourceURL, t);
}
}
/**
* 缓存type接口的所有实现类 如 gzip（key）=全类名(value)
* @param extensionClasses
* @param clazz
* @param name
*/
private void loadClass(Map<String, Class<?>> extensionClasses, Class<?> clazz, String name) {
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);
}
private ClassLoader findClassLoader() {
return ExtensionLoader.class.getClassLoader();
}
/**
* 加载SPI注解中指定的实现类
* @return
*/
private T getDefaultExtension() {
if (cachedDefaultName==null)
{
SPI defaultAnnotation=type.getAnnotation(SPI.class);
if (defaultAnnotation !=null)
{
String value=defaultAnnotation.value();
if((value=value.trim()).length()>0)
{
cachedDefaultName=value;
}
}else{
throw new IllegalArgumentException("The implementation class must be specified");
}
}
return getExtension(cachedDefaultName);
}
}
SPI:
package com.fastrpc.extension;
import java.lang.annotation.*;
/**
*
* @author: @zyz
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface SPI {
/**
* default extension name
*/
String value() default "";
}
Holder:
package com.fastrpc.extension;
/**
*
* @author: @zyz
*/
public class Holder<T> {
private volatile T value;
public T get() {
return value;
}
public void set(T value) {
this.value = value;
}
}