Java SPI (Service Provider Interface) 机制详解

概述

关键词:解耦,可拔插,面向接口编程,动态类加载。

本质:Java SPI 实际上是“基于接口的编程+策略模式+约定配置文件” 组合实现的动态加载机制,在JDK中提供了工具类:“java.util.ServiceLoader”来实现服务查找。

什么是SPI ?

SPI 全称:Service Provider Interface,是Java提供的一套用来被第三方实现或者扩展的接口,它可以用来启用框架扩展和替换组件。

面向的对象的设计里,我们一般推荐模块之间基于接口编程,模块之间不对实现类进行硬编码。一旦代码里涉及具体的实现类,就违反了可拔插的原则,如果需要替换一种实现,就需要修改代码。

为了实现在模块装配的时候不用在程序里动态指明,这就需要一种服务发现机制。java spi就是提供这样的一个机制:为某个接口寻找服务实现的机制。这有点类似IOC的思想,将装配的控制权移到了程序之外。

SPI的作用就是为被扩展的API寻找服务实现。

SPI(Service Provider Interface),是JDK内置的一种 服务提供发现机制,可以用来启用框架扩展和替换组件,主要是被框架的开发人员使用,比如java.sql.Driver接口,其他不同厂商可以针对同一接口做出不同的实现,MySQL和PostgreSQL都有不同的实现提供给用户,而Java的SPI机制可以为某个接口寻zhao服务实现。Java中SPI机制主要思想是将装配的控制权移到程序之外,在模块化设计中这个机制尤其重要,其核心思想就是 解耦。

SPI整体机制图如下:

当服务的提供者提供了一种接口的实现之后,需要在classpath下的META-INF/services/目录里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的实现类。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。

SPI 的不足

1.不能按需加载,需要遍历所有的实现,并实例化,然后在循环中才能找到我们需要的实现。如果不想用某些实现类,或者某些类实例化很耗时,它也被载入并实例化了,这就造成了浪费。

2.获取某个实现类的方式不够灵活,只能通过 Iterator 形式获取,不能根据某个参数来获取对应的实现类。(Spring 的BeanFactory,ApplicationContext 就要高级一些了。)

3.多个并发多线程使用 ServiceLoader 类的实例是不安全的。

API 与 SPI

SPI与API区别:

API是调用并用于实现目标的类、接口、方法等的描述;

SPI是扩展和实现以实现目标的类、接口、方法等的描述;

换句话说,API 为操作提供特定的类、方法,SPI 通过操作来符合特定的类、方法。

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

SPI和API的使用场景解析:

  • API (Application Programming Interface)在大多数情况下,都是实现方制定接口并完成对接口的实现,调用方仅仅依赖接口调用,且无权选择不同实现。 从使用人员上来说,API 直接被应用开发人员使用。
  • SPI (Service Provider Interface)是调用方来制定接口规范,提供给外部来实现,调用方在调用时则选择自己需要的外部实现。 从使用人员上来说,SPI 被框架扩展人员使用。

SPI 应用场景

SPI扩展机制应用场景有很多,比如Common-Logging,JDBC,Dubbo等等。

SPI流程:

有关组织和公式定义接口标准

第三方提供具体实现: 实现具体方法, 配置 META-INF/services/${interface_name} 文件

开发者使用

比如JDBC场景下:

首先在Java中定义了接口java.sql.Driver,并没有具体的实现,具体的实现都是由不同厂商提供。

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。

同样在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。

项目案例

Java 工程目录:

下面我们来简单实现一个 JDK 的SPI的简单实现。

Java代码开发

首先第一步,定义一个接口:

Phone.java

package com.light.sword;

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

这个接口分别有两个实现:

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";
    }
}

约定配置:新建 META-INF/services 目录

注意:这个META-INF/services 目录是写死的约定,在 java.util.ServiceLoader 源码实现中, java.util.ServiceLoader#PREFIX 可以看到这个目录的硬编码。

