JDK SPI机制ServiceLoader源码浅析-蒲公英云

1.load

返回创建的ServiceLoader对象

清理providers

lookupIterator指向创建的LazyIterator对象

public static <S> ServiceLoader<S> load(Class<S> service) { 
    // 获取当前线程的上下文类加载器
    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) { 
    // ServiceLoader的字段 private final Class<S> service; private final ClassLoader loader;
    service = Objects.requireNonNull(svc, "Service interface cannot be null");
    loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
    // acc为null,不用关注
    acc = (System.getSecurityManager() != null) ? AccessController.getContext() : null;
    // 每次创建ServiceLoader对象都会清除缓存
    reload();
}
public void reload() { 
    // private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
    // 清理缓存的实现类
    providers.clear();
    // 实例化LazyIterator
    lookupIterator = new LazyIterator(service, loader);
}
private LazyIterator(Class<S> service, ClassLoader loader) { 
    this.service = service;
    this.loader = loader;
}

2.iterator

通过iterator()获取所有的实现类

public Iterator<S> iterator() { 
    return new Iterator<S>() { 
        // 获取已有的缓存，如果有的话，会先遍历这些
        Iterator<Map.Entry<String,S>> knownProviders
            = providers.entrySet().iterator();
        public boolean hasNext() { 
            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();
        }
    };
}

3.hasNext

是否有下一个实现类

// hasNext()调用hasNextServcie
private boolean hasNextService() { 
    // 如果有下一个要加载的实现类，返回ture 
    // String nextName是LazyIterator的字段
    if (nextName != null) { 
        return true;
    }
    // LazyIterator字段 Enumeration<URL> configs = null;
    if (configs == null) { 
        try { 
            // ServiceLoader的字段 private static final String PREFIX = "META-INF/services/"
            // 接口名和services下的文件名一样
            String fullName = PREFIX + service.getName();
            if (loader == null)
                configs = ClassLoader.getSystemResources(fullName);
            else
                // LazyIterator的字段 Enumeration<URL> configs = null;
                // 获取文件资源
                configs = loader.getResources(fullName);
        } catch (IOException x) { 
            fail(service, "Error locating configuration files", x);
        }
    }
    // LazyIterator的字段 Iterator<String> pending = null;
    // 解析文件内容
    while ((pending == null) || !pending.hasNext()) { 
        if (!configs.hasMoreElements()) { 
            return false;
        }
        pending = parse(service, configs.nextElement());
    }
    // 取出一个实现类
    nextName = pending.next();
    return true;
}
// 解析文件内容，返回实现类名List的迭代器
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;
        // 一行一行解析
        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();
}
// 解析一行，将实现类的名字加到List中，并且返回加一的lc
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('#');
    // 截取#之前的内容并且去除前后空格
    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);
        }
        // 主要是这里，将实现类名加到List中
        if (!providers.containsKey(ln) && !names.contains(ln))
            names.add(ln);
    }
    return lc + 1;
}

4.next

加载实现类

// next()调用nextService()
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);
    }
}
private S nextService() { 
    if (!hasNextService())
        throw new NoSuchElementException();
    // 获取hasNextServcie()取出的实现类
    String cn = nextName;
    nextName = null;
    Class<?> c = null;
    try { 
        // 加载实现类
        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());
        // 放到提供者列表中并返回加载的实现类
        providers.put(cn, p);
        return p;
    } catch (Throwable x) { 
        fail(service,
             "Provider " + cn + " could not be instantiated",
             x);
    }
    throw new Error();          // This cannot happen
}