JDK有個ServiceLoader類，在java.util包裡，支援按約定目錄/META-INF/services去找到介面全路徑命名的檔案，讀取檔案內容得到介面實現類的全路徑，載入並例項化。如果我們在自己的程式碼中定義一個介面，別人按介面實現並打包好了，那麼我們只需要引入jar包，通過ServiceLoader就能夠把別人的實現用起來。舉個例子，JDK中的JDBC提供一個數據庫連線驅動介面，不同的廠商可以有不同的實現，如果它們給的jar包裡按規定提供了配置和實現類，那麼我們就可以執行不同的資料庫連線操作，比如MySql的jar包裡就會有自己的配置：

　　這裡檔名就是介面：

　　 檔案內容是實現類：

　　我們自己實現一個簡單例子，不需要打jar包，把目錄放到spring boot的resources下即可，這裡就是classpath，跟你放jar包裡效果一樣。

　　1、定義一個介面：

package com.wlf.service;

public interface ITest {
  void saySomething();
}

　　2、定義兩個實現：

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、按預定新增/META-INF/services/com.wlf.service.ITest檔案：

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

　　4、定義一個執行類，通過ServiceLoader載入並例項化，呼叫實現類方法，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<ITest> serviceLoader = ServiceLoader.load(ITest.class);
    Iterator<ITest> iTests = serviceLoader.iterator();
    while (iTests.hasNext()) {
      ITest iTest = iTests.next();
      System.out.printf("loading %s\n",iTest.getClass().getName());
      iTest.saySomething();
    }
  }
}

　　列印結果：

　　ServiceLoader原始碼比較簡單，可以看下上面我們使用到的標黃了的方法：

/**
   * Lazily loads the available providers of this loader's service.
   *
   * <p> 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.
   *
   * <p> 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.
   *
   * <p> 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.
   *
   * <blockquote style="font-size: smaller; line-height: 1.2"><span
   * style="padding-right: 1em; font-weight: bold">Design Note</span>
   * 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.</blockquote>
   *
   * <p> 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<S> iterator() {
    return new Iterator<S>() {

      Iterator<Map.Entry<String,S>> 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();
      }

    };
  }

　　我們用到的迭代器其實是一個Map：

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

　　它用來快取載入的實現類，真正執行的是lookupIterator：

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

　　我們看下它的hasNext和next方法：

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);
      }
    }

    public S next() {
      if (acc == null) {
        return nextService();
      } else {
        PrivilegedAction<S> action = new PrivilegedAction<S>() {
          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<Boolean> action = new PrivilegedAction<Boolean>() {
          public Boolean run() { return hasNextService(); }
        };
        return AccessController.doPrivileged(action,acc);
      }
    }

　　hasNext查詢實現類，並指定了類路徑：

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

　　具體查詢操作看這裡：

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

　　next則是例項化載入到的實現類，使用反射Class.forName載入類、newInstance例項化物件。

以上就是Java SPI的簡單小例項的詳細內容，更多關於Java SPI例項的資料請關注我們其它相關文章！