JAVA设计模式第十讲：SPI - 业务差异解决方案

public interface Search {
    public List<String> searchDoc(String keyword);   
}

public class FileSearch implements Search{
    @Override
    public List<String> searchDoc(String keyword) {
        System.out.println("文件搜索 "+keyword);
        return null;
    }
}

public class DatabaseSearch implements Search{
    @Override
    public List<String> searchDoc(String keyword) {
        System.out.println("数据搜索 "+keyword);
        return null;
    }
}

com.cainiao.ys.spi.learn.FileSearch

public class TestCase {
    public static void main(String[] args) {
        ServiceLoader<Search> s = ServiceLoader.load(Search.class);
        Iterator<Search> iterator = s.iterator();
        while (iterator.hasNext()) {
            Search search =  iterator.next();
            search.searchDoc("hello world");
        }
    }
}

String url = "jdbc:xxxx://xxxx:xxxx/xxxx";
Connection conn = DriverManager.getConnection(url,username,password);
.....

public static Driver getDriver(String url) throws SQLException {
        println("DriverManager.getDriver(\"" + url + "\")");
        ensureDriversInitialized();
        ...
 }

private static void ensureDriversInitialized() {
    if (driversInitialized) {
        return;
    }
    synchronized (lockForInitDrivers) {
        if (driversInitialized) {
            return;
        }
        String drivers;
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
                public String run() {
                    // 从系统变量"jdbc.drivers"中获取有关驱动的定义
                    return System.getProperty(JDBC_DRIVERS_PROPERTY);
                }
            });
        } catch (Exception ex) {
            drivers = null;
        }
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {
                // 使用SPI来获取驱动的实现
                ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
                // 遍历使用SPI获取到的具体实现，实例化各个实现类
                Iterator<Driver> driversIterator = loadedDrivers.iterator();
                try {
                    while (driversIterator.hasNext()) {
                        driversIterator.next();
                    }
                } catch (Throwable t) {
                    // Do nothing
                }
                return null;
            }
        });
        println("DriverManager.initialize: jdbc.drivers = " + drivers);
        if (drivers != null && !drivers.equals("")) {
            String[] driversList = drivers.split(":");
            println("number of Drivers:" + driversList.length);
            // 根据第一步获取到的驱动列表来实例化具体实现类
            for (String aDriver : driversList) {
                try {
                    println("DriverManager.Initialize: loading " + aDriver);
                    Class.forName(aDriver, true, ClassLoader.getSystemClassLoader());
                } catch (Exception ex) {
                    println("DriverManager.Initialize: load failed: " + ex);
                }
            }
        }
        driversInitialized = true;
        println("JDBC DriverManager initialized");
    }
}

ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);

//获取迭代器
Iterator<Driver> driversIterator = loadedDrivers.iterator();
//遍历所有的驱动实现
while(driversIterator.hasNext()) {
    driversIterator.next();
}

public static getLog(Class clazz) throws LogConfigurationException {
    return getFactory().getInstance(clazz);
}

