Java SPI的简单小实例

　　JDK有个ServiceLoader类，在java.util包里，支持按约定目录/meta-INF/services去找到接口全路径命名的文件，读取文件内容得到接口实现类的全路径，加载并实例化。如果我们在自己的代码中定义一个接口，别人按接口实现并打包好了，那么我们只需要引入jar包，通过ServiceLoader就能够把别人的实现用起来。举个例子，JDK中的JDBC提供一个数据库连接驱动接口，不同的厂商可以有不同的实现，如果它们给的jar包里按规定提供了配置和实现类，那么我们就可以执行不同的数据库连接操作，比如MySql的jar包里就会有自己的配置：

　　这里文件名就是接口：

　　 文件内容是实现类：

　　我们自己实现一个简单例子，不需要打jar包，把目录放到spring boot的resources下即可，这里就是classpath，跟你放jar包里效果一样。

　　1、定义一个接口：

package com.wlf.service;

public interface ITest {
  void saySomething();
}

　　2、定义两个实现：

package com.wlf.service.impl;

import com.wlf.service.ITest;

public class ITestImpl1 implements ITest {
  @Override
  public void saySomething() {
    System.out.println("Hi, mia.");
  }
}

package com.wlf.service.impl;

import com.wlf.service.ITest;

public class ITestImpl2 implements ITest {
  @Override
  public void saySomething() {
    System.out.println("Hello, world.");
  }
}

　　3、按预定新增/meta-INF/services/com.wlf.service.ITest文件：

com.wlf.service.impl.ITestImpl1
com.wlf.service.impl.ITestImpl2

　　4、定义一个执行类，通过ServiceLoader加载并实例化，调用实现类方法，跑一下：

package com.wlf.service;

import java.util.Iterator;
import java.util.ServiceLoader;

public class TestServiceLoader {
  public static void main(String[] args) {
    ServiceLoader serviceLoader = ServiceLoader.load(ITest.class);
    Iterator iTests = serviceLoader.iterator();
    while (iTests.hasNext()) {
      ITest iTest = iTests.next();
      System.out.printf("loading %sn", iTest.getClass().getName());
      iTest.saySomething();
    }
  }
}

　　打印结果：

　　ServiceLoader源码比较简单，可以看下上面我们使用到的标黄了的方法：

  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();
      }

    };
  }

  // Cached providers, in instantiation order
  private linkedHashMap providers = new linkedHashMap<>();

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

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);
      }
    }

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
    }

    public boolean hasNext() {
      if (acc == null) {
 return hasNextService();
      } else {
 PrivilegedAction action = new PrivilegedAction() {
   public Boolean run() { return hasNextService(); }
 };
 return AccessController.doPrivileged(action, acc);
      }
    }

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

pending = parse(service, configs.nextElement());