概念

SPI（service provider interface），是JDK内置的一种服务提供发现机制。是一种动态替换发现机制，比如有个接口，想在运行时动态地给它添件实现，只需要添加一个实现。

然后在META-INF/services目录创建一个以服务接口命名的文件，这个文件里的内容就是这个接口的具体的实现类（全限定名）。

源码实现

• 创建ServiceLoader

  ServiceLoader.load(Driver.class);
  
  
   public void reload() {
          providers.clear();
          lookupIterator = new LazyIterator(service, loader);
      }
  
      private ServiceLoader(Class<S> svc, ClassLoader cl) {
          service = Objects.requireNonNull(svc, "Service interface cannot be null");
          loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
          acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
          reload();
      }
  

• 服务实现获取

• 初始化LazyIterator

  public Iterator<S> iterator() {
      return new Iterator<S>() {
  				//ServiceLoader.LazyIterator的封装对象
          Iterator<Map.Entry<String,S>> knownProviders
              = providers.entrySet().iterator();
  
          public boolean hasNext() {
          //ServiceLoader已经从classPath读取过从构造参数传递进来的指定接口的子类，则会将全限定类名和对应实例缓存起来
              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();
          }
  
      };
  }
  

• 调用layIterator的forEachRemaining方法

  default void forEachRemaining(Consumer<? super E> action) {
          Objects.requireNonNull(action);
          while (hasNext())
              action.accept(next());
      }
  
  private class LazyIterator
      implements Iterator<S>
  {
      private boolean hasNextService() {
         ....
      }
  
      private S nextService() {
         ....
      }
  
      public boolean hasNext() {
          if (acc == null) {
              return hasNextService();
          } else {
              PrivilegedAction<Boolean> action = new PrivilegedAction<Boolean>() {
                  public Boolean run() { return hasNextService(); }
              };
              return AccessController.doPrivileged(action, acc);
          }
      }
  
      public S next() {
          if (acc == null) {
              return nextService();
          } else {
              PrivilegedAction<S> action = new PrivilegedAction<S>() {
                  public S run() { return nextService(); }
              };
              return AccessController.doPrivileged(action, acc);
          }
      }
  
      public void remove() {
          throw new UnsupportedOperationException();
      }
  
  }
  

  • 调用hasNext ->hasNextService
  • next -> nextService

• 判断是否还有服务提供

  private boolean hasNextService() {
              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;
          }
  

• 加载下一个服务

  private S nextService() {
              if (!hasNextService())
                  throw new NoSuchElementException();
              //nextName是在hasNextService中赋值的
              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 {
                  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
          }
  

• hasNextService() -> getResource()

  public Enumeration<URL> getResources(String name) throws IOException {
          @SuppressWarnings("unchecked")
          Enumeration<URL>[] tmp = (Enumeration<URL>[]) new Enumeration<?>[2];
          if (parent != null) {
         			//ExtensionClassLoader
              tmp[0] = parent.getResources(name);
          } else {
          		//BootstrapClassLoader
              tmp[0] = getBootstrapResources(name);
          }
          //加载当前类加载器
          tmp[1] = findResources(name);
  
          return new CompoundEnumeration<>(tmp);
      }
  

  这里是递归调用，需注意类加载器的层级关系 Application ClassLoader -> Extension ClassLoader -> BootStrap ClassLoader

• findResource

  public Enumeration<URL> findResources(final String name)
          throws IOException
      {
          final Enumeration<URL> e = ucp.findResources(name, true);
  
          return new Enumeration<URL>() {
              private URL url = null;
  
              private boolean next() {
                  if (url != null) {
                      return true;
                  }
                  do {
                      URL u = AccessController.doPrivileged(
                          new PrivilegedAction<URL>() {
                              public URL run() {
                                  if (!e.hasMoreElements())
                                      return null;
                                  return e.nextElement();
                              }
                          }, acc);
                      if (u == null)
                          break;
                      url = ucp.checkURL(u);
                  } while (url == null);
                  return url != null;
              }
  
              public URL nextElement() {
                  if (!next()) {
                      throw new NoSuchElementException();
                  }
                  URL u = url;
                  url = null;
                  return u;
              }
  
              public boolean hasMoreElements() {
                  return next();
              }
          };
      }
  

  从UrlClassPath中获取Enumeration

• UrlClassPath.findResource（）

  public Enumeration<URL> findResources(final String name,
                                       final boolean check) {
          return new Enumeration<URL>() {
          		// 懒加载的方式
              private int index = 0;
              private int[] cache = getLookupCache(name);
              private URL url = null;
  
              private boolean next() {
                  if (url != null) {
                      return true;
                  } else {
                      Loader loader;
                      //遍历UrlClassPath的所有loader，直到为空，并用index记录位置，因为再次调用时要获取
                      while ((loader = getNextLoader(cache, index++)) != null) {
                      //用获取到的loader加载指定资源
                          url = loader.findResource(name, check);
                          if (url != null) {
                              return true;
                          }
                      }
                      return false;
                  }
              }
          };
      }
  

• getLoader()

  private synchronized Loader getLoader(int index) {
          if (closed) {
              return null;
          }
          while (loaders.size() < index + 1) {
              URL url;
              synchronized (urls) {
                  if (urls.empty()) {
                      return null;
                  } else {
                      url = urls.pop();
                  }
              }
              String urlNoFragString = URLUtil.urlNoFragString(url);
              if (lmap.containsKey(urlNoFragString)) {
                  continue;
              }
              Loader loader;
              try {
                  loader = getLoader(url);
                  URL[] urls = loader.getClassPath();
                  if (urls != null) {
                      push(urls);
                  }
              } catch (IOException e) {
                  continue;
              } catch (SecurityException se) {
                  if (DEBUG) {
                      System.err.println("Failed to access " + url + ", " + se );
                  }
                  continue;
              }
              // Finally, add the Loader to the search path.
              validateLookupCache(loaders.size(), urlNoFragString);
              loaders.add(loader);
              lmap.put(urlNoFragString, loader);
          }
          if (DEBUG_LOOKUP_CACHE) {
              System.out.println("NOCACHE: Loading from : " + index );
          }
          return loaders.get(index);
      }
  

  UrlClassPath构造函数会传入待加载的url，并将其存入stack，如果处理了该url就将其抛出，所有url只会被处理一次。

  //代办为空 或 代办中没有元素开始遍历下一个类加载器获取代办
  while ((pending == null) || !pending.hasNext()) {
                  if (!configs.hasMoreElements()) {
                      return false;
                  }
                  pending = parse(service, configs.nextElement());
              }
              nextName = pending.next();
  

• hasNextService返回True，forEachRemaining()方法中的hasNext也为True，继续遍历