Java SPI (Service Provider Interface) 機制詳解 概述 什麼是SPI ？ API 與 SPI SPI 應用場景 項目案例 SPI 實現原理解析 參考資料

概述

關鍵詞：解耦，可拔插，面向接口編程，動態類加載。

本質：Java SPI 實際上是“基於接口的編程＋策略模式＋約定配置文件” 組合實現的動態加載機制，在JDK中提供了工具類：“java.util.ServiceLoader”來實現服務查找。

什麼是SPI ？

SPI 全稱：Service Provider Interface，是Java提供的一套用來被第三方實現或者擴展的接口，它可以用來啓用框架擴展和替換組件。

面向的對象的設計裏，我們一般推薦模塊之間基於接口編程，模塊之間不對實現類進行硬編碼。一旦代碼裏涉及具體的實現類，就違反了可拔插的原則，如果需要替換一種實現，就需要修改代碼。

爲了實現在模塊裝配的時候不用在程序裏動態指明，這就需要一種服務發現機制。java spi就是提供這樣的一個機制：爲某個接口尋找服務實現的機制。這有點類似IOC的思想，將裝配的控制權移到了程序之外。

SPI的作用就是爲被擴展的API尋找服務實現。

SPI（Service Provider Interface），是JDK內置的一種 服務提供發現機制，可以用來啓用框架擴展和替換組件，主要是被框架的開發人員使用，比如java.sql.Driver接口，其他不同廠商可以針對同一接口做出不同的實現，MySQL和PostgreSQL都有不同的實現提供給用戶，而Java的SPI機制可以爲某個接口尋zhao服務實現。Java中SPI機制主要思想是將裝配的控制權移到程序之外，在模塊化設計中這個機制尤其重要，其核心思想就是 解耦。

SPI整體機制圖如下：

當服務的提供者提供了一種接口的實現之後，需要在classpath下的META-INF/services/目錄裏創建一個以服務接口命名的文件，這個文件裏的內容就是這個接口的具體的實現類。當其他的程序需要這個服務的時候，就可以通過查找這個jar包（一般都是以jar包做依賴）的META-INF/services/中的配置文件，配置文件中有接口的具體實現類名，可以根據這個類名進行加載實例化，就可以使用該服務了。JDK中查找服務的實現的工具類是：java.util.ServiceLoader。

SPI 的不足

1.不能按需加載，需要遍歷所有的實現，並實例化，然後在循環中才能找到我們需要的實現。如果不想用某些實現類，或者某些類實例化很耗時，它也被載入並實例化了，這就造成了浪費。

2.獲取某個實現類的方式不夠靈活，只能通過 Iterator 形式獲取，不能根據某個參數來獲取對應的實現類。（Spring 的BeanFactory，ApplicationContext 就要高級一些了。）

3.多個併發多線程使用 ServiceLoader 類的實例是不安全的。

API 與 SPI

SPI與API區別：

API是調用並用於實現目標的類、接口、方法等的描述；

SPI是擴展和實現以實現目標的類、接口、方法等的描述；

換句話說，API 爲操作提供特定的類、方法，SPI 通過操作來符合特定的類、方法。

參考： https://stackoverflow.com/questions/2954372/difference-between-spi-and-api?answertab=votes#tab-top

SPI和API的使用場景解析：

• API （Application Programming Interface）在大多數情況下，都是實現方制定接口並完成對接口的實現，調用方僅僅依賴接口調用，且無權選擇不同實現。 從使用人員上來說，API 直接被應用開發人員使用。

• SPI （Service Provider Interface）是調用方來制定接口規範，提供給外部來實現，調用方在調用時則選擇自己需要的外部實現。 從使用人員上來說，SPI 被框架擴展人員使用。

SPI 應用場景

SPI擴展機制應用場景有很多，比如Common-Logging，JDBC，Dubbo等等。

SPI流程：

有關組織和公式定義接口標準

第三方提供具體實現: 實現具體方法, 配置 META-INF/services/${interface_name} 文件

開發者使用

比如JDBC場景下：

首先在Java中定義了接口java.sql.Driver，並沒有具體的實現，具體的實現都是由不同廠商提供。

在MySQL的jar包mysql-connector-java-6.0.6.jar中，可以找到META-INF/services目錄，該目錄下會有一個名字爲java.sql.Driver的文件，文件內容是com.mysql.cj.jdbc.Driver，這裏面的內容就是針對Java中定義的接口的實現。

同樣在PostgreSQL的jar包PostgreSQL-42.0.0.jar中，也可以找到同樣的配置文件，文件內容是org.postgresql.Driver，這是PostgreSQL對Java的java.sql.Driver的實現。

項目案例

Java 工程目錄：

下面我們來簡單實現一個 JDK 的SPI的簡單實現。

Java代碼開發

首先第一步，定義一個接口：

Phone.java

package com.light.sword;

/**
 * @author: Jack
 * 2021/1/31 上午1:44
 */
public interface Phone {
    String getSystemInfo();
}

