Class Loading and Unloading in JVM

Class Loader is one of the major component (sub system) of the Java Virtual Machine(JVM). This posts talks about loading and unloading of a class (or interface) into virtual machine. JVM specification refers to classes and interfaces as type.

A Java program is composed of many individual class files, unlike C/C++ which has single executable file. Each of these individual class files correspond to a single Java class and it gets loaded the first time you create an object from the class or the first time you access a static component (block, field or method) of the class.

A class loader's basic objective is to service a request for a class. JVM needs a class (ofcourse for instantiating it), so it asks the class loader by passing full name of the class. And in return class loader gives back a Class object representing the class(more detail here). Below diagram shows sequence of steps:

Class Loading

In the JVM architecture tutorial (link), I have discussed class loader subsystem briefly. JVM has a flexible class loader architecture that enables a Java application to load classes in custom ways. Each JVM has at 

least two class loaders. Let's cover both of them:

• Bootstrap class loader : Part of JVM implementation and it is used to load the Java API classes only. It "bootstraps" the JVM and is also known as primordial, system or default class loader.
• User-defined class loaders : There could be multiple user-defined class loaders. Class loaders are like normal Java classes, so application can install user-defined class loaders to load classes in custom way. You can dynamically extend Java application at run time with the help of user-defined class loaders.

JVM keeps track of which class loader loaded a given class. Classes can only see other classes loaded by the same class loader (for security concerns). Java architecture achieves this by maintaining name-space inside a Java application. Each class loader in a running Java application has its own name-space and class inside one class loader can't access a class from another class loader unless the application explicitly permits this access. This name-space restriction means :

• You can load only ONE class named as say Fruit in a given name-space. 
• But you can create multiple name-space by creating multiple class loaders in the same application. So, you can load three Fruit classes in three different class loaders. And all these Fruit classes will be unaware of the presence of rest two.

Now, obvious question is, how these multiple class loaders in the same application load classes ?

Class loaders use Parent-delegation technique to load classes. Each class loader except bootstrap class loader has a parent. Class loaders asks its parent to load a particular class. This delegation continues all the way to the bootstrap class loader, which is the last class loader in the chain. If the parent class loader can load a type, the class loader returns that type. Otherwise current class loader attempts to load the class itself. This approach ensures that you can't load your own String class, because request to load a new String class will always lead to the System/bootstrap class loader which is responsible for loading Java API classes. Also classes from Java API get loaded only when there is a request for one. 

What if, I create my own class in Java.util package ?

Java gives special privileges to classes in the same package. So does it mean my class say Jerk inside Java.util package will get special access and security will get compromised ?  

Java only grants this special access to class in the same package which gets loaded by the same class loader. So your Jerk class which would have got loaded by an user-class loader will NOT get access to java.lang classes of Java API. So code found on class path by the class path class loader can't gain access to package-visible members of the Java API. 

Class Unloading

Lifetime of a class is similar to the lifetime of an object. As JVM may garbage collects the objects after they are no longer referenced by the program. Similarly virtual machine can optionally unload the classes after they are no longer referenced by the program. 

Java program can be dynamically extended at run time by loading new types through user-defined class loaders. All these loaded types occupy space in the method area. So just like normal heaps the memory footprints of method areas grows and hence types can be freed as well by unloading if they are no longer needed. If the application has no references to a given type, then the type can be unloaded or garbage collected (like heap memory).

Until Java SE 7; classes (and Constant pool) were stored in Permanent Generation (or PermGen).  Tuning PermGen size was complicated so Java SE 8 has completely removed PermGen and now classes (Java HotSpot VM representation of class) are moved into native or heap memory, link.

Types (Class/Interface) loaded through the bootstrap loader will always be reachable and will never be unloaded. Only types which are loaded by user-defined class loaders can become unreachable and hence can be unloaded by JVM. A class instance can be reachable in following case:

• Class instance will be reachable if the application holds an explicit reference to the instance. 
• Class instance will be reachable if there is a reachable object on the heap whose type data in the method area refers to the Class instance. 

