聊聊如何实现一个支持键值对的SPI

前言

如果用过JDK提供的SPI机制的朋友，大概就会知道它无法按需加载。之前写过一篇文章聊聊基于jdk实现的spi如何与spring整合实现依赖注入。利用spring的依赖注入来实现spi按需加载，这种方案就是要借用spring。今天我们在聊聊另外一种实现方式，就是我们自己手写一个

实现思路

整体思路和jdk实现spi差不多，如果对jdk实现的spi不了解，可以查看我之前写的文章java之spi机制简介。差别就是我们在配置文件是以key-value的形式存在，形如

springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect 

实现逻辑

1、约定好要进行解析的目录，比如META-INF/services/

 private static final String SERVICE_DIRECTORY = "META-INF/services/"; 

2、约定好要解析的文件名命名，比如

com.github.lybgeek.dialect.SpringSqlDialect 

3、约定好文件内容格式，比如

springMysql=com.github.lybgeek.dialect.mysql.SpringMysqlDialect 

4、获取约定好的目录，解析文件，并将相应内容放入缓存

 /** * Load files under SERVICE_DIRECTORY. */ private void loadDirectory(final Map<String, Class<?>> classes) { String fileName = SERVICE_DIRECTORY + clazz.getName(); try { ClassLoader classLoader = ExtensionLoader.class.getClassLoader(); Enumeration<URL> urls = classLoader != null ? classLoader.getResources(fileName) : ClassLoader.getSystemResources(fileName); if (urls != null) { 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, Class<?>> 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, Class<?>> classes, final String name, final String classPath) throws ClassNotFoundException { Class<?> subClass = Class.forName(classPath); if (!clazz.isAssignableFrom(subClass)) { throw new IllegalStateException("load extension resources error," + subClass + " subtype is not of " + clazz); } Activate annotation = subClass.getAnnotation(Activate.class); if (annotation == null) { throw new IllegalStateException("load extension resources error," + subClass + " with Activate annotation"); } Class<?> oldClass = classes.get(name); if (oldClass == null) { classes.put(name, subClass); } else if (oldClass != subClass) { throw new IllegalStateException("load extension resources error,Duplicate class " + clazz.getName() + " name " + name + " on " + oldClass.getName() + " or " + subClass.getName()); } } 

5、根据key，去缓存查找相应的类实例

 public T getActivate(final String name) { if (StringUtils.isBlank(name)) { throw new NullPointerException("get Activate name is null"); } Holder<Object> objectHolder = cachedInstances.get(name); if (objectHolder == null) { cachedInstances.putIfAbsent(name, new Holder<>()); objectHolder = cachedInstances.get(name); } Object value = objectHolder.getValue(); if (value == null) { synchronized (cachedInstances) { value = objectHolder.getValue(); if (value == null) { value = createExtension(name); objectHolder.setValue(value); } } } return (T) value; } 

