The example for this recipe has been implemented using the Eclipse IDE. If you use Eclipse or a different IDE, such as NetBeans, open it and create a new Java project.

Follow these steps to implement the example:

        public class UnsafeTask implements Runnable{ 
          private Date startDate;

        @Override 
        public void run() { 
          startDate=new Date(); 
          System.out.printf("Starting Thread: %s : %s\n",
                            Thread.currentThread().getId(),startDate); 
          try { 
            TimeUnit.SECONDS.sleep( (int)Math.rint(Math.random()*10)); 
          } catch (InterruptedException e) { 
            e.printStackTrace(); 
          } 
          System.out.printf("Thread Finished: %s : %s\n",
                            Thread.currentThread().getId(),startDate); 
        }

        public class Main { 
          public static void main(String[] args) { 
            UnsafeTask task=new UnsafeTask(); 
            for (int i=0; i<10; i++){ 
              Thread thread=new Thread(task); 
              thread.start(); 
              try { 
                TimeUnit.SECONDS.sleep(2); 
              } catch (InterruptedException e) { 
                e.printStackTrace(); 
              } 
            } 
          } 
        }

        public class SafeTask implements Runnable {

           private static ThreadLocal<Date> startDate=new
                                                ThreadLocal<Date>(){ 
          protected Date initialValue(){ 
            return new Date(); 
          } 
        };

        @Override 
        public void run() { 
          System.out.printf("Starting Thread: %s : %s\n",
                       Thread.currentThread().getId(),startDate.get()); 
          try { 
            TimeUnit.SECONDS.sleep((int)Math.rint(Math.random()*10)); 
          } catch (InterruptedException e) { 
            e.printStackTrace(); 
          } 
          System.out.printf("Thread Finished: %s : %s\n",
                       Thread.currentThread().getId(),startDate.get()); 
        }

In the following screenshot, you can see the results of the execution of the safe sample. The ten Thread objects have their own value of the startDate attribute:

The thread-local variables mechanism stores a value of an attribute for each thread that uses one of these variables. You can read the value using the get() method and change the value using the set() method. The first time you access the value of a thread-local variable, if it has no value for the thread object that it is calling, the thread-local variable will call the initialValue() method to assign a value for that thread and return the initial value.

The thread-local class also provides the remove() method that deletes the value stored in a thread-local variable for the thread that it's calling.

The Java Concurrency API includes the InheritableThreadLocal class that provides inheritance of values for threads created from a thread. If thread A has a value in a thread-local variable and it creates another thread B, then thread B will have the same value as thread A in the thread-local variable. You can override the childValue() method that is called to initialize the value of the child thread in the thread-local variable. It receives the value of the parent thread as a parameter in the thread-local variable.