SPI：Java的高可扩展利器

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() + "]";
 }
}