核心代码

@Slf4j @SuppressWarnings("all") public final class ExtensionLoader<T> { private static final String SERVICE_DIRECTORY = "META-INF/services/"; private static final Map<Class<?>, ExtensionLoader<?>> LOADERS = new ConcurrentHashMap<>(); private final Class<T> clazz; private final Holder<Map<String, Class<?>>> cachedClasses = new Holder<>(); private final Map<String, Holder<Object>> cachedInstances = new ConcurrentHashMap<>(); private final Map<Class<?>, Object> ActivateInstances = new ConcurrentHashMap<>(); private String cachedDefaultName; /** * Instantiates a new Extension loader. * * @param clazz the clazz. */ private ExtensionLoader(final Class<T> clazz) { this.clazz = clazz; if (clazz != ExtensionFactory.class) { ExtensionLoader.getExtensionLoader(ExtensionFactory.class).getExtensionClasses(); } } /** * Gets extension loader. * * @param <T> the type parameter * @param clazz the clazz * @return the extension loader. */ public static <T> ExtensionLoader<T> getExtensionLoader(final Class<T> clazz) { if (clazz == null) { throw new NullPointerException("extension clazz is null"); } if (!clazz.isInterface()) { throw new IllegalArgumentException("extension clazz (" + clazz + ") is not interface!"); } if (!clazz.isAnnotationPresent(SPI.class)) { throw new IllegalArgumentException("extension clazz (" + clazz + ") without @" + SPI.class + " Annotation"); } ExtensionLoader<T> extensionLoader = (ExtensionLoader<T>) LOADERS.get(clazz); if (extensionLoader != null) { return extensionLoader; } LOADERS.putIfAbsent(clazz, new ExtensionLoader<>(clazz)); return (ExtensionLoader<T>) LOADERS.get(clazz); } /** * Gets default Activate. * * @return the default Activate. */ public T getDefaultActivate() { getExtensionClasses(); if (StringUtils.isBlank(cachedDefaultName)) { return null; } return getActivate(cachedDefaultName); } /** * Gets Activate. * * @param name the name * @return the Activate. */ public T getActivate(final String name) { if (StringUtils.isBlank(name)) { throw new NullPointerException("get Activate name is null"); } Holder<Object> objectHolder = cachedInstances.get(name); if (objectHolder == null) { cachedInstances.putIfAbsent(name, new Holder<>()); objectHolder = cachedInstances.get(name); } Object value = objectHolder.getValue(); if (value == null) { synchronized (cachedInstances) { value = objectHolder.getValue(); if (value == null) { value = createExtension(name); objectHolder.setValue(value); } } } return (T) value; } public Set<String> getSupportedExtensions() { Map<String, Class<?>> clazzes = getExtensionClasses(); return Collections.unmodifiableSet(new TreeSet<>(clazzes.keySet())); } @SuppressWarnings("unchecked") private T createExtension(final String name) { Class<?> aClass = getExtensionClasses().get(name); if (aClass == null) { throw new IllegalArgumentException("name is error"); } Object o = ActivateInstances.get(aClass); if (o == null) { try { ActivateInstances.putIfAbsent(aClass, aClass.newInstance()); o = ActivateInstances.get(aClass); } catch (InstantiationException | IllegalAccessException e) { throw new IllegalStateException("Extension instance(name: " + name + ", class: " + aClass + ") could not be instantiated: " + e.getMessage(), e); } } return (T) o; } /** * Gets extension classes. * * @return the extension classes */ public Map<String, Class<?>> getExtensionClasses() { Map<String, Class<?>> classes = cachedClasses.getValue(); if (classes == null) { synchronized (cachedClasses) { classes = cachedClasses.getValue(); if (classes == null) { classes = loadExtensionClass(); cachedClasses.setValue(classes); } } } return classes; } private Map<String, Class<?>> loadExtensionClass() { SPI annotation = clazz.getAnnotation(SPI.class); if (annotation != null) { String value = annotation.value(); if (StringUtils.isNotBlank(value)) { cachedDefaultName = value; } } Map<String, Class<?>> classes = new HashMap<>(16); loadDirectory(classes); return classes; } /** * Load files under SERVICE_DIRECTORY. */ private void loadDirectory(final Map<String, Class<?>> classes) { String fileName = SERVICE_DIRECTORY + clazz.getName(); try { ClassLoader classLoader = ExtensionLoader.class.getClassLoader(); Enumeration<URL> urls = classLoader != null ? classLoader.getResources(fileName) : ClassLoader.getSystemResources(fileName); if (urls != null) { 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, Class<?>> 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, Class<?>> classes, final String name, final String classPath) throws ClassNotFoundException { Class<?> subClass = Class.forName(classPath); if (!clazz.isAssignableFrom(subClass)) { throw new IllegalStateException("load extension resources error," + subClass + " subtype is not of " + clazz); } Activate annotation = subClass.getAnnotation(Activate.class); if (annotation == null) { throw new IllegalStateException("load extension resources error," + subClass + " with Activate annotation"); } Class<?> oldClass = classes.get(name); if (oldClass == null) { classes.put(name, subClass); } else if (oldClass != subClass) { throw new IllegalStateException("load extension resources error,Duplicate class " + clazz.getName() + " name " + name + " on " + oldClass.getName() + " or " + subClass.getName()); } } /** * The type Holder. * * @param <T> the type parameter. */ public static 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; } } } 

使用示例

1、定义服务接口

@SPI("mysql") public interface SqlDialect { String dialect(); } 

2、定义具体实现类

@Activate public class MysqlDialect implements SqlDialect { @Override public String dialect() { return "mysql"; } } 

@Activate public class OracleDialect implements SqlDialect { @Override public String dialect() { return "oracle"; } } 

3、src/main/resources/下建立/META-INF/services 目录，新增一个以接口命名的文件

4、接口命名的文件填入如下内容

5、加载服务类

 SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql"); 

6、测试

 @Test public void testSpi(){ SqlDialect sqlDialect = ExtensionLoader.getExtensionLoader(SqlDialect.class).getActivate("mysql"); Assert.assertEquals("mysql",sqlDialect.dialect()); } 

总结

如果有用过dubbo的spi的朋友，就会发现上面实现的思路基本上就是dubbo的spi简化版。如果是有了解过shenyu网关的spi机制的朋友，就会发现上面的实现思路和shenyu网关基本上是一样了。

demo链接

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