package demo; 
 
import java.util.concurrent.locks.Lock; 
import java.util.concurrent.locks.ReentrantLock; 
 
/** 
 * 参考使用 
 * @author Watson_Xu 
 * @date 2012-1-8 下午04:31:09 
 */ 
public final class SingletonContext { 
  private static SingletonContext sc = null; 
  private SingletonContext() { 
   
  } 
   
  private static Lock lock = new ReentrantLock(); 
  public static SingletonContext getInstance() { 
    if(sc == null) { 
      lock.lock(); 
      if(sc == null) { 
        try{ 
          sc = new SingletonContext(); 
        } finally { 
          lock.unlock(); 
        } 
      } 
    } 
    return sc; 
  } 
}

package demo; 
 
/** 
 * 参考使用 
 * 
 * @author Watson_Xu 
 * @date 2012-1-8 下午04:31:09 
 */ 
public final class SingletonContext { 
  private static SingletonContext sc = null; 
 
  private SingletonContext() { 
 
  } 
 
  public static synchronized SingletonContext getInstance() { 
    if(null == sc) { 
      sc = new SingletonContext(); 
    } 
    return new SingletonContext(); 
  } 
}