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> classes) {
        String fileName = SERVICE_DIRECTORY + clazz.getName();
        try {
            ClassLoader classLoader = ExtensionLoader.class.getClassLoader();
            Enumeration 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> 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> 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