這個接口分別有兩個實現：

Huawei.java

package com.light.sword;

/**
 * @author: Jack
 * 2021/1/31 上午1:48
 */
public class Huawei implements Phone {
    @Override
    public String getSystemInfo() {
        return "Hong Meng";
    }
}

IPhone.java

package com.light.sword;

/**
 * @author: Jack
 * 2021/1/31 上午1:48
 */
public class IPhone implements Phone {
    @Override
    public String getSystemInfo() {
        return  "iOS";
    }
}

約定配置：新建 META-INF/services 目錄

注意：這個META-INF/services 目錄是寫死的約定，在 java.util.ServiceLoader 源碼實現中, java.util.ServiceLoader#PREFIX 可以看到這個目錄的硬編碼。

然後需要在resources目錄下新建 META-INF/services 目錄，並且在這個目錄下新建一個與上述接口的全限定名一致的文件:

com.light.sword.Phone (這是一個文件，是的，一切皆是文件。)

在這個文件中寫入接口的實現類的全限定名（文件 com.light.sword.Phone 中寫死的內容）：

com.light.sword.Huawei
com.light.sword.IPhone

如下圖所示：

加載實現類並調用服務

這時，通過ServiceLoader 加載實現類並調用服務：

Main.java

package com.light.sword;

import java.util.ServiceLoader;

public class Main {

    public static void main(String[] args) {
        ServiceLoader<Phone> phoneServiceLoader = ServiceLoader.load(Phone.class);
        phoneServiceLoader.forEach(provider -> {
            String systemInfo = provider.getSystemInfo();
            System.out.println(systemInfo);
        });
    }
}

輸出如下：

Hong Meng
iOS

工程源代碼：https://gitee.com/universsky/java-spi-demo

這樣一個簡單的 Java SPI 的demo就完成了。可以看到其中最爲核心的就是通過一系列的約定（其實，就是按照人家 java.util.ServiceLoader 的規範標準來）， 然後，通過ServiceLoader 這個類來加載具體的實現類，進而調用實現類的服務。

---

知識拓展：

其實，我們在Spring框架中，可以通過 component-scan 標籤來對指定包路徑進行掃描，只要掃到 Spring 制定的 @Service、@Controller 等註解，spring自動會把它注入容器。 這就相當於spring制定了註解規範，我們按照這個註解規範開發相應的實現類或controller，spring並不需要感知我們是怎麼實現的，他只需要根據註解規範和scan標籤注入相應的bean，這正是 spi 理念的體現。

SPI 實現原理解析

首先，ServiceLoader實現了Iterable接口，所以它有迭代器的屬性，這裏主要都是實現了迭代器的hasNext和next方法。這裏主要都是調用的lookupIterator的相應hasNext和next方法，lookupIterator是懶加載迭代器。

其次，LazyIterator中的hasNext方法，靜態變量PREFIX就是”META-INF/services/”目錄，這也就是爲什麼需要在classpath下的META-INF/services/目錄裏創建一個以服務接口命名的文件。

最後，通過反射方法Class.forName()加載類對象，並用newInstance方法將類實例化，並把實例化後的類緩存到providers對象中，(LinkedHashMap<String,S>類型） 然後返回實例對象。

---

java.util.ServiceLoader.java 源代碼如下：

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.AccessController;
import java.security.AccessControlContext;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

// ServiceLoader實現了Iterable接口，可以遍歷所有的服務實現者
public final class ServiceLoader<S> implements Iterable<S>
{

    // 約定的配置文件的存放目錄
    private static final String PREFIX = "META-INF/services/";

    // The class or interface representing the service being loaded
    private final Class<S> service;

    // The class loader used to locate, load, and instantiate providers
    private final ClassLoader loader;

    // The access control context taken when the ServiceLoader is created
    private final AccessControlContext acc;

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

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

    /**
     * Clear this loader's provider cache so that all providers will be
     * reloaded.
     *
     * <p> After invoking this method, subsequent invocations of the {@link
     * #iterator() iterator} method will lazily look up and instantiate
     * providers from scratch, just as is done by a newly-created loader.
     *
     * <p> This method is intended for use in situations in which new providers
     * can be installed into a running Java virtual machine.
     */
    public void reload() {
        providers.clear();
        lookupIterator = new LazyIterator(service, loader);
    }

    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();
    }

    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<String> 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;
    }

    // Parse the content of the given URL as a provider-configuration file.
    //
    // @param  service
    //         The service type for which providers are being sought;
    //         used to construct error detail strings
    //
    // @param  u
    //         The URL naming the configuration file to be parsed
    //
    // @return A (possibly empty) iterator that will yield the provider-class
    //         names in the given configuration file that are not yet members
    //         of the returned set
    //
    // @throws ServiceConfigurationError
    //         If an I/O error occurs while reading from the given URL, or
    //         if a configuration-file format error is detected
    //
    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();
    }

    // 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 {
                // 用反射機制，創建接口實現對象
                S p = service.cast(c.newInstance());
               // 放進 ServiceLoader的providers容器裏面
                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();
        }

