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 %s\n", 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：

/**
   * 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();
      }

    };
  }

　　我们用到的迭代器其实是一个Map：

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

　　它用来缓存加载的实现类，真正执行的是lookupIterator：

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

　　我们看下它的hasNext和next方法：

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);
      }
    }

　　hasNext查找实现类，并指定了类路径：

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

　　具体查找操作看这里：

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

　　next则是实例化加载到的实现类，使用反射Class.forName加载类、newInstance实例化对象。

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