What?
SPI机制（Service Provider Interface)其实源自服务提供者框架（Service Provider Framework，参考【EffectiveJava】page6)，是一种将服务接口与服务实现分离以达到解耦、大大提升了程序可扩展性的机制。引入服务提供者就是引入了spi接口的实现者，通过本地的注册发现获取到具体的实现类，轻松可插拔
典型实例：jdbc的设计
通常各大厂商（如Mysql、Oracle）会根据一个统一的规范(java.sql.Driver)开发各自的驱动实现逻辑。客户端使用jdbc时不需要去改变代码，直接引入不同的spi接口服务即可。
Mysql的则是com.mysql.jdbc.Drive,Oracle则是oracle.jdbc.driver.OracleDriver。
伪代码如下:
//注:从jdbc4.0之后无需这个操作,spi机制会自动找到相关的驱动实现
//Class.forName(driver);
//1.getConnection()方法，连接MySQL数据库。有可能注册了多个Driver，这里通过遍历成功连接后返回。
con = DriverManager.getConnection(mysqlUrl,user,password);
//2.创建statement类对象，用来执行SQL语句！！
Statement statement = con.createStatement();
//3.ResultSet类，用来存放获取的结果集！！
ResultSet rs = statement.executeQuery(sql);jdbc连接源码分析
1. java.sql.DriverManager静态块初始执行，其中使用spi机制加载jdbc具体实现
//java.sql.DriverManager.java
//当调用DriverManager.getConnection(..)时，static会在getConnection(..)执行之前被触发执行
/**
* 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");
}2.loadInitialDrivers()中完成了引入的数据库驱动的查找以及载入，本示例只引入了oracle厂商的mysql，我们具体看看。
//java.util.serviceLoader.java
private static void loadInitialDrivers() {
String drivers;
try {
drivers = AccessController.doPrivileged(new PrivilegedAction<String>() {
public String run() {
//使用系统变量方式加载
return System.getProperty("jdbc.drivers");
}
});
} catch (Exception ex) {
drivers = null;
}
//如果spi 存在将使用spi方式完成提供的Driver的加载
// 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() {
//查找具体的provider,就是在META-INF/services/***.Driver文件中查找具体的实现。
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.
*/
//查找具体的实现类的全限定名称
try{
while(driversIterator.hasNext()) {
driversIterator.next();//加载并初始化实现类
}
} catch(Throwable t) {
// Do nothing
}
return null;
}
});
println("DriverManager.initialize: jdbc.drivers = " + drivers);
if (drivers == null || drivers.equals("")) {
return;
}
String[] driversList = drivers.split(":");
....
}
}3.java.util.ServiceLoader 加载spi实现类.
上一步的核心代码如下，我们接着分析：
//java.util.serviceLoader.java
ServiceLoader<Driver> loadedDrivers = ServiceLoader.load(Driver.class);
Iterator<Driver> driversIterator = loadedDrivers.iterator();
try{
//查找具体的实现类的全限定名称
while(driversIterator.hasNext()) {
//加载并初始化实现
driversIterator.next();
}
} catch(Throwable t) {
// Do nothing
}主要是通过ServiceLoader来完成的,我们按照执行顺序来看看ServiceLoader实现：
//初始化一个ServiceLoader,load参数分别是需要加载的接口class对象,当前类加载器
public static <S> ServiceLoader<S> load(Class<S> service) {
ClassLoader cl = Thread.currentThread().getContextClassLoader();
return ServiceLoader.load(service, cl);
}
public static <S> ServiceLoader<S> load(Class<S> service,
ClassLoader loader)
{
return new ServiceLoader<>(service, loader);
}遍历所有存在的service实现
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);
}
} //写死的一个目录
private static final String PREFIX = "META-INF/services/";
private boolean hasNextService() {
if (nextName != null) {
return true;
}
if (configs == null) {
try {
String fullName = PREFIX + service.getName();
//通过相对路径读取classpath中META-INF目录的文件，也就是读取服务提供者的实现类全限定名
if (loader == null)
configs = ClassLoader.getSystemResources(fullName);
else
configs = loader.getResources(fullName);
} catch (IOException x) {
fail(service, "Error locating configuration files", x);
}
}
//判断是否读取到实现类全限定名,比如mysql的“com.mysql.jdbc.Driver
”
while ((pending == null) || !pending.hasNext()) {