    }

    /**
     * Lazily loads the available providers of this loader's service.
     *
     * <p> The iterator returned by this method first yields all of the
     * elements of the provider cache, in instantiation order.  It then lazily
     * loads and instantiates any remaining providers, adding each one to the
     * cache in turn.
     *
     * <p> To achieve laziness the actual work of parsing the available
     * provider-configuration files and instantiating providers must be done by
     * the iterator itself.  Its {@link java.util.Iterator#hasNext hasNext} and
     * {@link java.util.Iterator#next next} methods can therefore throw a
     * {@link ServiceConfigurationError} if a provider-configuration file
     * violates the specified format, or if it names a provider class that
     * cannot be found and instantiated, or if the result of instantiating the
     * class is not assignable to the service type, or if any other kind of
     * exception or error is thrown as the next provider is located and
     * instantiated.  To write robust code it is only necessary to catch {@link
     * ServiceConfigurationError} when using a service iterator.
     *
     * <p> If such an error is thrown then subsequent invocations of the
     * iterator will make a best effort to locate and instantiate the next
     * available provider, but in general such recovery cannot be guaranteed.
     *
     * <blockquote style="font-size: smaller; line-height: 1.2"><span
     * style="padding-right: 1em; font-weight: bold">Design Note</span>
     * Throwing an error in these cases may seem extreme.  The rationale for
     * this behavior is that a malformed provider-configuration file, like a
     * malformed class file, indicates a serious problem with the way the Java
     * virtual machine is configured or is being used.  As such it is
     * preferable to throw an error rather than try to recover or, even worse,
     * fail silently.</blockquote>
     *
     * <p> The iterator returned by this method does not support removal.
     * Invoking its {@link java.util.Iterator#remove() remove} method will
     * cause an {@link UnsupportedOperationException} to be thrown.
     *
     * @implNote When adding providers to the cache, the {@link #iterator
     * Iterator} processes resources in the order that the {@link
     * java.lang.ClassLoader#getResources(java.lang.String)
     * ClassLoader.getResources(String)} method finds the service configuration
     * files.
     *
     * @return  An iterator that lazily loads providers for this loader's
     *          service
     */
    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();
            }

        };
    }

    /**
     * Creates a new service loader for the given service type and class
     * loader.
     *
     * @param  <S> the class of the service type
     *
     * @param  service
     *         The interface or abstract class representing the service
     *
     * @param  loader
     *         The class loader to be used to load provider-configuration files
     *         and provider classes, or <tt>null</tt> if the system class
     *         loader (or, failing that, the bootstrap class loader) is to be
     *         used
     *
     * @return A new service loader
     */
    public static <S> ServiceLoader<S> load(Class<S> service,
                                            ClassLoader loader)
    {
        return new ServiceLoader<>(service, loader);
    }

    /**
     * Creates a new service loader for the given service type, using the
     * current thread's {@linkplain java.lang.Thread#getContextClassLoader
     * context class loader}.
     *
     * <p> An invocation of this convenience method of the form
     *
     * <blockquote><pre>
     * ServiceLoader.load(<i>service</i>)</pre></blockquote>
     *
     * is equivalent to
     *
     * <blockquote><pre>
     * ServiceLoader.load(<i>service</i>,
     *                    Thread.currentThread().getContextClassLoader())</pre></blockquote>
     *
     * @param  <S> the class of the service type
     *
     * @param  service
     *         The interface or abstract class representing the service
     *
     * @return A new service loader
     */
    public static <S> ServiceLoader<S> load(Class<S> service) {
        ClassLoader cl = Thread.currentThread().getContextClassLoader();
        return ServiceLoader.load(service, cl);
    }

    /**
     * Creates a new service loader for the given service type, using the
     * extension class loader.
     *
     * <p> This convenience method simply locates the extension class loader,
     * call it <tt><i>extClassLoader</i></tt>, and then returns
     *
     * <blockquote><pre>
     * ServiceLoader.load(<i>service</i>, <i>extClassLoader</i>)</pre></blockquote>
     *
     * <p> If the extension class loader cannot be found then the system class
     * loader is used; if there is no system class loader then the bootstrap
     * class loader is used.
     *
     * <p> This method is intended for use when only installed providers are
     * desired.  The resulting service will only find and load providers that
     * have been installed into the current Java virtual machine; providers on
     * the application's class path will be ignored.
     *
     * @param  <S> the class of the service type
     *
     * @param  service
     *         The interface or abstract class representing the service
     *
     * @return A new service loader
     */
    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);
    }

    /**
     * Returns a string describing this service.
     *
     * @return  A descriptive string
     */
    public String toString() {
        return "java.util.ServiceLoader[" + service.getName() + "]";
    }

}

參考資料

https://www.cnblogs.com/jy107600/p/11464985.html
http://blog.itpub.net/69912579/viewspace-2656555/
https://segmentfault.com/a/1190000020422160