关于java:SPI

尝试 pigeon 拦截器的时候，发现加载自定义拦截器的时候是通过 spi serviceloader 加载的，学习一波 spi。

某个接口，实现有多种，spi 能够为某个接口提供具体的实现类。先看下在 spi 机制下怎么在 pigeon 中自定义拦截器：

// 自定义了一个拦截器，实现了 pigeon 提供了拦截器接口
public class BusinessProviderInterceptor implements ProviderInterceptor {
    @Override
    public void preInvoke(ProviderContext providerContext) {LogTagContext.setLogTag("mobileno", "12345678");
    }
    @Override
    public void postInvoke(ProviderContext providerContext) {}

META-INF/services 下创立一个以接口命名的文件，文件里按行排列具体的实现类；文件里内容如下：

xxx.xxx.xxx.xxx.BusinessProviderInterceptor
#other selfinterceptor which implement ProviderInterceptor

以上，能够在 pigeon 中实现自定义拦截器。

pigeon 是如何加载实例化这些拦截器的，具体如下：

//providerinterceptor 的执行在 BuinessProcessFilter 里，ProviderProcessInterceptorFactory 在废除，新的是 ProviderInterceptorFactory
List<ProviderProcessInterceptor> interceptors = ProviderProcessInterceptorFactory.getInterceptors();
for (ProviderProcessInterceptor interceptor : interceptors) {interceptor.preInvoke(request);
}
List<ProviderInterceptor> contextInterceptors = ProviderInterceptorFactory.getInterceptors();
for (ProviderInterceptor interceptor : contextInterceptors) {interceptor

public class ProviderInterceptorFactory {
   // 通过 ExtensionLoader 来加载 interceptor
   private static List<ProviderInterceptor> interceptors = ExtensionLoader
         .getExtensionList(ProviderInterceptor.class);
   // 也能够手动调用这个办法来注册
   public static boolean registerInterceptor(ProviderInterceptor providerContextInterceptor) {if (!interceptors.contains(providerContextInterceptor)) {return interceptors.add(providerContextInterceptor);
      }
      return false;
   }
   public static boolean unregisterInterceptor(ProviderInterceptor providerContextInterceptor) {return interceptors.remove(providerContextInterceptor);
   }
   public static List<ProviderInterceptor> getInterceptors() {return interceptors;}
}

public final class ExtensionLoader {

   // 应用 map 来存储接口对应的实现，如果 map 中没有，则通过 serviceloader 来加载
   private static Map<Class<?>, Object> extensionMap = new ConcurrentHashMap<Class<?>, Object>();

   private static Map<Class<?>, List<?>> extensionListMap = new ConcurrentHashMap<Class<?>, List<?>>();

   private ExtensionLoader() {}

   public static <T> T getExtension(Class<T> clazz) {T extension = (T) extensionMap.get(clazz);
      if (extension == null) {extension = newExtension(clazz);
         if (extension != null) {extensionMap.put(clazz, extension);
         }
      }
      return extension;
   }

   public static <T> List<T> getExtensionList(Class<T> clazz) {List<T> extensions = (List<T>) extensionListMap.get(clazz);
      if (extensions == null) {extensions = newExtensionList(clazz);
         if (!extensions.isEmpty()) {extensionListMap.put(clazz, extensions);
         }
      }
      return extensions;
   }

   public static <T> T newExtension(Class<T> clazz) {
      //  here
      ServiceLoader<T> serviceLoader = ServiceLoader.load(clazz);
      for (T service : serviceLoader) {return service;}
      return null;
   }

   public static <T> List<T> newExtensionList(Class<T> clazz) {ServiceLoader<T> serviceLoader = ServiceLoader.load(clazz);
      List<T> extensions = new ArrayList<T>();
      for (T service : serviceLoader) {extensions.add(service);
      }
      return extensions;
   }
}

重点就是 ServiceLoader.load

// ServiceLoader 实现了 Iterable
public final class ServiceLoader<S>
    implements Iterable<S>
{
    // 目录门路
    private static final String PREFIX = "META-INF/services/";

    // 正在加载的类或接口
    private Class<S> service;

    // 用来实例化的 classloader
    private ClassLoader loader;

    // 按实例化的程序缓存实现类
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

    //ServiceLoader 自定义的一个外部类，次要实现了懒加载
    private LazyIterator lookupIterator;

    
    public void reload() {providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

    private ServiceLoader(Class<S> svc, ClassLoader cl) {
        service = svc;
        loader = cl;
        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);
    }

    // 解析文件，将文件中的 name 放到 list 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);
            }
            // 加载过的不放在这个 list 里
            if (!providers.containsKey(ln) && !names.contains(ln))
                names.add(ln);
        }
        return lc + 1;
    }

   
    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();}

    // 懒加载的实现
    private class LazyIterator
        implements Iterator<S>
    {

        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;

        private LazyIterator(Class<S> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }

        public boolean hasNext() {if (nextName != null) {return true;}
            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);
                }
            }
            while ((pending == null) || !pending.hasNext()) {if (!configs.hasMoreElements()) {return false;}
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

        public S next() {if (!hasNext()) {throw new NoSuchElementException();
            }
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {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 {
                // 在 next 到某个实现类时，才真正的加载实例
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider" + cn + "could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen}

        public void remove() {throw new UnsupportedOperationException();
        }

    }

    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();
            }

        };
    }
    public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader)
    {return new ServiceLoader<>(service, loader);
    }

  
    public static <S> ServiceLoader<S> load(Class<S> service) {ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

    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);
    }
}