motan扩展机制

系统里抽象的各个模块，往往有很多不同的实现方案，
比如日志模块的方案，xml解析模块、jdbc模块的方案等。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能不在程序里动态指明，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。
有点类似IOC的思想，就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。

What is the difference between Service Provider Interface (SPI) and Application Programming Interface (API)?
More specifically, for Java libraries, what makes them an API and/or SPI?the API is the description of classes/interfaces/methods/... 
that you call and use to achieve a goalthe SPI is the description of classes/interfaces/methods/... 
that you extend and implement to achieve a goal
Put differently, the API tells you what a specific class/method does for you and the SPI tells you what you must do to conform.
Sometimes SPI and API overlap. 
For example in JDBC the Driver class is part of the SPI: 
If you simply want to use JDBC, you don't need to use it directly, but everyone who implements a JDBC driver must implement that class.
The Connection interface on the other hand is both SPI and API: 
You use it routinely when you use a JDBC driver and it needs to be implemented by the developer of the JDBC driver。

public interface Spi {    public void provide();
}

public class DefaultSpi implements Spi{    @Override
    public void provide() {
        System.out.println("默认spi实现");
    }
}

/**
 * 一个spi的demo
 *
 */
public class SpiDemoMain{
    public static void main( String[] args )
    {        ServiceLoader<Spi> spiServiceLoader = ServiceLoader.load(Spi.class);        Iterator<Spi> spiIterator = spiServiceLoader.iterator();        while ( spiIterator.hasNext()) {
            spiIterator.next().provide();
        }
    }
}

private static final String PREFIX = "META-INF/services/";

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 {            // 调用next方法时，才实例化
                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
        }