public static org.apache.commons.logging.LogFactory getFactory() throws LogConfigurationException {
    // Identify the class loader we will be using
    ClassLoader contextClassLoader = getContextClassLoaderInternal();
    if (contextClassLoader == null) {
        // This is an odd enough situation to report about. This
        // output will be a nuisance on JDK1.1, as the system
        // classloader is null in that environment.
        if (isDiagnosticsEnabled()) {
            logDiagnostic("Context classloader is null.");
        }
    }
    // Return any previously registered factory for this class loader
    org.apache.commons.logging.LogFactory factory = getCachedFactory(contextClassLoader);
    if (factory != null) {
        return factory;
    }
    if (isDiagnosticsEnabled()) {
        logDiagnostic(
                "[LOOKUP] LogFactory implementation requested for the first time for context classloader " +
                        objectId(contextClassLoader));
        logHierarchy("[LOOKUP] ", contextClassLoader);
    }
    // Load properties file.
    //
    // If the properties file exists, then its contents are used as
    // "attributes" on the LogFactory implementation class. One particular
    // property may also control which LogFactory concrete subclass is
    // used, but only if other discovery mechanisms fail..
    //
    // As the properties file (if it exists) will be used one way or
    // another in the end we may as well look for it first.
    // classpath根目录下寻找commons-logging.properties
    Properties props = getConfigurationFile(contextClassLoader, FACTORY_PROPERTIES);
    // Determine whether we will be using the thread context class loader to
    // load logging classes or not by checking the loaded properties file (if any).
    // classpath根目录下commons-logging.properties是否配置use_tccl
    ClassLoader baseClassLoader = contextClassLoader;
    if (props != null) {
        String useTCCLStr = props.getProperty(TCCL_KEY);
        if (useTCCLStr != null) {
            // The Boolean.valueOf(useTCCLStr).booleanValue() formulation
            // is required for Java 1.2 compatibility.
            if (Boolean.valueOf(useTCCLStr).booleanValue() == false) {
                // Don't use current context classloader when locating any
                // LogFactory or Log classes, just use the class that loaded
                // this abstract class. When this class is deployed in a shared
                // classpath of a container, it means webapps cannot deploy their
                // own logging implementations. It also means that it is up to the
                // implementation whether to load library-specific config files
                // from the TCCL or not.
                baseClassLoader = thisClassLoader;
            }
        }
    }
    // 这里真正开始决定使用哪个factory
    // 首先，尝试查找vm系统属性org.apache.commons.logging.LogFactory，其是否指定factory
    // Determine which concrete LogFactory subclass to use.
    // First, try a global system property
    if (isDiagnosticsEnabled()) {
        logDiagnostic("[LOOKUP] Looking for system property [" + FACTORY_PROPERTY +
                "] to define the LogFactory subclass to use...");
    }
    try {
        String factoryClass = getSystemProperty(FACTORY_PROPERTY, null);
        if (factoryClass != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[LOOKUP] Creating an instance of LogFactory class '" + factoryClass +
                        "' as specified by system property " + FACTORY_PROPERTY);
            }
            factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
        } else {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[LOOKUP] No system property [" + FACTORY_PROPERTY + "] defined.");
            }
        }
    } catch (SecurityException e) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[LOOKUP] A security exception occurred while trying to create an" +
                    " instance of the custom factory class" + ": [" + trim(e.getMessage()) +
                    "]. Trying alternative implementations...");
        }
        // ignore
    } catch (RuntimeException e) {
        // This is not consistent with the behaviour when a bad LogFactory class is
        // specified in a services file.
        //
        // One possible exception that can occur here is a ClassCastException when
        // the specified class wasn't castable to this LogFactory type.
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[LOOKUP] An exception occurred while trying to create an" +
                    " instance of the custom factory class" + ": [" +
                    trim(e.getMessage()) +
                    "] as specified by a system property.");
        }
        throw e;
    }
    // 第二，尝试使用java spi服务发现机制，载META-INF/services下寻找 org.apache.commons.logging.LogFactory 实现
    // Second, try to find a service by using the JDK1.3 class
    // discovery mechanism, which involves putting a file with the name
    // of an interface class in the META-INF/services directory, where the
    // contents of the file is a single line specifying a concrete class
    // that implements the desired interface.
    if (factory == null) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("[LOOKUP] Looking for a resource file of name [" + SERVICE_ID +
