Java ThreadLocal - How to use and code sample

Source:http://veerasundar.com/blog/2010/11/java-thread-local-how-to-use-and-code-sample/
Thread Local is an interesting and useful concept, yet most of the Java developers are not aware of how to use that. In this post, I’ll explain what is Thread Local and when to use it, with an example code.
Since it’ll be little tough to understand this concept at first, I’ll keep the explanation as simple as possible (corollary: you shouldn’t use this code as it is in a production environment. Grasp the concept and improve upon it!)
Let’s begin.

What is Thread Local?

Thread Local can be considered as a scope of access, like a request scope or session scope. It’s a thread scope. You can set any object in Thread Local and this object will be global and local to the specific thread which is accessing this object. Global and local!!? Let me explain:

• Values stored in Thread Local are global to the thread, meaning that they can be accessed from anywhere inside that thread. If a thread calls methods from several classes, then all the methods can see the Thread Local variable set by other methods (because they are executing in same thread). The value need not be passed explicitly. It’s like how you use global variables.
• Values stored in Thread Local are local to the thread, meaning that each thread will have it’s own Thread Local variable. _disibledevent=>ThreadLocal object using which you can set/get thread scoped variables. Below is a code example demonstrating what I’d explained above.
  Lets first have the Context.java file which will hold the transactionId field.
  

  package com.veerasundar;
  public class Context {
  private String transactionId = null;
  /* getters and setters here */
  }
  

  Now create the MyThreadLocal.java file which will act as a container to hold our context object.
  

  package com.veerasundar;
  /**
  * this class acts as a container to our thread local variables.
  * @author vsundar
  *
  */
  public class MyThreadLocal {
  public static final ThreadLocal userThreadLocal = new ThreadLocal();
  public static void set(Context user) {
  userThreadLocal.set(user);
  }
  public static void unset() {
  userThreadLocal.remove();
  }
  public static Context get() {
  return userThreadLocal.get();
  }
  }
  

  In the above code, you are creating a ThreadLocal object as a static field which can be used by rest of the code to set/get thread local variables.
  Let’s create our main class file which will generate and set the transaction ID in thread local and then call the business method.
  

  package com.veerasundar;
  public class ThreadLocalDemo extends Thread {
  public static void main(String args[]) {
  Thread threadOne = new ThreadLocalDemo();
  threadOne.start();
  Thread threadTwo = new ThreadLocalDemo();
  threadTwo.start();
  }
  @Override
  public void run() {
  // sample code to simulate transaction id
  Context context = new Context();
  context.setTransactionId(getName());
  // set the context object in thread local to access it somewhere else
  MyThreadLocal.set(context);
  /* note that we are not explicitly passing the transaction id */
  new BusinessService().businessMethod();
  MyThreadLocal.unset();
  }
  }
  

  Finally, here’s the code for the BusinessService.java which will read from thread local and use the value.
  

  package com.veerasundar;
  public class BusinessService {
  public void businessMethod() {
  // get the context from thread local
  Context context = MyThreadLocal.get();
  System.out.println(context.getTransactionId());
  }
  }
  

  When you run the ThreadLocalDemo file, you’ll get the below output:
  Thread-0
  Thread-1
  As you might see, even though we are not explicitly passing the transaction id, the value can be accessed from the business method and printed _disibledevent=>http://www.ibm.com/developerworks/java/library/j-threads3/index.html
  
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