Daemon Thread in Java

Important points about Daemon threads in Java

In one line main difference between daemon thread and user thread is that as soon as all user thread finish execution java program or JVM terminates itself, JVM doesn't wait for daemon thread to finish there execution. As soon as last non daemon thread finished JVM terminates no matter how many Daemon thread exists or running inside JVM.

Daemon thread in Java are those thread which runs in background and mostly created by JVM for performing background task like Garbage collection and other house keeping tasks.
1. Any thread created by main thread, which runs main method in Java is by default non daemon because Thread inherits its daemon nature from the Thread which creates it i.e. parent Thread and since main thread is a non daemon thread, any other thread created from it will remain non-daemon until explicitly made daemon by calling setDaemon(true).  

2. Thread.setDaemon(true) makes a Thread daemon but it can only be called before starting Thread in Java. It will throw IllegalThreadStateException if corresponding Thread is already started and running. 

3. Daemon Threads are suitable for doing background jobs like housekeeping, Though I have yet to use it for any practical purpose in application code. let us know if you have used daemon thread in your java application for any practical purpose. 

Example:

public class DaemonTest {

    public static void main(String[] args) {
        new WorkerThread().start();
        try {
            Thread.sleep(7500);
        } catch (InterruptedException e) {}
        System.out.println("Main Thread ending") ;
    }

}
class WorkerThread extends Thread {

    public WorkerThread() {
        setDaemon(true) ;   // When false, (i.e. when it's a user thread),
                // the Worker thread continues to run.
                // When true, (i.e. when it's a daemon thread),
                // the Worker thread terminates when the main 
                // thread terminates.
    }

    public void run() {
        int count=0 ;
        while (true) {
            System.out.println("Hello from Worker "+count++) ;
            try {
                sleep(5000);
            } catch (InterruptedException e) {}
        }
    }
}

Read more: http://javarevisited.blogspot.com/2012/03/what-is-daemon-thread-in-java-and.html#ixzz3B1Dayfvh

Make a class Immutable in Java

An immutable class is one whose state can not be changed once created. There are certain guidelines to create an class immutable. In this post, we will revisit these guidelines.

Sections:

• Benefits of making a class immutable
• Guidelines to make a class immutable

Benefits of making a class immutable

Lets first identify benefits of making a class immutable. Immutable classes are

1. are simple to construct, test, and use
2. are automatically thread-safe and have no synchronization issues
3. do not need a copy constructor
4. do not need an implementation of clone
5. allow hashCode to use lazy initialization, and to cache its return value
6. do not need to be copied defensively when used as a field
7. make good Map keys and Set elements (these objects must not change state while in the collection)
8. have their class invariant established once upon construction, and it never needs to be checked again
9. always have “failure atomicity” (a term used by Joshua Bloch) : if an immutable object throws an exception, it’s never left in an undesirable or indeterminate state

Guidelines to make a class immutable

Java documentation itself has some guidelines identified in this link. We will understand what they mean actually:

1) Don’t provide “setter” methods — methods that modify fields or objects referred to by fields.

This principle says that for all mutable properties in your class, do not provide setter methods. Setter methods are meant to change the state of object and this is what we want to prevent here.

2) Make all fields final and private

This is another way to increase immutability. Fields declared private will not be accessible outside the class and making them final will ensure the even accidentally you can not change them.

3) Don’t allow subclasses to override methods

The simplest way to do this is to declare the class as final. Final classes in java can not be overridden.

4) Special attention when having mutable instance variables

Always remember that your instance variables will be either mutable or immutable. Identify them and return new objects with copied content for all mutable objects. Immutable variables can be returned safely without extra effort.

A more sophisticated approach is to make the constructor private and construct instances in factory methods.

Lets add all above rules and make something concrete class implementation.

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import java.util.Date;   /** * Always remember that your instance variables will be either mutable or immutable. * Identify them and return new objects with copied content for all mutable objects. * Immutable variables can be returned safely without extra effort. * */ public final class ImmutableClass {       /**     * Integer class is immutable as it does not provide any setter to change its content     * */     private final Integer immutableField1;     /**     * String class is immutable as it also does not provide setter to change its content     * */     private final String immutableField2;     /**     * Date class is mutable as it provide setters to change various date/time parts     * */     private final Date mutableField;       //Default private constructor will ensure no unplanned construction of class     private ImmutableClass(Integer fld1, String fld2, Date date)     {         this.immutableField1 = fld1;         this.immutableField2 = fld2;         this.mutableField = new Date(date.getTime());     }       //Factory method to store object creation logic in single place     public static ImmutableClass createNewInstance(Integer fld1, String fld2, Date date)     {         return new ImmutableClass(fld1, fld2, date);     }       //Provide no setter methods       /**     * Integer class is immutable so we can return the instance variable as it is     * */     public Integer getImmutableField1() {         return immutableField1;     }       /**     * String class is also immutable so we can return the instance variable as it is     * */     public String getImmutableField2() {         return immutableField2;     }       /**     * Date class is mutable so we need a little care here.     * We should not return the reference of original instance variable.     * Instead a new Date object, with content copied to it, should be returned.     * */     public Date getMutableField() {         return new Date(mutableField.getTime());     }       @Override     public String toString() {         return immutableField1 +" - "+ immutableField2 +" - "+ mutableField;     } }

Now its time to test our class:

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class TestMain {     public static void main(String[] args)     {         ImmutableClass im = ImmutableClass.createNewInstance(100,"test", new Date());         System.out.println(im);         tryModification(im.getImmutableField1(),im.getImmutableField2(),im.getMutableField());         System.out.println(im);     }       private static void tryModification(Integer immutableField1, String immutableField2, Date mutableField)     {         immutableField1 = 10000;         immutableField2 = "test changed";         mutableField.setDate(10);     } }   Output: (content is unchanged)   100 - test - Tue Oct 30 21:34:08 IST 2012 100 - test - Tue Oct 30 21:34:08 IST 2012

As it can be seen that even changing the instance variables using their references does not change their value, so the class is immutable.

http://howtodoinjava.com/2012/10/28/how-to-make-a-java-class-immutable/