然后需要在resources目录下新建 META-INF/services 目录,并且在这个目录下新建一个与上述接口的全限定名一致的文件:

com.light.sword.Phone (这是一个文件,是的,一切皆是文件。)

在这个文件中写入接口的实现类的全限定名(文件 com.light.sword.Phone 中写死的内容):

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

如下图所示:

加载实现类并调用服务

这时,通过ServiceLoader 加载实现类并调用服务:

Main.java

package com.light.sword;

import java.util.ServiceLoader;

public class Main {

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

输出如下:

Hong Meng
iOS

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

这样一个简单的 Java SPI 的demo就完成了。可以看到其中最为核心的就是通过一系列的约定(其实,就是按照人家 java.util.ServiceLoader 的规范标准来), 然后,通过ServiceLoader 这个类来加载具体的实现类,进而调用实现类的服务。


知识拓展:

其实,我们在Spring框架中,可以通过 component-scan 标签来对指定包路径进行扫描,只要扫到 Spring 制定的 @Service@Controller 等注解,spring自动会把它注入容器。 这就相当于spring制定了注解规范,我们按照这个注解规范开发相应的实现类或controller,spring并不需要感知我们是怎么实现的,他只需要根据注解规范和scan标签注入相应的bean,这正是 spi 理念的体现。

SPI 实现原理解析

首先,ServiceLoader实现了Iterable接口,所以它有迭代器的属性,这里主要都是实现了迭代器的hasNext和next方法。这里主要都是调用的lookupIterator的相应hasNext和next方法,lookupIterator是懒加载迭代器。

其次,LazyIterator中的hasNext方法,静态变量PREFIX就是”META-INF/services/”目录,这也就是为什么需要在classpath下的META-INF/services/目录里创建一个以服务接口命名的文件。

最后,通过反射方法Class.forName()加载类对象,并用newInstance方法将类实例化,并把实例化后的类缓存到providers对象中,(LinkedHashMap类型) 然后返回实例对象。


java.util.ServiceLoader.java 源代码如下:


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实现了Iterable接口,可以遍历所有的服务实现者
public final class ServiceLoader implements Iterable
{

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

    // The class or interface representing the service being loaded
    private final Class 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 providers = new LinkedHashMap<>();

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

    /**
     * Clear this loader's provider cache so that all providers will be
     * reloaded.
     *
     * 

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. * *

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 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 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 parse(Class service, URL u) throws ServiceConfigurationError { InputStream in = null; BufferedReader r = null; ArrayList 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 { Class service; ClassLoader loader; Enumeration configs = null; Iterator pending = null; String nextName = null; private LazyIterator(Class 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 { // 用反射机制,创建接口实现对象 S p = service.cast(c.newInstance()); // 放进 ServiceLoader的providers容器里面 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 action = new PrivilegedAction() { public Boolean run() { return hasNextService(); } }; return AccessController.doPrivileged(action, acc); } } public S next() { if (acc == null) { return nextService(); } else { PrivilegedAction action = new PrivilegedAction() { 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. * *

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. * *

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. * *

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. * *

Design Note * 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.
* *

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 iterator() { return new Iterator() { Iterator> 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 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 null if the system class * loader (or, failing that, the bootstrap class loader) is to be * used * * @return A new service loader */ public static ServiceLoader load(Class 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}. * *

An invocation of this convenience method of the form * *

     * ServiceLoader.load(service)
* * is equivalent to * *
     * ServiceLoader.load(service,
     *                    Thread.currentThread().getContextClassLoader())
* * @param the class of the service type * * @param service * The interface or abstract class representing the service * * @return A new service loader */ public static ServiceLoader load(Class 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. * *

This convenience method simply locates the extension class loader, * call it extClassLoader, and then returns * *

     * ServiceLoader.load(service, extClassLoader)
* *

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. * *

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 the class of the service type * * @param service * The interface or abstract class representing the service * * @return A new service loader */ public static ServiceLoader loadInstalled(Class 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() + "]"; } }

参考资料

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

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