Monday, 21 July 2014

Java Thread Local with simple example


Java ThreadLocal is an interesting and important Class in Java where most the developers unaware of it. Lets see what is Java ThreadLocal by its importance and how to use it with simple sample code.
Java Thread Local with simple example


What is Java ThreadLocal?
Java ThreadLocal gives the scope of an Object which stored in the ThreadLocal. We can set any Object in the Thread Local which inturns to be global access for the Thread from anywhere. 
Only thing which we need to keep in mind that to maintain a separate instance copy for each thread, so that Object conflict won't occur between multiple threads.
Values stored in ThreadLocal are local to each threads, i.e., each thread will have it's own ThreadLocal Objects where one thread can not access/modify other thread's Thread Local Objects unless it satisfies preview statement. 

Where to use Java ThreadLocal?
Lets a take situation that we need to use some global value for each thread which can be access across and anywhere the Thread is been called (For example in any methods). In those cases we can value or Object in ThreadLocal and same value or Object can be accessed anywhere in particular Thread call. 

There are 4 methods in ThreadLocal class and they are

  • initialValue() - Returns the current thread's "initial value" for this thread-local variable.
  • get() - Returns the value in the current thread's copy of this thread-local variable.
  • set() - Sets the current thread's copy of this thread-local variable to the specified value.
  • remove() - Removes the current thread's value for this thread-local variable.

Lets see simple example how to use ThreadLocal in Java.


public class MyLocalThreadHolder{
 
 private static final ThreadLocal<String> myThreadLocal = new ThreadLocal<String>();

 public static void set(String val) {
  myThreadLocal.set(val);
 }
 
 public static void delete() {
  myThreadLocal.remove();
 }
 
 public static String get() {
 return myThreadLocal.get();
 }
}



public class TestThreadLocal extends Thread{

 public TestThreadLocal(String tName) {
  this.setName(tName);
 }
 
 public static void main(String[] args) {
 
  TestThreadLocal t1 = new TestThreadLocal("First thread...");
  TestThreadLocal t2 = new TestThreadLocal("Second thread...");
  TestThreadLocal t3 = new TestThreadLocal("Third thread...");
  
  t1.start();
  t2.start();
  t3.start();
 }
 
 @Override
 public void run() {
  //Setting thread name in ThreadLocal
  MyLocalThreadHolder.set(currentThread().getName());
  new PrintLocalThread().printThreadName(); 
 }
}



public class PrintLocalThread {
  
 public void printThreadName(){
  //Getting thread name from ThreadLocal and printing
  String currentThreadName = MyLocalThreadHolder.get();
  System.out.println("Before ----> : "+currentThreadName);
  
  //Removing value set in ThreadLocal
  MyLocalThreadHolder.delete();
  
  currentThreadName = MyLocalThreadHolder.get();
  System.out.println("After ----> : "+currentThreadName);
 }  
}


OUTPUT:


Before ----> : Second thread...
After ----> : null
Before ----> : First thread...
After ----> : null
Before ----> : Third thread...
After ----> : null



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