                    "] to define the LogFactory subclass to use...");
        }
        try {
            // META-INF/services/org.apache.commons.logging.LogFactory, SERVICE_ID
            final InputStream is = getResourceAsStream(contextClassLoader, SERVICE_ID);
            if (is != null) {
                // This code is needed by EBCDIC and other strange systems.
                // It's a fix for bugs reported in xerces
                BufferedReader rd;
                try {
                    rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));
                } catch (java.io.UnsupportedEncodingException e) {
                    rd = new BufferedReader(new InputStreamReader(is));
                }
                String factoryClassName = rd.readLine();
                rd.close();
                if (factoryClassName != null && !"".equals(factoryClassName)) {
                    if (isDiagnosticsEnabled()) {
                        logDiagnostic("[LOOKUP]  Creating an instance of LogFactory class " +
                                factoryClassName +
                                " as specified by file '" + SERVICE_ID +
                                "' which was present in the path of the context classloader.");
                    }
                    factory = newFactory(factoryClassName, baseClassLoader, contextClassLoader);
                }
            } else {
                // is == null
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("[LOOKUP] No resource file with name '" + SERVICE_ID + "' found.");
                }
            }
        } catch (Exception ex) {
            // note: if the specified LogFactory class wasn't compatible with LogFactory
            // for some reason, a ClassCastException will be caught here, and attempts will
            // continue to find a compatible class.
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] A security exception occurred while trying to create an" +
                                " instance of the custom factory class" +
                                ": [" + trim(ex.getMessage()) +
                                "]. Trying alternative implementations...");
            }
            // ignore
        }
    }
    // 第三，尝试从classpath根目录下的commons-logging.properties中查找org.apache.commons.logging.LogFactory属性指定的factory
    // Third try looking into the properties file read earlier (if found)
    if (factory == null) {
        if (props != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic(
                        "[LOOKUP] Looking in properties file for entry with key '" + FACTORY_PROPERTY +
                                "' to define the LogFactory subclass to use...");
            }
            String factoryClass = props.getProperty(FACTORY_PROPERTY);
            if (factoryClass != null) {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic(
                            "[LOOKUP] Properties file specifies LogFactory subclass '" + factoryClass + "'");
                }
                factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
                // TODO: think about whether we need to handle exceptions from newFactory
            } else {
                if (isDiagnosticsEnabled()) {
                    logDiagnostic("[LOOKUP] Properties file has no entry specifying LogFactory subclass.");
                }
            }
        } else {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("[LOOKUP] No properties file available to determine" + " LogFactory subclass from..");
            }
        }
    }
    // 最后，使用后备factory实现，org.apache.commons.logging.impl.LogFactoryImpl
    // Fourth, try the fallback implementation class
    if (factory == null) {
        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                    "[LOOKUP] Loading the default LogFactory implementation '" + FACTORY_DEFAULT +
                            "' via the same classloader that loaded this LogFactory" +
                            " class (ie not looking in the context classloader).");
        }
        // Note: unlike the above code which can try to load custom LogFactory
        // implementations via the TCCL, we don't try to load the default LogFactory
        // implementation via the context classloader because:
        // * that can cause problems (see comments in newFactory method)
        // * no-one should be customising the code of the default class
        // Yes, we do give up the ability for the child to ship a newer
        // version of the LogFactoryImpl class and have it used dynamically
        // by an old LogFactory class in the parent, but that isn't
        // necessarily a good idea anyway.
        factory = newFactory(FACTORY_DEFAULT, thisClassLoader, contextClassLoader);
    }
    if (factory != null) {
        /**
         * Always cache using context class loader.
         */
        cacheFactory(contextClassLoader, factory);
        if (props != null) {
            Enumeration names = props.propertyNames();
            while (names.hasMoreElements()) {
                String name = (String) names.nextElement();
                String value = props.getProperty(name);
                factory.setAttribute(name, value);
            }
        }
    }
    return factory;
}