References:
http://www.artima.com/insidejvm/ed2/jvm5.html 

Related Articles:
how-garbage-collection-works
jvm-architecture
life-cycle-of-object-in-java

---
calling it a post; do give your feedback!!!

Life Cycle of an Object in Java

In Java, objects play the pivotal role; so understanding its instantiation, and how and when it gets garbage collected is important. This post covers the life cycle of objects inside Java Virtual Machine. But keep in mind that, successful creation of the object is possible only if class is already loaded in JVM (by class loader).

Object Creation 

Class instantiation activity gives life to an object and thereafter you can call its methods to manipulate states/attributes. Object creation is done by Java Virtual Machine(after class has been loaded). When JVM creates a new instance of a class, it allocates memory on the heap to hold the object's instance. Memory is allocated for all variables/attributes declared in object's class and in all its superclasses (including hidden attributes). Once the virtual machine has allocated memory for the new object, the machine initializes the instance variable to default initial values. After this VM gives proper initial value depending on how the object gets created:

   1.  new keyword

        Person p = new Person();

        String s = new String("life");

        This is one of the most frequently used approach to create an instance of a class. 

   2.  newInstance() on a Class / Reflection

        Class class = Class.forName("fully.qualified.class.name");

        Object obj = class.newInstance();

        Person p = (Person)obj;

         

        Refer to the detailed Reflection tutorial. 

   3. Cloning         
       Person pClone = (Person)p.clone(); 

       Make sure that, Person class implements Cloneable interface and also provides the definition of the clone() method. 

Refer to the detailed cloning tutorial.

   4. Deserialization      
       FileInputStream fis = new FileInputStream("person.ser");

       ObjectInputStream ois = new ObjectInputStream(fis);

       Person per =(Person)ois.readObject(); 

      

       Deserialization is a technique to convert the encoded byte stream into the corresponding object as 

       shown above. Refer to the detailed tutorial.

   5. Implicit Instantiation

       All above techniques explicitly creates an object of a class. Apart from above, there are multiple implicit ways of object creation as well :

          

       1. Class loading : For every type that a JVM loads, it implicitly creates a new Class object to                  represent that type.

       2. String Literals : When JVM loads a class which has String literals (i.e. String PI = "3.14"), 

           it might create a new String object. 

       3. Expression Evaluation : Process of evaluation an expression like String concatenation

           can also create new objects. 

               

 So above explicit/implicit object instantiation techniques give proper initial value to the object. JVM uses one of the three techniques to do this task, depending on how the object is being created.

If the object is being created because of a clone() invocation, the virtual machine copies the values of the instance variables of the object being cloned into the new object. If the object is deserialized via a readObject() invocation on an ObjectInputStream, the virtual machine initializes non-transient instance object variables from values read from the input stream. Otherwise, the virtual machine invokes an instance initialization method on the object. The instance initialization method initializes the object's instance variables to their proper initial value. 

The Java compiler generates at least one instance initialization method for every class it compiles. In the  Java class file, the instance initialization method is called "<init>". For each constructor in the source code of a class, the Java compiler generates one <init>() method.  

Source : Inside Java 2 Virtual Machine (Lifetime of a type)

Object in action

Once object is created successfully, its ready for the real action.
obj.haveFun();

Garbage Collection of Object / End of life

Application can allocate memory for object via explicit and implicit ways described under object creation, but they can not explicitly free that memory. To signal to JVM that an object is ready for garbage collection, object needs to get unrefrenced in either of below ways :

1. Explicit unreferencing

    person = null;  //person is an instance of Person   

2. Object goes out of scope

    An Object created inside a method goes out of scope once method returns to the calling method.

    So in below case once method() is over, p implicitly becomes null. 

    public void method(){

       Person p = new Person();

       //method action

    } 

   After unrefrencing, VM can reclaim that memory. Virtual machine can decide when to garbage collect unrefrenced object. But specification doesn't guarantee any predictable behavior. It is up-to the VM to decide when to reclaim memory from unrefrenced object or it might NOT reclaim memory at all.

