SPI机制
根本概述
SPI全称Service Provider Interface，是一种服务发现机制。通过提供接口、预约义的加载器(Loader)以及约定俗称的配置(个别在META-INF目录下)，能够实现动静加载服务实现类。
类图
通过类图能够剖析出， ServiceLoader 实现了 Iterable 接口，提供了迭代的性能。
而 ServiceLoader 将迭代的实现委托给 LazyIterator 。
LazyIterator 提供了延时迭代的能力，当有须要的时候，才去加载。
在 Skywalking 模块中的应用
接口定义
org.apache.skywalking.oap.server.library.module.ModuleDefine
package org.apache.skywalking.oap.server.library.module;import java.lang.reflect.Field;import java.util.Enumeration;import java.util.Properties;import java.util.ServiceLoader;import org.slf4j.Logger;import org.slf4j.LoggerFactory;/** * A module definition. */public abstract class ModuleDefine implements ModuleProviderHolder { private static final Logger LOGGER = LoggerFactory.getLogger(ModuleDefine.class); private ModuleProvider loadedProvider = null; private final String name; public ModuleDefine(String name) { this.name = name; } /** * @return the module name * */ public final String name() { return name; } /** * @return the {@link Service} provided by this module. */ public abstract Class[] services(); /** * Run the prepare stage for the module, including finding all potential providers, and asking them to prepare. * * @param moduleManager of this module * @param configuration of this module * @throws ProviderNotFoundException when even don't find a single one providers. */ void prepare(ModuleManager moduleManager, ApplicationConfiguration.ModuleConfiguration configuration, ServiceLoader<ModuleProvider> moduleProviderLoader) throws ProviderNotFoundException, ServiceNotProvidedException, ModuleConfigException, ModuleStartException { // etc... } // etc... @Override public final ModuleProvider provider() throws DuplicateProviderException, ProviderNotFoundException { if (loadedProvider == null) { throw new ProviderNotFoundException("There is no module provider in " + this.name() + " module!"); } return loadedProvider; }}接口实现
org.apache.skywalking.oap.server.library.module.BaseModuleA
package org.apache.skywalking.oap.server.library.module;public class BaseModuleA extends ModuleDefine { public BaseModuleA() { super("BaseA"); } // 须要提供服务的接口 @Override public Class<? extends Service>[] services() { return new Class[] { ServiceABusiness1.class, ServiceABusiness2.class }; } public interface ServiceABusiness1 extends Service { void print(); } public interface ServiceABusiness2 extends Service { }}META-INF 定义
META-INF/services/org.apache.skywalking.oap.server.library.module.ModuleDefine
org.apache.skywalking.oap.server.library.module.BaseModuleA应用形式
org.apache.skywalking.oap.server.library.module.ModuleManager#init
/** * Init the given modules */ public void init(ApplicationConfiguration applicationConfiguration) /* etc... */ { // SPI机制加载 ServiceLoaderModuleDefine> moduleServiceLoader = ServiceLoader.load(ModuleDefine.class); // 迭代器获取 for (ModuleDefine module : moduleServiceLoader) { // do something // etc... } // etc... }源码解析
package java.util;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.net.URL;import java.security.AccessController;import java.security.AccessControlContext;import java.security.PrivilegedAction;import java.util.ArrayList;import java.util.Enumeration;import java.util.Iterator;import java.util.List;import java.util.NoSuchElementException;public final class ServiceLoader<S> implements Iterable<S> { // 目录前缀 private static final String PREFIX = "META-INF/services/"; // 须要被加载对象的Class对象 private final Class<S> service; // 类加载器 private final ClassLoader loader; // The access control context taken when the ServiceLoader is created private final AccessControlContext acc; // 加载对象缓存(按实例化程序排序) private LinkedHashMap<String,S> providers = new LinkedHashMap<>(); // 以后应用的懒加载迭代器 private LazyIterator lookupIterator; // 重载 public void reload() { // 革除加载对象缓存 providers.clear(); // 重置懒加载迭代器 lookupIterator = new LazyIterator(service, loader); } // 不容许间接创立ServiceLoader对象，只能通过loadXXX获取ServiceLoader对象 private ServiceLoader(Class<S> svc, ClassLoader cl) { service = Objects.requireNonNull(svc, "Service interface cannot be null"); loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl; acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null; reload(); } private static void fail(Class<?> service, String msg, Throwable cause) throws ServiceConfigurationError { throw new ServiceConfigurationError(service.getName() + ": " + msg, cause); } private static void fail(Class<?> service, String msg) throws ServiceConfigurationError { throw new ServiceConfigurationError(service.getName() + ": " + msg); } private static void fail(Class<?> service, URL u, int line, String msg) throws ServiceConfigurationError { fail(service, u + ":" + line + ": " + msg); } // 解析配置文件中的一行，如果没有正文，则退出到类名列表中 private int parseLine(Class<?