netty补充NIO的SelectableChannel和SelectorProvider

/**
 * A channel that can be multiplexed via a {@link Selector}.
 * <p> In order to be used with a selector, an instance of this class must
 * first be <i>registered</i> via the {@link #register(Selector,int,Object)
 * register} method.  This method returns a new {@link SelectionKey} object
 * that represents the channel's registration with the selector.
 *其他略。。。
 */

    public static Selector open() throws IOException {
        return SelectorProvider.provider().openSelector();
    }

 public static SelectorProvider provider() {
        synchronized (lock) {
            if (provider != null)
                return provider;
//AccessController.doPrivileged属于特权操作，下面详说
            return AccessController.doPrivileged(
                new PrivilegedAction<SelectorProvider>() {
                    public SelectorProvider run() {
 //loadProviderFromProperty方法是通过JDK的参数//-Djava.nio.channels.spi.SelectorProvider=class设置的class来反射构造SelectorProvider
                            if (loadProviderFromProperty())
                                return provider;
//loadProviderAsService从jar中的目录META-INF/services配置文件中找参数//java.nio.channels.spi.SelectorProvider=class设置的第一个class来反射构造SelectorProvider
                            if (loadProviderAsService())
                                return provider;
//最后都没有则调用不同操作系统版本的JDK里自带的sun.nio.ch.DefaultSelectorProvider来创建SelectorProvider
                            provider = sun.nio.ch.DefaultSelectorProvider.create();
                            return provider;
                        }
                    });
        }
    }

1、根据当前有效的安全策略，决定是允许还是拒绝对关键系统资源的访问，
2、将代码标记为“特权”，从而影响后续访问确定
3、获取当前调用上下文的“快照”，因此可以针对保存的上下文进行来自不同上下文的访问控制决策。

//类注释
/**
 * somemethod() {
 *     ...normal code here...
 *     String user = AccessController.doPrivileged(
 *         new PrivilegedAction<String>() {
 *         public String run() {
 *             return System.getProperty("user.name");
 *             }
 *         });
 *     ...normal code here...
 * }}
* 其他略。。
**/
public final class AccessController {
//方法注释
 /**
     * Performs the specified {@code PrivilegedAction} with privileges
     * enabled. The action is performed with <i>all</i> of the permissions
     * possessed by the caller's protection domain.
     * 其他略。。
     */
//简言之就是让调用者有权执行方法代码中的action
    @CallerSensitive
    public static native <T> T doPrivileged(PrivilegedAction<T> action);
//其他略。。
}

public class DefaultSelectorProvider {
//
    public static SelectorProvider create() {
        return new WindowsSelectorProvider();
    }
}

public class DefaultSelectorProvider {
 public static SelectorProvider create()
  {
    return new KQueueSelectorProvider();
  }
}

public class DefaultSelectorProvider {
public static SelectorProvider create()
  {
    String str1 = (String)AccessController.doPrivileged(new GetPropertyAction("os.name"));
 
    if ("SunOS".equals(str1)) {
      return new DevPollSelectorProvider();
    }
 
    if ("Linux".equals(str1)) {
      String str2 = (String)AccessController.doPrivileged(new GetPropertyAction("os.version"));
      String[] arrayOfString = str2.split("\\.", 0);
      if (arrayOfString.length >= 2) {
        try {
          int i = Integer.parseInt(arrayOfString[0]);
          int j = Integer.parseInt(arrayOfString[1]);
          if ((i > 2) || ((i == 2) && (j >= 6))) {
            return new EPollSelectorProvider();
          }
        }
        catch (NumberFormatException localNumberFormatException)
        {
        }
      }
    }
    return new PollSelectorProvider();
  }
}