If the class declares a finalizer (i.e. public void finalize() method ) then Garbage Collector will execute the finalize() method on the instance of the class before it frees the memory space occupied by that instance. So clearly, exact time of garbage collection is unpredictable.

Related Article:
how-garbage-collection-works
class-loading-and-unloading-in-jvm
jvm-architecture.html

Latch API in Java : CountDownLatch

Literally, latch means a device for keeping a door or gate closed.  Its meaning is analogous to a gate in Java as well. So if latch (gate) is open, everyone can pass through it but when it's shut, no one is allowed to cross over. With this as background let's go in detail:

It's one of the advanced threading/concurrency concepts of Java. Java provides a Latch API named as  CountDownLatch which got introduced in Java 5 ( java.util.concurrent package). So now on; I will refer Latch and CountDownLatch interchangeably to refer to the same thing. Latch is a synchronizer that can delay the progress of threads until it reaches its terminal state. [A synchronizer is any object that coordinates the control flow of threads based on its state] So it is used to synchronize one or more tasks by forcing them to wait for the completion of a set of operation being performed by other tasks.

CountDownLatch in action

Let me start directly with a simple example to stress on the fundamentals of this API. Below sample program has two tasks (as taskone() and tasktwo()) represented as methods. And I want to make sure that taskone() should get completed before tasktwo(). 

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class CountDownLatchTest {
 static CountDownLatch latch;

 CountDownLatchTest(final int count) {
  latch = new CountDownLatch(count);
 }

 public void firstTask() {
  Runnable s1 = new Runnable() {
   public void run() {
    try {
         System.out.println("waiting....");
         TimeUnit.SECONDS.sleep(5);
    } catch (InterruptedException e) {
         e.printStackTrace();
    }
    // finish first activity before last line
    latch.countDown();
   }
  };
  Thread t = new Thread(s1);
  t.start();
 }

 public void secondTask() {
  Runnable s2 = new Runnable() {
   public void run() {
    try {
         System.out.println("wait....");
         latch.await();
         // perform task here
         System.out.println("after wait.... done");
    } catch (InterruptedException e1) {
         e1.printStackTrace();
    }
   }
  };
  Thread t = new Thread(s2);
  t.start();
 }

 public static void main(String[] args) throws InterruptedException {
  CountDownLatchTest cdlt = new CountDownLatchTest(1);
  cdlt.secondTask();
  TimeUnit.SECONDS.sleep(5);
  cdlt.firstTask();
 }
}

 Output:

 wait....
waiting....
after wait.... done

Above example shows CountDownLatch attribute getting initialized to a value of 1 through constructor. Two tasks in above example are synchronized though CountDownLatch. Task2 i.e. secondTask() should wait for completion of Task1 i.e. firstTask(). Run above example and notice the sequence in which output appears on the console. 

Please note few important points

1. Any task that calls await() on the object will block until the count reaches zero or it's interrupted by another thread. secondTask() gets blocked after call to await(); evident from output.
2. Call countDown() on the object to reduce the count. The task that call countDown() are not blocked.  This cal signals the end of the task. This method need to be called at the end of the task.
3. As soon as count reaches zero; threads awaiting starts running. 
4. The value of count which is passed during creation of latch object is very important. It should be same as the number of task which needs to be finished first. If count is 5 then first task should be called five times to make sure that count has reduced to 0.
5. You can also use wait and notify mechanism of Java to achieve the same behavior but code will become quite complicated. 
6. One of the disadvantage of CountDownLatch is that its not reusable once count reaches to zero. But Java provides another concurrency API called CyclicBarrier for such cases. 

Usage of CountDownLatch

1. Use this when your current executing thread/main thread needs to wait for the completion of other dependent activities. 
2. Ensure that a service doesn't start until other services on which it depends have not completed.
3. In a multi-player game like RoadRash; wait for all players to get ready to start the race. 

---
do post your comments/questions !!!

Semaphore in Java

Let's start with below problems; before starting on Semaphore.

