Object lock=new Object();
synchronized(lock)
{
  double d = 1.0；
}

------解决方案--------------------
http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-double.html?page=1

class SomeClass {
 private Resource resource = null;
 public Resource getResource() {
   if (resource == null) {
     synchronized {
       if (resource == null)  
         resource = new Resource();
     }
   }
   return resource;
 }
}
DCL relies on an unsynchronized use of the resource field. That appears to be harmless, but it is not. To see why, imagine that thread A is inside the synchronized block, executing the statement resource = new Resource(); while thread B is just entering getResource(). Consider the effect on memory of this initialization. Memory for the new Resource object will be allocated; the constructor for Resource will be called, initializing the member fields of the new object; and the field resource of SomeClass will be assigned a reference to the newly created object.  

However, since thread B is not executing inside a synchronized block, it may see these memory operations in a different order than the one thread A executes. It could be the case that B sees these events in the following order (and the compiler is also free to reorder the instructions like this): allocate memory, assign reference to resource, call constructor. Suppose thread B comes along after the memory has been allocated and the resource field is set, but before the constructor is called. It sees that resource is not null, skips the synchronized block, and returns a reference to a partially constructed Resource! Needless to say, the result is neither expected nor desired.  



------解决方案--------------------
JMM 的关系，由于共享变量在本地线程栈中有个复制缓存，可能会因为不能及时更新的关系，而使用得双重检查同步锁同样会产生多个对象的实例。