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Tip of the Day
Language: Java
Expertise: Advanced
May 8, 2002

Creating and Using Thread-Scoped Variables


Sometimes you need variables scoped by thread so that each thread has an independent copy. There's an easy way to create and manage such variables.

The java.lang.ThreadLocal class provides a wrapper around an object that's guaranteed to be independent in each accessing thread. To create a ThreadLocal object, implement the initialValue() method to create and initialize the object you want to maintain. For example:
 
ThreadLocal threadVar = new
adLocal(){  
   protected Object initialValue(){ 
      return new MyObject(); //do any default initialization here 
   } 
};

The threadVar variable that's created is then accessible as any other variable; you can declare it to be static, give it an access restriction like private or public, etc. To retrieve the maintained object, just call the get() method on the ThreadLocal instance (the class cast is optional):
 
MyObject myObject = (MyObject) threadVar.get();

The myObject object, when retrieved this way, is independent in each thread that runs this code and initialized to its default value as set in the initialValue() method. When using this technique, you can avoid having to synchronize the encapsulated object and utilize truly "thread-safe" variables in your applications.
Ramneek Handa
 
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