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
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