一文讲透Java核心技术之高可扩展利器SPI

最后，给出整个 java.util.ServiceLoader 的类，如下所示：

package java.util;
 
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.security.AccessControlContext;
import java.security.AccessController;
import java.security.PrivilegedAction;
 
 
public final class ServiceLoader  implements Iterable {
 
    // 加载具体实现类信息的前缀，也就是以接口命名的文件需要放到Jar包中的META-INF/services/目录下
    private static final String PREFIX = "META-INF/services/";
 
    // 需要加载的接口
    private final Class service;
 
    // 类加载器，用于加载以接口命名的文件中配置的接口的实现类
    private final ClassLoader loader;
 
    // 创建ServiceLoader时采用的访问控制上下文环境
    private final AccessControlContext acc;
 
    // 用来缓存已经加载的接口实现类，其中，Key是接口实现类的完整类名，Value为实现类对象
    private LinkedHashMap providers = new LinkedHashMap<>();
 
    // 用于延迟加载实现类的迭代器
    private LazyIterator lookupIterator;
 
    //重新加载
    public void reload() {
 
        //清空保存加载的实现类的LinkedHashMap
        providers.clear();
        //构造延迟加载的迭代器
        lookupIterator = new LazyIterator(service, loader);
    }
 
    //构造ServiceLoader对象
    private ServiceLoader(Class svc, ClassLoader cl) {
 
        //如果传入的Class对象为空，则判处空指针异常
        service = Objects.requireNonNull(svc, "Service interface cannot be null");
        //如果传入的ClassLoader为空，则通过ClassLoader.getSystemClassLoader()获取，否则直接使用传入的ClassLoader
        loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
        acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
        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);
    }
 
    // Parse a single line from the given configuration file, adding the name
    // on the line to the names list.
    //
    private int parseLine(Class service, URL u, BufferedReader r, int lc,
                          List 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;
    }
 
    private Iterator parse(Class service, URL u)
        throws ServiceConfigurationError
    {
 
        InputStream in = null;
        BufferedReader r = null;
        ArrayList 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();
    }
 
    // Private inner class implementing fully-lazy provider lookupload
    private class LazyIterator
        implements Iterator
    {
 
 
        Class service;
        ClassLoader loader;
        Enumeration configs = null;
        Iterator pending = null;
        String nextName = null;
 
        private LazyIterator(Class service, ClassLoader loader) {
 
            this.service = service;
            this.loader = loader;
        }
 
        //判断是否拥有下一个实例
        private boolean hasNextService() {
 
            //如果拥有下一个实例，直接返回true
            if (nextName != null) {
 
                return true;
            }
            //如果实现类的全名为null
            if (configs == null) {
 
                try {
 
                    //获取全文件名，文件相对路径+文件名称（包名+接口名）
                    String fullName = PREFIX + service.getName();
                    //类加载器为空，则通过ClassLoader.getSystemResources()方法获取
                    if (loader == null)
                        configs = ClassLoader.getSystemResources(fullName);
                    else
                        //类加载器不为空，则直接通过类加载器获取
                        configs = loader.getResources(fullName);
                } catch (IOException x) {
 
                    fail(service, "Error locating configuration files", x);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
 
                //如果configs中没有更过的元素，则直接返回false
                if (!configs.hasMoreElements()) {
 
                    return false;
                }
                //解析包结构
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }
 
        private S nextService() {
 
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class c = null;
            try {
 
                //加载类对象
                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 {
 
                //通过c.newInstance()生成对象实例
                S p = service.cast(c.newInstance());
                //将生成的对象实例保存到缓存中（LinkedHashMap）
                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 boolean hasNext() {
 
            if (acc == null) {
 
                return hasNextService();
            } else {
 
                PrivilegedAction action = new PrivilegedAction() {
 
                    public Boolean run() {
  return hasNextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }
 
        public S next() {
 
            if (acc == null) {
 
                return nextService();
            } else {
 
                PrivilegedAction action = new PrivilegedAction() {
 
                    public S run() {
  return nextService(); }
                };
                return AccessController.doPrivileged(action, acc);
            }
        }
 
        public void remove() {
 
            throw new UnsupportedOperationException();
        }
 
    }
 
    //迭代ServiceLoader的方法
    public Iterator iterator() {
 
        return new Iterator() {
 
            //获取保存实现类的LinkedHashMap的迭代器
            Iterator> knownProviders = providers.entrySet().iterator();
            //判断是否有下一个元素
            public boolean hasNext() {
 
                //如果knownProviders存在元素，则直接返回true
                if (knownProviders.hasNext())
                    return true;
                //返回延迟加载器是否存在元素
                return lookupIterator.hasNext();
            }
            //获取下一个元素
            public S next() {
 
                //如果knownProviders存在元素，则直接获取
                if (knownProviders.hasNext())
                    return knownProviders.next().getValue();
                //获取延迟迭代器lookupIterator中的元素
                return lookupIterator.next();
            }
 
            public void remove() {
 
                throw new UnsupportedOperationException();
            }
 
        };
    }
 
    //通过ClassLoader加载指定类的Class，并将返回结果封装到ServiceLoader对象中
    public static  ServiceLoader load(Class service,
                                            ClassLoader loader)
    {
 
        return new ServiceLoader<>(service, loader);
    }
 
    //根据类的Class对象加载指定的类，返回ServiceLoader对象
    public static  ServiceLoader load(Class service) {
 
        //获取当前线程的类加载器
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        //动态加载指定的类，将类加载到ServiceLoader中
        return ServiceLoader.load(service, cl);
    }
    
    public static  ServiceLoader loadInstalled(Class service) {
 
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        ClassLoader prev = null;
        while (cl != null) {
 
            prev = cl;
            cl = cl.getParent();
        }
        return ServiceLoader.load(service, prev);
    }
 
    /**
     * Returns a string describing this service.
     *
     * @return  A descriptive string
     */
    public String toString() {
 
        return "java.util.ServiceLoader[" + service.getName() + "]";
    }
 
}