Service Provider Interface 介绍

package com.example.demo;
//相关类
public interface MyService {
    void say();
}

public class HelloWorldService implements MyService {
    @Override
    public void say() {
        System.out.println("Hello world!");
    }
}

public class WelcomeService implements MyService {
    @Override
    public void say() {
        System.out.println("Welcome!");
    }
}
//其他更多的服务类...

com.example.demo.HelloWorldService
com.example.demo.WelcomeService

ServiceLoader<MyService> serviceLoader = ServiceLoader.load(MyService.class);
Iterator<MyService> iterator = serviceLoader.iterator();
while (iterator.hasNext()) {
  iterator.next().say();
}

for (MyService myService : serviceLoader) {
	myService.say();
}

//自定义类加载器
load(Class<S>, ClassLoader)
//使用当前线程的Context Class Loader
load(Class<S>)
//使用扩展类加载器加载
loadInstalled(Class<S>)

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();
}

public void reload() {
  //清空缓存
  //providers类型为LinkedHashMap<String,S>,键为我们的类全名，值为对应
  providers.clear();
  //实例化赋值LazyIterator
  lookupIterator = new LazyIterator(service, loader);
}

//configs为资源集合
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);
  }
}

private static final String PREFIX = "META-INF/services/";
//所代表的需要加载的服务类或者接口类
// The class or interface representing the service being loaded
private final Class<S> service;

//全类名
String fullName = PREFIX + service.getName();
//获取资源
if (loader == null)
  configs = ClassLoader.getSystemResources(fullName);
else
  configs = loader.getResources(fullName);

private boolean hasNextService() {
  //省略其他代码....
  while ((pending == null) || !pending.hasNext()) {
    if (!configs.hasMoreElements()) {
      return false;
    }
    //读取每个资源
    //每个成功资源完全枚举结束后，再加载额外的其他资源
    //摊薄io时间复杂度
    pending = parse(service, configs.nextElement());
  }
  nextName = pending.next();
  return true;
}

 //
 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);
            }
            if (!providers.containsKey(ln) && !names.contains(ln))
                names.add(ln);
        }
        return lc + 1;
    }

while(iterator.hasNext()) {
	//do nothing.
}

Class<?> c = null;
//查找加载类
try {
  c = Class.forName(cn, false, loader);
} catch (ClassNotFoundException x) {
  fail(service,
       "Provider " + cn + " not found");
}
//判定类或接口的继承性
if (!service.isAssignableFrom(c)) {
  fail(service,
       "Provider " + cn  + " not a subtype");
}
//创建类
//类型转换
//并将结果类放入providers缓存
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);
}