在Android中再次检查了锁定。

volatile

// Broken multithreaded version
// \"Double-Checked Locking\" idiom
class Foo { 
  private Helper helper = null;
  public Helper getHelper() {
    if (helper == null) 
      synchronized(this) {
        if (helper == null) 
          helper = new Helper();
      }    
    return helper;
    }
  // other functions and members...
  }