ServiceLoader整理

• ServiceLoader简单使用
  Search search = null; ServiceLoader<Search> serviceLoader = ServiceLoader.load(Search.class); Iterator<Search> searchs = serviceLoader.iterator(); if (searchs.hasNext()) { search = searchs.next(); }

• 源码分析

  • ServiceLoader.load
    • public static <S> ServiceLoader<S> load(Class<S> service) {
      ClassLoader cl = Thread.currentThread().getContextClassLoader();
      return ServiceLoader.load(service, cl);
      }
      public static <S> ServiceLoader<S> load(Class<S> service, ClassLoader loader)
      {
      return new ServiceLoader<>(service, loader);
      }
      private ServiceLoader(Class<S> svc, ClassLoader cl) {
      service = svc;
      loader = cl;
      reload();
      }
      public void reload() {
      providers.clear();//private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
      lookupIterator = new LazyIterator(service, loader);
      }
      private class LazyIterator implements Iterator<S>//这里叫lazy其实就是调用hasNext的时候才去加载
      {
      ...
      private LazyIterator(Class<S> service, ClassLoader loader) {
      this.service = service;
      this.loader = loader;
      }
      ...
      }
  • iterator()

    • public Iterator<S> iterator() {
        return new Iterator<S>() {//返回匿名内部类
            Iterator<Map.Entry<String,S>> knownProviders = providers.entrySet().iterator();//目前这个linkedhashmap还是空的
            public boolean hasNext() {
                if (knownProviders.hasNext())
                    return true;
                return lookupIterator.hasNext();//knownProviders空时走这个
            }
            public S next() {
                if (knownProviders.hasNext())
                    return knownProviders.next().getValue();
                return lookupIterator.next();
            }
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
      

      }
  • hasNext()
    • public boolean hasNext() {
      if (nextName != null) {//null 调用next后会置nextName为null,不调用next则nextName始终不为null
      return true;
      }
      if (configs == null) {//null
      try {
      String fullName = PREFIX + service.getName();
      if (loader == null)//null
      configs = ClassLoader.getSystemResources(fullName);
      else
      //这里加载资源: META-INF/services/packageName.className
      //Finds all the resources with the given name.
      //A resource is some data (images, audio, text, etc) that can be accessed by class code in a way that is independent of the location of the code.
      configs = loader.getResources(fullName);
      } catch (IOException x) {
      fail(service, "Error locating configuration files", x);
      }
      }
      while ((pending == null) || !pending.hasNext()) {//null
      if (!configs.hasMoreElements()) {
      return false;
      }
      pending = parse(service, configs.nextElement());//go here
      }
      nextName = pending.next();// Iterator<String> pending
      return true;
      }
    • 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();
      }
  • next()
    • public S next() {
      if (!hasNext()) {
      throw new NoSuchElementException();
      }
      String cn = nextName;
      nextName = null;// *** 置nextName为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 {
      S p = service.cast(c.newInstance());// Casts an object to the class or interface represented by this Class object
      providers.put(cn, p);// add cache 放入到knownProviders里面
      return p;
      } catch (Throwable x) {
      fail(service, "Provider " + cn + " could not be instantiated: " + x, x);
      }
      throw new Error();// This cannot happen
      }
  • 小结
    • hasNext和next必须搭配一起使用
      • hasNext获取nextName从一个Iterator<String>的pending中
      • next则拿这个nextName去加载创建Class并创建对象返回
    • 首次从lookupIterator中获取,后面从knownProviders缓存里面获取

• References

  • Java中的SPI(Service Provider Interface)介绍及示例
  • java中的SPI机制