1. Implement a Database connection pool which will block if no more connection is available instead of failing and handover connection as soon as it's available
2. Implement a thread pool
3. Create a blocking bounded collection
4. Limiting number of http connection to an external site

Note : Bounded puts an upper limit on the size of collection. 

And block(ing) here means that, wait for the collection to become non-empty when retrieving an element, and wait for space to become available when storing an element. 

(One of the) Solution to all above problems is, Semaphore!
Recall that, in normal monitor lock (implicit lock), only one thread can access a resource. So, if you want more than one thread to access a resource at the same time, Semaphore comes into play. Semaphores are used to restrict number of threads which can access a resource or perform a given action at the same time. To achieve this, semaphore maintains a counter which keeps track of the number of resources available.

When a thread requests access to resource, semaphore checks the variable count, and if it's less than total count, then grants access and subsequently reduces the available count. If count is equal to maximum allowed count then it asks thread to wait. If resource count is one (0/1 or on/off) then its called as Binary Semaphore, otherwise its called as counting semaphore.

Semaphore got introduced into standard Java library in version 5.

Limit Number of Http Connections

Below example show how can you limit the number of http connection in your system.

import java.io.IOException;
import java.net.URL;
import java.net.URLConnection;
import java.util.concurrent.Semaphore;

/**
 * Limit Maximum number of URL connection allowed through Counting Semaphore
 * @author Sid
 *
 */
class UrlConnectionManager {
 private final Semaphore semaphore;
 private final int DEFAULT_ALLOWEED = 10;

 UrlConnectionManager(int maxConcurrentRequests) {
  semaphore = new Semaphore(maxConcurrentRequests);
 }
 
 UrlConnectionManager() {
  semaphore = new Semaphore(DEFAULT_ALLOWEED);
 }

 public URLConnection acquire(URL url) throws InterruptedException,
   IOException {
  semaphore.acquire();
  return url.openConnection();

 }

 public void release(URLConnection conn) {
  try {
   // clean up activity if required
  } finally {
   semaphore.release();
  }
 }
}

In above example; semphore maintains a set of permits or pass. Method, semaphore.acquire(), blocks if necessary until permit is available and then takes it. And each semaphore.release() adds/returns a permit. Semaphore is like a gatekeeper which keeps track of number of visitors allowed in a building.

Blocking Bounded LinkedList

You can also use semaphore to turn any collection into blocking bounded collection. Let's see how to do the same for LinkedList. Assume that, only add and remove operations are allowed. Below class provides main method to test it. Note that, at most 5 add operations are allowed.

import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

/**
 * Minimilistic Bounded List  
 * @author Sid
 *
 */
public class BoundedLinkedList<T> {
 private List<T> list;
 private Semaphore semaphore;

 public BoundedLinkedList(final int bound) {
  list = Collections.synchronizedList(new LinkedList<T>());
  semaphore = new Semaphore(bound);
 }

 public void add(T o) throws InterruptedException {
  semaphore.acquire();
  list.add(o);
  System.out.println(" Added element :"+ o);
 }
 
 public void remove(T o) {
  if (list.contains(o)) {
   list.remove(o);
   semaphore.release();
   System.out.println(" Removed element :"+ o);
  }
 }
 
 /**
  * Test Method
  * Adds 6 elements; then waits for 5 seconds; removes one element
  */
 public static void main(String[] args){
     final BoundedLinkedList<Integer> bll = new BoundedLinkedList<>(5);
  new Thread(new Runnable(){
   public void run(){
    for(int i = 1; i <= 6; i++)
     try {
      bll.add(i);
     } catch (InterruptedException e) {
      e.printStackTrace();
     }
   }
  }).start();
  
  try {
   TimeUnit.SECONDS.sleep(5);
  } catch (InterruptedException e) {
   e.printStackTrace();
  } 
  
  bll.remove(3);
 }

}

Output
 Added element :1
 Added element :2
 Added element :3
 Added element :4
 Added element :5
 Removed element :3
 Added element :6

Important points to remember

1. You should be very careful to make sure that you are releasing after acquire. You can miss it due to programming error or any exception. 
2. Long held lock can cause starvation 
3. Method, release doesn't have to be called by the same thread which called acquire. This is an important property that we don't have with normal mutex in Java.
4. You can increase number of permits at runtime (you should be careful though). This is because number of permits in a semaphore is not fixed, and call to release will always increase the number of permits, even if no corresponding acquire call was made. 