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
// spring.factories文件的格式为：key=value1,value2,value3
// 从所有的jar包中找到META-INF/spring.factories文件
// 然后从文件中解析出key=factoryClass类名称的所有value值
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    // 取得资源文件的URL
    Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) : ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
    List<String> result = new ArrayList<String>();
    // 遍历所有的URL
    while (urls.hasMoreElements()) {
        URL url = urls.nextElement();
        // 根据资源文件URL解析properties文件，得到对应的一组@Configuration类
        Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
        String factoryClassNames = properties.getProperty(factoryClassName);
        // 组装数据，并返回
        result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
    }
    return result;
}

ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
//获取迭代器
Iterator<Driver> driversIterator = loadedDrivers.iterator();
//遍历
while(driversIterator.hasNext()) {
    driversIterator.next();
    //可以做具体的业务逻辑
}

//ServiceLoader实现了Iterable接口，可以遍历所有的服务实现者
public final class ServiceLoader<S>
    implements Iterable<S>
{
    //查找配置文件的目录
    private static final String PREFIX = "META-INF/services/";
    //表示要被加载的服务的类或接口
    private final Class<S> service;
    //这个ClassLoader用来 定位，加载，实例化服务提供者
    private final ClassLoader loader;
    // 访问控制上下文
    private final AccessControlContext acc;
    // 缓存已经被实例化的服务提供者，按照实例化的顺序存储
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();
    // 迭代器
    private LazyIterator lookupIterator;
    // 重新加载，就相当于重新创建 ServiceLoader了，用于新的服务提供者安装到正在运行的Java虚拟机中的情况。
    public void reload() {
        // 清空缓存中所有已实例化的服务提供者
        providers.clear();
        // 新建一个迭代器，该迭代器会从头查找和实例化服务提供者
        lookupIterator = new LazyIterator(service, loader);
    }
    //私有构造器
    //使用指定的类加载器和服务创建服务加载器
    //如果没有指定类加载器，使用系统类加载器，就是应用类加载器。
    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();
    }
    //解析失败处理的方法
    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);
    }
    //解析服务提供者配置文件中的一行
    //首先去掉注释校验，然后保存
    //返回下一行行号
    //重复的配置项和已经被实例化的配置项不会被保存
    private int parseLine(Class<?> service, URL u, BufferedReader r, int lc,
                          List<String> names)
        throws IOException, ServiceConfigurationError {
        //读取一行
        String ln = r.readLine();
        if (ln == null) {
            return -1;
        }
        //#号代表注释行
        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;
    }
    //解析配置文件，解析指定的url配置文件
    //使用parseLine方法进行解析，未被实例化的服务提供者会被保存到缓存中去
    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;
            while ((lc = parseLine(service, u, r, lc, names)) >= 0);
        }
        return names.iterator();
    }
    //服务提供者查找的迭代器
    private class LazyIterator
        implements Iterator<S> {
        Class<S> service;//服务提供者接口
        ClassLoader loader;//类加载器
        Enumeration<URL> configs = null;//保存实现类的url
        Iterator<String> pending = null;//保存实现类的全名
        String nextName = null;//迭代器中下一个实现类的全名
        private LazyIterator(Class<S> service, ClassLoader loader) {
            this.service = service;
            this.loader = loader;
        }
        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);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                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 {
                c = Class.forName(cn, false, loader);
            }
            if (!service.isAssignableFrom(c)) {
                fail(service, "Provider " + cn  + " not a subtype");
            }
            try {
                S p = service.cast(c.newInstance());
                providers.put(cn, p);
                return p;
            }
        }
        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();
        }
    }
    //获取迭代器
    //返回遍历服务提供者的迭代器
    //以懒加载的方式加载可用的服务提供者
    // 懒加载的实现是：解析配置文件和实例化服务提供者的工作由迭代器本身完成
    public Iterator<S> iterator() {
        return new Iterator<S>() {
            //按照实例化顺序返回已经缓存的服务提供者实例
            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();
            }
        };
    }
    // 为指定的服务使用指定的类加载器来创建一个ServiceLoader
    public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader) {
        return new ServiceLoader<>(service, loader);
    }
    // 使用线程上下文的类加载器来创建ServiceLoader
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }
    // 使用扩展类加载器为指定的服务创建ServiceLoader
    // 只能找到并加载已经安装到当前Java虚拟机中的服务提供者，应用程序类路径中的服务提供者将被忽略
    public static <S> ServiceLoader<S> loadInstalled(Class<S> service) {
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        ClassLoader prev = null;
        while (cl != null) {
            prev = cl;
            cl = cl.getParent();
        }
        return ServiceLoader.load(service, prev);
    }
    public String toString() {
        return "java.util.ServiceLoader[" + service.getName() + "]";
    }
}

// 判断校验对应的sku集合是否允许购买
switch (businessCode) {
    case NETSUPER:
        // 校验网超是否有协议限制
        netsuperItemValidate(buyer, multiMap, ids);
        result.put(ItemTagEnum.NETSUPER.getTagName(),ids);
        break;
    case VACCINE:
        // 检查疫苗的禁售期, 禁售期内不可买
        vaccineItemValidate(buyer,multiMap,ids);
        result.put(ItemTagEnum.VACCINE.getTagName(),ids);
        break;
    case BLOCKTRADE:
        // 检查大宗商品的下浮率
        blockItemValidate(buyer, multiMap, ids);
        result.put(ItemTagEnum.BLOCKTRADE.getTagName(), ids);
        break;
    default:
        // 默认校验逻辑
        defaultItemValidate(buyer, multiMap, ids);
}

// 可以在dubbo嵌入渠道信息
String channelContext = "vaccine";
ThreadLocal.set(channelContext);
ItemValidateStrategy auto = SpiProviderManager.getProvider(SimpleProvider.class);
class SpiProviderManager {
    public <T> T getProvider(Class clazz){
        String channel = ThreadLocal.get();
        // getProvider 去读配置并根据 channel 加载实现
        T provider = getProvider(clazz, channel);
    }
}