java SPI

概念

SPI(Service Provider Interface)，是JDK内置的一种服务提供发现机制。SPI是一种动态替换发现的机制，框架制定标准，不同的服务提供厂商可以用不同的实现。如java.sql.Driver接口，mysql和postgresql都有不同的实现提供给用户，java的spi机制可以为这个接口寻找服务发现。

使用方法

服务提供者提供一个接口的实现后，需要在classpath下的META-INF/services目录下创建一个以服务接口命名的文件，这个文件里的内容就是这个接口的具体实现类的全限定名。当其他的程序需要这个服务的时候，就可以通过查找这个jar包（一般都是以jar包做依赖）的META-INF/services/中的配置文件，配置文件中有接口的具体实现类名，可以根据这个类名进行加载实例化，就可以使用该服务了。JDK中查找服务的实现的工具类是：java.util.ServiceLoader。

JDBC实例

在JDBC4.0之前，我们开发有连接数据库的时候，通常会用Class.forName("com.mysql.jdbc.Driver")这句先加载数据库相关的驱动，然后再进行获取连接等的操作。而JDBC4.0之后不需用Class.forName("com.mysql.jdbc.Driver")来加载驱动，直接获取连接就可以了，现在这种方式就是使用了Java的SPI扩展机制来实现。

• mysql

  在mysql的jar包mysql-connector-java-6.0.6.jar中，可以找到META-INF/services目录，该目录下会有一个名字为java.sql.Driver的文件，文件内容是com.mysql.cj.jdbc.Driver，这里面的内容就是针对Java中定义的接口的实现。

• Postgresql

  在postgresql的jar包postgresql-42.0.0.jar中，也可以找到同样的配置文件，文件内容是org.postgresql.Driver，这是postgresql对Java的java.sql.Driver的实现。

使用方法

不需要再使用Class.forName(“com.mysql.jdbc.Driver”)来加载驱动了，直接使用如下代码：

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

源码实现

首先程序的入口在DriverManger的getConnection函数

public static Connection getConnection(String url,
       String user, String password) throws SQLException {
       java.util.Properties info = new java.util.Properties();

       if (user != null) {
           info.put("user", user);
       }
       if (password != null) {
           info.put("password", password);
       }
			//传入调用者的类
       return (getConnection(url, info, Reflection.getCallerClass()));
   }

这里就是迭代registeredDrivers来获取驱动，并返回第一个成功获取到的连接

//  Worker method called by the public getConnection() methods.
 private static Connection getConnection(
     String url, java.util.Properties info, Class<?> caller) throws SQLException {
     /*
      * When callerCl is null, we should check the application's
      * (which is invoking this class indirectly)
      * classloader, so that the JDBC driver class outside rt.jar
      * can be loaded from here.
      * caller有可能是bootstrap classloader
      */
     ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
     synchronized(DriverManager.class) {
         // synchronize loading of the correct classloader.
         if (callerCL == null) {
             callerCL = Thread.currentThread().getContextClassLoader();
         }
     }

     if(url == null) {
         throw new SQLException("The url cannot be null", "08001");
     }

     println("DriverManager.getConnection(\"" + url + "\")");

     // Walk through the loaded registeredDrivers attempting to make a connection.
     // Remember the first exception that gets raised so we can reraise it.
     SQLException reason = null;

     for(DriverInfo aDriver : registeredDrivers) {
         // If the caller does not have permission to load the driver then
         // skip it.
         if(isDriverAllowed(aDriver.driver, callerCL)) {
             try {
                 println("    trying " + aDriver.driver.getClass().getName());
                 Connection con = aDriver.driver.connect(url, info);
                 if (con != null) {
                     // Success!
                     println("getConnection returning " + aDriver.driver.getClass().getName());
                     return (con);
                 }
             } catch (SQLException ex) {
                 if (reason == null) {
                     reason = ex;
                 }
             }

         } else {
             println("    skipping: " + aDriver.getClass().getName());
         }

     }

     // if we got here nobody could connect.
     if (reason != null)    {
         println("getConnection failed: " + reason);
         throw reason;
     }

     println("getConnection: no suitable driver found for "+ url);
     throw new SQLException("No suitable driver found for "+ url, "08001");
 }

现在的关键就是registeredDrivers是如何生成的，DriverManger有一个静态代码块

/**
* Load the initial JDBC drivers by checking the System property
* jdbc.properties and then use the {@code ServiceLoader} mechanism
*/
static {
   loadInitialDrivers();
   println("JDBC DriverManager initialized");
}

private static void loadInitialDrivers() {
  			//1. 从系统变量中获取有关驱动的定义
        String drivers;
        try {
            drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
                public String run() {
                    return System.getProperty("jdbc.drivers");
                }
            });
        } catch (Exception ex) {
            drivers = null;
        }
        // If the driver is packaged as a Service Provider, load it.
        // Get all the drivers through the classloader
        // exposed as a java.sql.Driver.class service.
        // ServiceLoader.load() replaces the sun.misc.Providers()
			 	2. 使用SPI来获取驱动
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            public Void run() {

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

                /* Load these drivers, so that they can be instantiated.
                 * It may be the case that the driver class may not be there
                 * i.e. there may be a packaged driver with the service class
                 * as implementation of java.sql.Driver but the actual class
                 * may be missing. In that case a java.util.ServiceConfigurationError
                 * will be thrown at runtime by the VM trying to locate
                 * and load the service.
                 *
                 * Adding a try catch block to catch those runtime errors
                 * if driver not available in classpath but it's
                 * packaged as service and that service is there in classpath.
                 */
                //遍历使用SPI获取到的具体实现，实例化各个实现类
              	try{
                    while(driversIterator.hasNext()) {
                        driversIterator.next();
                    }
                } catch(Throwable t) {
                // Do nothing
                }
                return null;
            }
        });

        println("DriverManager.initialize: jdbc.drivers = " + drivers);

        if (drivers == null || drivers.equals("")) {
            return;
        }
  			//4. 根据第一步获取到的驱动列表来实例化具体实现类
        String[] driversList = drivers.split(":");
        println("number of Drivers:" + driversList.length);
        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);
            }
        }
    }

首先SPI源码部分

//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() + "]";
    }

}

利用SPI做RPC调用，与HSF区别