---
keep coding !!!

Implementing Single Linked List in Java

Before I start, let me caution you that this post is not about LinkedList [Java Doc] API of Java. LinkedList API of Java is a specialized implementation of doubly linked list. This post talks in general about linked data structures (i.e. linked list) and implementation of single linked list in Java.

Definition

Linked data structures are composed of distinct chunks of memory; and these chunks are bounded/linked through pointers. These memory chunks are referred as nodes. As nodes are not stored in contiguous memory so adding or removing individual nodes is quite easier (unlike an array). But one drawback is that random access to node is not possible. 

typedef struct node {
         item_type item;  //data stored in node
         struct list *next;  //points to successor 
}node;

In C language; *p denotes the item that is pointed to by pointer p, and &x denotes the address (i.e. pointer) of a particular variable x. A special null value is used to denote the termination of the list. 

C pointers are similar to Java references; as both of them point to something.

Let's cover them in detail

C pointers:
    int var = 20; 
    int *ip;   //pointer to an integer
    ip = &var;  //store address of var in pointer ip

Java references:
     Integer x = new Integer(20);  //x is reference to Integer

Usually, Java references are implemented as pointers in C; but specification doesn't say it explicitly. Java reference should be just an abstraction on C pointers (i.e. references in Java will be implemented using C pointers). I am not going to stress if both are same or not; it's debatable!

Implementation

Below is custom single linked list implementation in Java. I have just provided add and print method.

package algo;  
   
 /**  
  * Generic single linked list implementation with generics  
  *   
  * @author Siddheshwar   
  */  
 public class SingleLinkedList<E> {  
      Node<E> start; // points to the head or first node  
   
      /**  
       * Node class    
       */  
      private class Node<E> {  
           E data;  
           Node<E> next;  
   
           public Node(E data, Node<E> next) {  
                this.data = data;  
                this.next = next;  
           }  
   
           public E getData() {  
                return data;  
           }  
   
           public Node<E> getNext() {  
                return next;  
           }  
   
           public void setNext(Node<E> next) {  
                this.next = next;  
           }  
      }  
   
      public void add(E d) { // add at the end of list  
           if (start == null) {  
                start = new Node<E>(d, null);  
           } else {  
                Node<E> tmp = start;  
                while (tmp.next != null) {  
                     tmp = tmp.next;  
                }  
                tmp.setNext(new Node<E>(d, null));  
           }  
      }  
   
      public void print() {  
           Node<E> current = start;  
           System.out.print(" values in link-list are :");  
           while (current != null) {  
                System.out.print(current.getData() + "--> ");  
                current = current.getNext();  
           }  
           System.out.println("null");  
      }  
   
      public static void main(String[] args) {  
           SingleLinkedList<String> sll = new SingleLinkedList<>();  
           sll.add("abc");  
           sll.add("def");  
           sll.print();  
      }  
 } 

Output : 
values in link-list are :abc--> def--> null

Complexity of common operations 

1.  Insert/Update/delete at end of list: O(n) . Need to traverse whole list.  
2.  Insert at the beginning/head of the list : O(1)
3.  Find the size of list : O(n). But it can be achieved in O(1) if you keep track of the count in a separate attribute (increment its value on each addition and decrement on each deletion).

References from Java

1. LinkedList  [Java Doc] : Doubly linked list implementation of the List and Deque interfaces.
2. LinkedHashMap [Java Doc] : Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. Linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map.
3. LinkedHashSet [Java Doc] : Hash table and linked list implementation of the Set interface, with predictable iteration order.

Related Post :Implementing Doubly linked list in Java