SPI概述
SPI全称为(Service Provider Interface) ，是JDK内置的一种服务提供发现机制；主要被框架的开发人员使用，比如java.sql.Driver接口，数据库厂商实现此接口即可，当然要想让系统知道具体实现类的存在，还需要使用固定的存放规则，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；下面以JDBC为实例来进行具体的分析。
JDBC驱动
1.准备驱动包
mysql
mysql-connector-java
5.1.47
org.postgresql
postgresql
42.2.2
com.microsoft.sqlserver
mssql-jdbc
7.0.0.jre8
分别准备了mysql，postgresql和sqlserver，可以打开jar，发现每个jar包的META-INF/services/都存在一个java.sql.Driver文件，文件里面存在一个或多个类名，比如mysql：
com.mysql.jdbc.Driver
com.mysql.fabric.jdbc.FabricMySQLDriver提供的每个驱动类占据一行，解析的时候会按行读取，具体使用哪个会根据url来决定；
2.简单实例
String url = "jdbc:mysql://localhost:3306/db3";
String username = "root";
String password = "root";
String sql = "update travelrecord set name=\'bbb\' where id=1";
Connection con = DriverManager.getConnection(url, username, password);类路径下存在多个驱动包，具体在使用DriverManager.getConnection应该使用哪个驱动类会解析url来识别，不同的数据库有不同的url前缀；
3.驱动类加载分析
具体META-INF/services/下的驱动类是什么时候加载的，DriverManager有一个静态代码块：
static {
loadInitialDrivers();
println("JDBC DriverManager initialized");
}
private static void loadInitialDrivers() {
String drivers;
try {
drivers = AccessController.doPrivileged(new PrivilegedAction() {
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() {
public Void run() {
ServiceLoader loadedDrivers = ServiceLoader.load(Driver.class);
Iterator 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);
}
}
} 在加载DriverManager类的时候会执行loadInitialDrivers方法，方法内通过了两种加载驱动类的方式，分别是：使用系统变量方式和ServiceLoader加载方式；系统变量方式其实就是在变量jdbc.drivers中配置好驱动类，然后使用Class.forName进行加载；下面重点看一下ServiceLoader方式，此处调用了load方法但是并没有真正去加载驱动类，而是返回了一个LazyIterator，后面的代码就是循环变量迭代器：
private static final String PREFIX = "META-INF/services/";
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 {
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
}
......
} 类中指定了一个静态常量PREFIX = “META-INF/services/”，然后和java.sql.Driver拼接组成了fullName，然后通过类加载器去获取所有类路径下java.sql.Driver文件，获取之后存放在configs中，里面的每个元素对应一个文件，每个文件中可能会存在多个驱动类，所以使用pending用来存放每个文件中的驱动信息，获取驱动信息之后在nextService中使用Class.forName加载类信息，并且指定不进行初始化；同时在下面使用newInstance对驱动类进行了实例化操作；每个驱动类中都提供了一个静态注册代码块，比如mysql：
static {
try {
java.sql.DriverManager.registerDriver(new Driver());
} catch (SQLException E) {
throw new RuntimeException("Can't register driver!");
}
}这里又实例化了一个驱动类，同时注册到DriverManager；接下来就是调用DriverManager的getConnection方法，代码如下：
private static Connection getConnection(
String url, java.util.Properties info, Class caller) throws SQLException {
/*
* When callerCl is null, we should check the application's
* (which is invoking this class indirectly)
* classloader, so that the JDBC driver class outside rt.jar
* can be loaded from here.
*/
ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
synchronized(DriverManager.class) {
// synchronize loading of the correct classloader.
if (callerCL == null) {
callerCL = Thread.currentThread().getContextClassLoader();
}
}
if(url == null) {
throw new SQLException("The url cannot be null", "08001");
}
println("DriverManager.getConnection(\"" + url + "\")");
// Walk through the loaded registeredDrivers attempting to make a connection.
// Remember the first exception that gets raised so we can reraise it.
SQLException reason = null;
for(DriverInfo aDriver : registeredDrivers) {
// If the caller does not have permission to load the driver then
// skip it.
if(isDriverAllowed(aDriver.driver, callerCL)) {
try {
println(" trying " + aDriver.driver.getClass().getName());
Connection con = aDriver.driver.connect(url, info);
if (con != null) {
// Success!
println("getConnection returning " + aDriver.driver.getClass().getName());
return (con);
}
} catch (SQLException ex) {
if (reason == null) {
reason = ex;
}
}
} else {
println(" skipping: " + aDriver.getClass().getName());
}
}
// if we got here nobody could connect.
if (reason != null) {
println("getConnection failed: " + reason);
throw reason;
}
println("getConnection: no suitable driver found for "+ url);
throw new SQLException("No suitable driver found for "+ url, "08001");
}此方法主要是遍历之前注册的DriverInfo，拿着url信息去每个驱动类中建立连接，当然每个驱动类中都会进行url匹配校验，成功之后返回Connection，如果中途有失败的连接并不影响尝试新的驱动连接，遍历完之后还是无法获取连接，则抛出异常；
4.扩展
如果想扩展新的驱动类也很简单，只需要在类路径下创建META-INF/services/文件夹，同时在里面创建java.sql.Driver文件，在文件中写入具体的驱动类名称，当然此类需要继承java.sql.Driver接口类；例如实例中提供的TestDriver。
序列化实战
1.准备接口类
public interface Serialization {
/**
* 序列化
*
* @param obj
* @return
*/
public byte[] serialize(Object obj) throws Exception;
/**
* 反序列化
*
* @param param
* @param clazz
* @return
* @throws Exception
*/
public T deserialize(byte[] param, Class clazz) throws Exception;
/**
* 序列化名称
*
* @return
*/
public String getName();
} 2.准备实现类
分别准备JsonSerialization和ProtobufSerialization
3.接口文件
在META-INF/services/目录下创建文件com.spi.serializer.Serialization，内容如下：
com.spi.serializer.JsonSerialization
com.spi.serializer.ProtobufSerialization4.提供Manager类
public class SerializationManager {
private static Map map = new HashMap<>();
static {
loadInitialSerializer();
}
private static void loadInitialSerializer() {
ServiceLoader loadedSerializations = ServiceLoader.load(Serialization.class);
Iterator iterator = loadedSerializations.iterator();
try {
while (iterator.hasNext()) {
Serialization serialization = iterator.next();
map.put(serialization.getName(), serialization);
}
} catch (Throwable t) {
t.printStackTrace();
}
}
public static Serialization getSerialization(String name) {
return map.get(name);
}
} 提供类似DriverManager的SerializationManager类，在加载类的时候加载所有配置的序列化方式；提供一个getSerialization的今天方法类似getConnection；
总结
本文以JDBC驱动为实例，重点对使用ServiceLoader方式服务发现进行分析，同时提供了序列化的简单实战；dubbo也提供了类似的SPI方式，核心类是ExtensionLoader，比起java官方提供的ServiceLoader功能更强大，后续继续分析一下dubbo的SPI方式，然后进行一个对比。
示例代码地址
https://github.com/ksfzhaohui...
https://gitee.com/OutOfMemory...