再聊SPI机制

前言

去年更新了一系列和SPI相关的内容，最近因为业务需要，我又基于业务场景，实现了一版。对于什么是spi，很久之前有写过一篇文章，java之spi机制简介感兴趣的朋友可以蛮看一下

需求分析

用过原生jdk提供的spi的朋友，应该会知道原生jdk的spi有个缺陷，就是没法实现按需加载，因此本文的实现就是来解决这个问题。

自定义SPI

核心代码

@Slf4j public final class SpiLoader<T> { private static final String SPI_DIRECTORY = "META-INF/spi/"; private static final Map<Class<?>, SpiLoader<?>> LOADERS = new ConcurrentHashMap<>(); private final Class<T> clazz; private final ClassLoader classLoader; private final Holder<Map<String, ClassEntity>> cachedClasses = new Holder<>(); private final Map<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>(); private final Map<Class<?>, Object> targetInstances = new ConcurrentHashMap<>(); /** * Instantiates a new Extension loader. * * @param clazz the clazz. */ private SpiLoader(final Class<T> clazz, final ClassLoader cl) { this.clazz = clazz; this.classLoader = cl; if (!Objects.equals(clazz, SpiFactory.class)) { SpiLoader.getExtensionLoader(SpiFactory.class).getExtensionClassesEntity(); } } /** * Gets extension loader. * * @param <T> the type parameter * @param clazz the clazz * @param cl the cl * @return the extension loader. */ public static <T> SpiLoader<T> getExtensionLoader(final Class<T> clazz, final ClassLoader cl) { Objects.requireNonNull(clazz, "extension clazz is null"); if (!clazz.isInterface()) { throw new IllegalArgumentException("extension clazz (" + clazz + ") is not interface!"); } SpiLoader<T> extensionLoader = (SpiLoader<T>) LOADERS.get(clazz); if (Objects.nonNull(extensionLoader)) { return extensionLoader; } LOADERS.putIfAbsent(clazz, new SpiLoader<>(clazz, cl)); return (SpiLoader<T>) LOADERS.get(clazz); } /** * Gets extension loader. * * @param <T> the type parameter * @param clazz the clazz * @return the extension loader */ public static <T> SpiLoader<T> getExtensionLoader(final Class<T> clazz) { return getExtensionLoader(clazz, SpiLoader.class.getClassLoader()); } /** * Gets target. * * @param name the name * @return the target. */ public T getTarget(final String name) { if (StringUtils.isBlank(name)) { throw new NullPointerException("get target name is null"); } Holder<Object> objectHolder = cachedInstances.get(name); if (Objects.isNull(objectHolder)) { cachedInstances.putIfAbsent(name, new Holder<>()); objectHolder = cachedInstances.get(name); } Object value = objectHolder.getValue(); if (Objects.isNull(value)) { synchronized (cachedInstances) { value = objectHolder.getValue(); if (Objects.isNull(value)) { createExtension(name, objectHolder); value = objectHolder.getValue(); } } } return (T) value; } /** * get all target spi. * * @return list. target */ public List<T> getTargets() { Map<String, ClassEntity> extensionClassesEntity = this.getExtensionClassesEntity(); if (extensionClassesEntity.isEmpty()) { return Collections.emptyList(); } if (Objects.equals(extensionClassesEntity.size(), cachedInstances.size())) { return (List<T>) this.cachedInstances.values().stream() .map(Holder::getValue).collect(Collectors.toList()); } List<T> targets = new ArrayList<>(); List<ClassEntity> classEntities = new ArrayList<>(extensionClassesEntity.values()); classEntities.forEach(v -> { T target = this.getTarget(v.getName()); targets.add(target); }); return targets; } @SuppressWarnings("unchecked") private void createExtension(final String name, final Holder<Object> holder) { ClassEntity classEntity = getExtensionClassesEntity().get(name); if (Objects.isNull(classEntity)) { throw new IllegalArgumentException(name + " name is error"); } Class<?> aClass = classEntity.getClazz(); Object o = targetInstances.get(aClass); if (Objects.isNull(o)) { try { targetInstances.putIfAbsent(aClass, aClass.newInstance()); o = targetInstances.get(aClass); } catch (InstantiationException | IllegalAccessException e) { throw new IllegalStateException("Extension instance(name: " + name + ", class: " + aClass + ") could not be instantiated: " + e.getMessage(), e); } } holder.setValue(o); } /** * Gets extension classes. * * @return the extension classes */ public Map<String, Class<?>> getTargetClassesMap() { Map<String, ClassEntity> classes = this.getExtensionClassesEntity(); return classes.values().stream().collect(Collectors.toMap(ClassEntity::getName, ClassEntity::getClazz, (a, b) -> a)); } private Map<String, ClassEntity> getExtensionClassesEntity() { Map<String, ClassEntity> classes = cachedClasses.getValue(); if (Objects.isNull(classes)) { synchronized (cachedClasses) { classes = cachedClasses.getValue(); if (Objects.isNull(classes)) { classes = loadExtensionClass(); cachedClasses.setValue(classes); } } } return classes; } private Map<String, ClassEntity> loadExtensionClass() { Map<String, ClassEntity> classes = new HashMap<>(16); loadDirectory(classes); return classes; } /** * Load files under SPI_DIRECTORY. */ private void loadDirectory(final Map<String, ClassEntity> classes) { String fileName = SPI_DIRECTORY + clazz.getName(); try { Enumeration<URL> urls = Objects.nonNull(this.classLoader) ? classLoader.getResources(fileName) : ClassLoader.getSystemResources(fileName); if (Objects.nonNull(urls)) { while (urls.hasMoreElements()) { URL url = urls.nextElement(); loadResources(classes, url); } } } catch (IOException t) { log.error("load extension class error {}", fileName, t); } } private void loadResources(final Map<String, ClassEntity> classes, final URL url) throws IOException { try (InputStream inputStream = url.openStream()) { Properties properties = new Properties(); properties.load(inputStream); properties.forEach((k, v) -> { String name = (String) k; String classPath = (String) v; if (StringUtils.isNotBlank(name) && StringUtils.isNotBlank(classPath)) { try { loadClass(classes, name, classPath); } catch (ClassNotFoundException e) { throw new IllegalStateException("load extension resources error", e); } } }); } catch (IOException e) { throw new IllegalStateException("load extension resources error", e); } } private void loadClass(final Map<String, ClassEntity> classes, final String name, final String classPath) throws ClassNotFoundException { Class<?> subClass = Objects.nonNull(this.classLoader) ? Class.forName(classPath, true, this.classLoader) : Class.forName(classPath); if (!clazz.isAssignableFrom(subClass)) { throw new IllegalStateException("load extension resources error," + subClass + " subtype is not of " + clazz); } ClassEntity oldClassEntity = classes.get(name); if (Objects.isNull(oldClassEntity)) { ClassEntity classEntity = new ClassEntity(name, subClass); classes.put(name, classEntity); } else if (!Objects.equals(oldClassEntity.getClazz(), subClass)) { throw new IllegalStateException("load extension resources error,Duplicate class " + clazz.getName() + " name " + name + " on " + oldClassEntity.getClazz().getName() + " or " + subClass.getName()); } } /** * The type Holder. * * @param <T> the type parameter. */ private static final class Holder<T> { private volatile T value; /** * Gets value. * * @return the value */ public T getValue() { return value; } /** * Sets value. * * @param value the value */ public void setValue(final T value) { this.value = value; } } private static final class ClassEntity { /** * name. */ private final String name; /** * class. */ private Class<?> clazz; private ClassEntity(final String name, final Class<?> clazz) { this.name = name; this.clazz = clazz; } /** * get class. * * @return class. */ public Class<?> getClazz() { return clazz; } /** * set class. * * @param clazz class. */ public void setClazz(final Class<?> clazz) { this.clazz = clazz; } /** * get name. * * @return name. */ public String getName() { return name; } } } 

