【Java实战】源码解析Java SPI（Service Provider Interface ）机制原理

一、背景知识

在阅读开源框架源码时，发现许多框架都支持SPI（Service Provider Interface ），前面有篇文章JDBC对Driver的加载时应用了SPI，参考【Hibernate实战】源码解析Hibernate参数绑定及PreparedStatement防SQL注入原理，于是借着JDBC对Driver的加载实现，分析下SPI机制。

二、什么是SPI

看下Wikipedia对其的解释

Service Provider Interface (SPI) is an API intended to be implemented or extended by a third party. It can be used to enable framework extension and replaceable components.

简单翻译下：

服务提供者接口(SPI)是一个API，它是由第三方实现或扩展的。它可以用于支持框架扩展和可替换组件。

简单来说SPI就是为了框架扩展而生的。在不修改原始应用程序的基础上扩展应用，可以看下Creating Extensible Applications With the Java Platform对其的解释，其中也有详细的示例。

A service provider interface (SPI) is the set of public interfaces and abstract classes that a service defines. The SPI defines the classes and methods available to your application.

A service provider implements the SPI. An application with extensible services will allow you, vendors, and perhaps even customers to add service providers without modifying the original application.

简单翻译如下：

服务提供者接口(SPI)是服务定义的公共接口和抽象类的集合。SPI定义了应用程序可用的类和方法。 服务提供者实现SPI。具有可扩展服务的应用程序将允许您、供应商甚至客户在不修改原始应用程序的情况下添加服务提供者。

自JDK1.6对外提供了对SPI的支持，java.util.ServiceLoader.java，下面以Driver加载为例分析SPI机制。jdk1.8、mySQLDriver5.1.38

三、SPI原理

1、JDBC示例程序

public static void JDBCExample(){
		try {
			//Class.forName("com.mysql.jdbc.Driver");
			Connection connection = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/hhl?useServerPrepStmts=true&cachePrepStmts=true&prepStmtCacheSize=25&prepStmtCacheSqlLimit=2048&characterEncoding=utf8&useSSL=false",
					"root", "123456");
			PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM product p WHERE p.productName=?");
			preparedStatement.setString(1,"Mango");
			ResultSet resultSet = preparedStatement.executeQuery();
			while (resultSet.next()){
				System.out.println(resultSet.getString(1));
			}
			resultSet.close();
			preparedStatement.close();
			connection.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

自JDBC4.0以后就支持SPI了，不在需要用Class.forName()加载数据库驱动了，当然以前程序中用Class.forName()加载数据库驱动的仍然可以正常工作（用数据库连接池的还都是用Class.forName()加载数据库驱动的）。那么加载数据库驱动的操作在哪儿实现呢，java.sql.DriverManager.java

/**
     * 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");
    }

根据注释看出从jdbc.properties和利用ServiceLoader机制加载JDBC驱动。jdbc.properties的先不管，看下ServiceLoader机制

private static void loadInitialDrivers() {
        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()

        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.
                 */
                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;
        }
        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);
            }
        }
    }

ServiceLoader关键代码如下几行

ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);         2
                Iterator<Driver> driversIterator = loadedDrivers.iterator();    3

                try{
                    while(driversIterator.hasNext()) {                          4
                        driversIterator.next();                                 5
                    }
                } catch(Throwable t) {
                // Do nothing
                }

2、逐行看下 java.util.ServiceLoader.java

public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

其中的classLoader是当前线程上下文的加载器，泛型S代表服务类型的类，本例中就是Driver；参数service 为代表服务的接口或者抽象类，本例中是Driver.class。

public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader)
    {
        return new ServiceLoader<>(service, loader);
    }

最终根据service和loader创建了一个ServiceLoader

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();
    }

本例中，其中acc为null，接着看reload方法

 public void reload() {
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

清空了providers缓存

// Cached providers, in instantiation order
    private LinkedHashMap<String,S> providers = new LinkedHashMap<>();

创建了lookupIterator

// The current lazy-lookup iterator
    private LazyIterator lookupIterator;

// Private inner class implementing fully-lazy provider lookup
    //
    private class LazyIterator
        implements Iterator<S>
    {

        Class<S> service;
        ClassLoader loader;
        Enumeration<URL> configs = null;
        Iterator<String> pending = null;
        String nextName = null;

        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);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            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);
            } 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());
                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<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();
        }

    }

创建改对象的原因就是为了实现延迟服务提供者查找。延迟到什么时候，继续看代码。

3、loadedDrivers.iterator()

public Iterator<S> iterator() {
        return new Iterator<S>() {

            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();
            }

        };
    }

创建了一个内部类Iterator，用于操作缓存providers和延迟加载类lookupIterator

4、driversIterator.hasNext()

操作的就是上面的hasNext

  public boolean hasNext() {
                if (knownProviders.hasNext())
                    return true;
                return lookupIterator.hasNext();
            }

用到了LazyIterator中的hasNext()

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);
            }
        }

这里acc为null，不需要特权继续执行hasNextService

 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);
                } catch (IOException x) {
                    fail(service, "Error locating configuration files", x);
                }
            }
            while ((pending == null) || !pending.hasNext()) {
                if (!configs.hasMoreElements()) {
                    return false;
                }
                pending = parse(service, configs.nextElement());
            }
            nextName = pending.next();
            return true;
        }

起初nextName为null，configs也为null，这个时候就会根据fullName获取包含其的资源，然后parse解析

private static final String PREFIX = "META-INF/services/";

本例中fullName就是META-INF/services/java.sql.Driver，mysql驱动中该文件内容如下：

com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver

 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);
        } 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();
    }

改文件需要utf-8编码，返回的就是包含有文件内容的集合迭代器。

5、driversIterator.next()

接着看Next

public S next() {
                if (knownProviders.hasNext())
                    return knownProviders.next().getValue();
                return lookupIterator.next();
            }

起初走lookupIterator.next()

 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);
            }
        }

直接走nextService

 private S nextService() {
            if (!hasNextService())
                throw new NoSuchElementException();
            String cn = nextName;
            nextName = null;
            Class<?> c = null;
            try {
                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());
                providers.put(cn, p);
                return p;
            } catch (Throwable x) {
                fail(service,
                     "Provider " + cn + " could not be instantiated",
                     x);
            }
            throw new Error();          // This cannot happen
        }

这里会加载并初始化获取到的驱动，例如com.mysql.jdbc.Driver，这里还是需要Class.forName。采用

 S p = service.cast(c.newInstance());
                providers.put(cn, p);

初始化驱动，因此驱动需要有一个默认的构造函数。

至此，利用ServiceLoader加载并初始化驱动的操作就完成了。那么那么多驱动，要选择哪个驱动呢，就是根据url确定

jdbc:mysql://127.0.0.1:3306/hhl

如上url就会选择com.mysql.jdbc.Driver进行连接操作，其判断由驱动自己去做，由驱动中的acceptsURL及parseURL判断驱动支持的url，具体可看代码com.mysql.jdbc.NonRegisteringDriver.java

总结：SPI的机制就是在不修改原有程序的基础上实现扩展

当服务的提供者，提供了服务接口（java.sql.Driver）的一种实现之后，在jar包的META-INF/services/目录里同时创建一个以服务接口命名的文件。该文件里就是实现该服务接口的具体实现类。而当外部程序装配这个模块的时候，就能通过该jar包META-INF/services/里的配置文件找到具体的实现类名，并装载实例化，完成模块的注入。 基于这样一个约定就能很好的找到服务接口的实现类，而不需要再代码里指定。