Skywalking-12：Skywalking SPI机制
SPI机制
基本概述
SPI全称Service Provider Interface，是一种服务发现机制。通过提供接口、预定义的加载器(Loader)以及约定俗称的配置(一般在META-INF目录下)，可以实现动态加载服务实现类。
类图
通过类图可以分析出， ServiceLoader 实现了 Iterable 接口，提供了迭代的功能。
而 ServiceLoader 将迭代的实现委托给 LazyIterator 。
LazyIterator 提供了延时迭代的能力，当有需要的时候，才去加载。
在 Skywalking 模块中的使用
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");
} // 需要提供服务的接口
@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
使用方式
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/"; // 需要被加载对象的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(按实例化顺序排序)
private LinkedHashMap<String,S> providers = new LinkedHashMap<>(); // 当前使用的懒加载迭代器
private LazyIterator lookupIterator; // 重载
public void reload() { // 清除加载对象缓存
providers.clear(); // 重置懒加载迭代器
lookupIterator = new LazyIterator(service, loader);
} // 不允许直接创建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，如果没有注释，则加入到类名列表中
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，当有需要的时候，才去加载
private class LazyIterator implements Iterator<S> { // 需要被加载对象的Class对象
Class<S> service; // 类加载器
ClassLoader loader; // 配置文件列表
Enumeration<URL> configs = null; // 当前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);
}
} // 当当前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()); // 加入到缓存中
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 { // 授权资源
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 { // 授权资源
PrivilegedAction<S> action = new PrivilegedAction<S>() { public S run() { return nextService(); }
}; return AccessController.doPrivileged(action, acc);
}
} // 不支持删除
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对象和扩展类加载器，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，必须要使用迭代器遍历获取需要的实现，而 Dubbo SPI 可以通过 #getExtension 获取指定实现类。
总结
通过源码分析，可以了解到 Skywalking 没有定义自己的 SPI 机制，但深入阅读 Skywalking 的使用场景后，发现用 JDK 提供的 SPI 机制也没什么问题。
个人认为，任何技术都应该根据场景选取，适合的才是最好的，如果没有那么复杂的需要，没必要像 dubbo 一样，定义自己的 SPI 机制。
来源 https://www.cnblogs.com/switchvov/p/15363105.html