if (!configs.hasMoreElements()) {
return false;
}
pending = parse(service, configs.nextElement());
}
nextName = pending.next();//nextName保存,后续初始化实现类使用
return true;//查到了 返回true，接着调用next()
} public S next() {
if (acc == null) {//用来判断serviceLoader对象是否完成初始化
return nextService();
} else {
PrivilegedAction<S> action = new PrivilegedAction<S>() {
public S run() { return nextService(); }
};
return AccessController.doPrivileged(action, acc);
}
}
private S nextService() {
if (!hasNextService())
throw new NoSuchElementException();
String cn = nextName;//上一步找到的服务实现者全限定名
nextName = null;
Class<?> c = null;
try {
//加载字节码返回class对象.但并不去初始化（换句话就是说不去执行这个类中的static块与static变量初始化）
//
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 {
//初始化这个实现类.将会通过static块的方式触发实现类注册到DriverManager(其中组合了一个CopyOnWriteArrayList的registeredDrivers成员变量)中
S p = service.cast(c.newInstance());
providers.put(cn, p);//本地缓存 （全限定名，实现类对象）
return p;
} catch (Throwable x) {
fail(service,
"Provider " + cn + " could not be instantiated",
x);
}
throw new Error(); // This cannot happen
}上一步中，Sp = service.cast(c.newInstance()) 将会导致具体实现者的初始化，比如mysqlJDBC，会触发如下代码：
//com.mysql.jdbc.Driver.java
......
private final static CopyOnWriteArrayList<DriverInfo> registeredDrivers = new CopyOnWriteArrayList<>();
......
static {
try {
//并发安全的想一个copyOnWriteList中方
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}4.最终Driver全部注册并初始化完毕，开始执行DriverManager.getConnection(url, “root”, “root”)方法并返回。
使用实例
四个项目:spiInterface、spiA、spiB、spiDemo
spiInterface中定义了一个com.zs.IOperation接口。
spiA、spiB均是这个接口的实现类，服务提供者。
spiDemo作为客户端,引入spiA或者spiB依赖，面向接口编程，通过spi的方式获取具体实现者并执行接口方法。
├─spiA
│ └─src
│ ├─main
│ │ ├─java
│ │ │ └─com
│ │ │ └─zs
│ │ ├─resources
│ │ │ └─META-INF
│ │ │ └─services
│ │ └─webapp
│ │ └─WEB-INF
│ └─test
│ └─java
├─spiB
│ └─src
│ ├─main
│ │ ├─java
│ │ │ └─com
│ │ │ └─zs
│ │ ├─resources
│ │ │ └─META-INF
│ │ │ └─services
│ │ └─webapp
│ │ └─WEB-INF
│ └─test
│ └─java
├─spiDemo
│ └─src
│ ├─main
│ │ ├─java
│ │ │ └─com
│ │ │ └─zs
│ │ ├─resources
│ │ └─webapp
│ │ └─WEB-INF
│ └─test
│ └─java
└─spiInterface
└─src
├─main
│ ├─java
│ │ └─com
│ │ └─zs
│ ├─resources
│ └─webapp
│ └─WEB-INF
└─test
└─java
└─spiInterfacespiDemo
package com.zs;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Iterator;
import java.util.ServiceLoader;
public class Launcher {
public static void main(String[] args) throws Exception {
// jdbcTest();
showSpiPlugins();
}
private static void jdbcTest() throws SQLException {
String url = "jdbc:mysql://localhost:3306/test";
Connection conn = DriverManager.getConnection(url, "root", "root");
Statement statement = conn.createStatement();
ResultSet set = statement.executeQuery("select * from test.user");
while (set.next()) {
System.out.println(set.getLong("id"));
System.out.println(set.getString("userName"));
System.out.println(set.getInt("age"));
}
}
private static void showSpiPlugins() {
ServiceLoader<IOperation> operations = ServiceLoader.load(IOperation.class);
Iterator<IOperation> operationIterator = operations.iterator();
while (operationIterator.hasNext()) {
IOperation operation = operationIterator.next();
System.out.println(operation.operation(6, 3));
}
}
}dubbo自定义的方式请参考：https://dubbo.apache.org/zh/docs/v2.7/dev/impls/