代码解读：

从classpath类路径下查找/META-INF/spi/接口文件，并解析相关文件，将解析后的key和class类名放入本地缓存，最后根据业务实际需要，按需将class实例化为对象

示例

以mock一个不同日志门面打印为例子

1、创建日志接口

public interface LogService { void info(String msg); } 

2、创建日志实现

public class Log4jService implements LogService { @Override public void info(String msg) { System.out.println(Log4jService.class.getName() + " info: " + msg); } } 

3、在具体实现的classpath目录下创建

/META-INF/spi/com.github.lybgeek.log.LogService文件，并填入如下内容

log4j=com.github.lybgeek.log.Log4jService 

4、测试

public class LogMainTest { public static void main(String[] args) { LogService logService = SpiLoader.getExtensionLoader(LogService.class).getTarget("log4j2"); logService.info("log4j2-hello"); logService = SpiFactoriesLoader.loadFactories().getTarget("log4j",LogService.class); logService.info("log4j-hello"); } } 

可以看到控制台输出如下内容

com.github.lybgeek.log.Log4j2Service info: log4j2-hello com.github.lybgeek.log.Log4jService info: log4j-hello 

总结

本文主要是实现原生SPI不支持按需加载的能力，其次本文的核心实现其实是搬dubbo的spi能力，因为我们业务场景比较简单，并不需要dubbo那么灵活的spi能力，因此在实现时，就仅仅搬了dubbo的一部分能力扩展

demo链接

https://github.com/lyb-geek/springboot-learning/tree/master/springboot-custom-spi