> service, URL u, BufferedReader r, int lc, List<String> names) throws IOException, ServiceConfigurationError { String ln = r.readLine(); if (ln == null) { return -1; } int ci = ln.indexOf('#'); if (ci >= 0) ln = ln.substring(0, ci); ln = ln.trim(); int n = ln.length(); if (n != 0) { if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0)) fail(service, u, lc, "Illegal configuration-file syntax"); int cp = ln.codePointAt(0); if (!Character.isJavaIdentifierStart(cp)) fail(service, u, lc, "Illegal provider-class name: " + ln); for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) { cp = ln.codePointAt(i); if (!Character.isJavaIdentifierPart(cp) && (cp != '.')) fail(service, u, lc, "Illegal provider-class name: " + ln); } if (!providers.containsKey(ln) && !names.contains(ln)) names.add(ln); } return lc + 1; } // 解析配置文件，返回实现类名列表 private Iterator<String> parse(Class<?> service, URL u) throws ServiceConfigurationError { InputStream in = null; BufferedReader r = null; ArrayList<String> names = new ArrayList<>(); try { in = u.openStream(); r = new BufferedReader(new InputStreamReader(in, "utf-8")); int lc = 1; while ((lc = parseLine(service, u, r, lc, names)) >= 0); } catch (IOException x) { fail(service, "Error reading configuration file", x); } finally { try { if (r != null) r.close(); if (in != null) in.close(); } catch (IOException y) { fail(service, "Error closing configuration file", y); } } return names.iterator(); } // 懒加载迭代器，提供了延时迭代的能力，当有须要的时候，才去加载 private class LazyIterator implements Iterator<S> { // 须要被加载对象的Class对象 Class<S> service; // 类加载器 ClassLoader loader; // 配置文件列表 Enumeration<URL> configs = null; // 以后迭代的配置文件中类名列表的迭代器 Iterator<String> pending = null; // 下一个实现类名 String nextName = null; private LazyIterator(Class<S> service, ClassLoader loader) { this.service = service; this.loader = loader; } // 是否有下一个Service private boolean hasNextService() { if (nextName != null) { return true; } // 加载所有配置文件 if (configs == null) { try { String fullName = PREFIX + service.getName(); if (loader == null) configs = ClassLoader.getSystemResources(fullName); else configs = loader.getResources(fullName); } catch (IOException x) { fail(service, "Error locating configuration files", x); } } // 当以后类名列表迭代完之后，加载下一个配置文件 while ((pending == null) || !pending.hasNext()) { if (!configs.hasMoreElements()) { return false; } pending = parse(service, configs.nextElement()); } // 获取下一个类名 nextName = pending.next(); return true; } // 获取下一个Service private S nextService() { if (!hasNextService()) throw new NoSuchElementException(); String cn = nextName; nextName = null; Class<?> c = null; try { // 类名 -> 类的Class对象 c = Class.forName(cn, false, loader); } catch (ClassNotFoundException x) { fail(service, "Provider " + cn + " not found"); } if (!service.isAssignableFrom(c)) { fail(service, "Provider " + cn + " not a subtype"); } try { // 实例化 S p = service.cast(c.newInstance()); // 退出到缓存中 providers.put(cn, p); return p; } catch (Throwable x) { fail(service, "Provider " + cn + " could not be instantiated", x); } throw new Error(); // This cannot happen } // 迭代器，是否有下个元素 public boolean hasNext() { if (acc == null) { return hasNextService(); } else { // 受权资源 PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } } // 迭代器，获取下个元素 public S next() { if (acc == null) { return nextService(); } else { // 受权资源 PrivilegedAction<S> action = new PrivilegedAction<S>() { public S run() { return nextService(); } }; return AccessController.doPrivileged(action, acc); } } // 不反对删除 public void remove() { throw new UnsupportedOperationException(); } } // 迭代器实现，如果有缓存从缓存中获取，没有则从懒加载迭代器加载 public Iterator<S> iterator() { return new Iterator<S>() { Iterator<Map.Entry<String,S>> knownProviders = providers.entrySet().iterator(); public boolean hasNext() { if (knownProviders.hasNext()) return true; return lookupIterator.hasNext(); } public S next() { if (knownProviders.hasNext()) return knownProviders.next().getValue(); return lookupIterator.next(); } public void remove() { throw new UnsupportedOperationException(); } }; } // 通过类的Class对象及类加载，获取ServiceLoader public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader) { return new ServiceLoader<>(service, loader); } // 通过类的Class对象，获取ServiceLoader public static <S> ServiceLoader<S> load(Class<S> service) { ClassLoader cl = Thread.currentThread().getContextClassLoader(); return ServiceLoader.load(service, cl); } // 通过类的Class对象和扩大类加载器，获取ServiceLoader public static <S> ServiceLoader<S> loadInstalled(Class<S> service) { ClassLoader cl = ClassLoader.getSystemClassLoader(); ClassLoader prev = null; while (cl != null) { prev = cl; cl = cl.getParent(); } return ServiceLoader.load(service, prev); } public String toString() { return "java.util.ServiceLoader[" + service.getName() + "]"; }}PS： JDK 提供的 SPI 机制，必须要应用迭代器遍历获取须要的实现，而 Dubbo SPI 能够通过 #getExtension 获取指定实现类。
总结
通过源码剖析，能够理解到 Skywalking 没有定义本人的 SPI 机制，但深刻浏览 Skywalking 的应用场景后，发现用 JDK 提供的 SPI 机制也没什么问题。
集体认为，任何技术都应该依据场景选取，适宜的才是最好的，如果没有那么简单的须要，没必要像 dubbo 一样，定义本人的 SPI 机制。
参考文档
- Java Service Provider Interface
Dubbo SPI
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