How To Resolve Java.Lang.Classnotfoundexception Inwards Java

What is ClassNotFoundException inwards Java
ClassNotFoundException is 1 of Java nightmare every Java developer confront inwards at that topographic point twenty-four hr menses to twenty-four hr menses life. java.lang.NoClassDefFoundError as well as java.lang.ClassNotFoundException are 2 errors  which occurs past times as well as straight off as well as chew upward of your precious fourth dimension piece finding as well as fixing beginning cause. From the get upward java.lang.ClassNotFoundException looks quite uncomplicated but underlying drive of it is e'er dissimilar and which classifies it every bit an environmental issue. In this coffee tutorial nosotros volition see what is ClassNotFoundException inwards java, what is existent drive of it and how to prepare it along alongside around to a greater extent than frequent as well as infamous examples of java.lang.ClassNotFoundException inwards Java or J2EE, Don’t mistake this exception alongside NoClassDefFoundError inwards Java which is likewise due to wrong classpath inwards Java.  Though both of them are related to missing course of didactics file when Java tries to charge course of didactics inwards Java they are completely dissimilar to each other.  Correct agreement of  When course of didactics is loaded inwards Java as well as How Classpath works  is must to troubleshoot as well as prepare this error quickly.

What is java.lang.classNotFoundException inwards Java

As the get upward suggests classNotFoundException in Java is a subclass of java.lang.Exception as well as Comes when Java Virtual Machine tries to charge a item course of didactics as well as doesn't institute the requested course of didactics inwards classpath. Another of import bespeak close this Exception is that, It is a checked Exception as well as yous ask to furnish explicitly Exception treatment piece using methods which tin dismiss perchance throw classnotfoundexception inwards coffee either past times using try-catch block or past times using throws clause. Though underlying concept of this exception is uncomplicated but it e'er manifest itself inwards such format that yous ask to pass around fourth dimension to figure out what just wrong alongside your classpath. If yous desire to know nasty secrets of coffee classpath  which tin dismiss drive consequence see the link. 

When ClassNotFoundException occurs inwards Java:

As per java physician java.lang.classNotFoundException comes inwards next cases:

1) When nosotros attempt to charge a course of didactics past times using Class.forName() method as well as .class file or binary of course of didactics is non available inwards classpath.
2) When Classloader try to charge a course of didactics past times using findSystemClass () method.
3) While using loadClass() method of course of didactics ClassLoader in Java.

Reflection is corking ability of Java but yous ask to last aware of java.lang.classNotFoundException piece using it or loading course of didactics inwards Java. See Understanding the Java Virtual Machine: Class Loading as well as Reflection to larn to a greater extent than close how just course of didactics loading plant within JVM.

Examples of classnotfoundexception inwards java

Though java.lang.classNotFoundException is really mutual as well as it tin dismiss come upward for whatever classes, I unremarkably see it piece doing JDBC connectivity similar when I was writing Java programme to connect Oracle database. I am going to listing around of the most mutual scenario where yous volition acquire classnotfoundexception inwards java.

java.lang.classnotfoundexception com.mysql.jdbc.driver

This is classical as well as most infamous illustration of  as well as likewise my starting fourth dimension meet with java.lang.ClassNotFoundException as well as comes when yous are writing JDBC connectivity code as well as trying to charge the JDBC driver. In this item instance of ClassNotFoundException looks similar mysql driver appal file is missing from Classpath. If yous pay attending yous volition uncovering that nosotros role method Class.forName (“driver”) to charge the driver course of didactics which resides inwards a item appal inwards instance of this its mysql-connector.jar as well as if that appal is non inwards classpath or non accessible to JVM it volition throw  java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

Here are few to a greater extent than infamous examples of java.lang.ClassnotFoundException which comes hither as well as at that topographic point piece doing whatever Java J2EE project. 

java.lang.classnotfoundexception org.hibernate.hql.ast.hqltoken

java.lang.classnotfoundexception org.springframework.web.context.contextloaderlistener

java.lang.classnotfoundexception org.eclipse.core.runtime.adaptor.eclipsestarter

java.lang.classnotfoundexception org.apache.catalina.startup.catalina

java.lang.classnotfoundexception javax.mail.messagingexception

java.lang.classnotfoundexception oracle.jdbc.driver.oracledriver

This ClassNotfoundException comes when yous are trying to connect Oracle database from Java programme using JDBC but yous don't direct keep corresponding Oracle driver e.g.ojdbc6.jar is non inwards classpath of your Java program

More Complicated ClassNotFoundException

With the advent of dynamic library e.g. OSGi and ClassLoader inwards Java, this exception tin dismiss last to a greater extent than tricky as well as difficult to find. Thanks to Mr. Anonymous who has summarized this beautifully, hither it is what he says

“It tin dismiss transcend a chip to a greater extent than complicated than that. In truth a course of didactics does non direct keep to last just visible past times the JVM through its classpath, but last visible past times the Classloader being used. When yous are inwards a multi-classloader environment (In a EE environment, for example, but non express to), each classloader may direct keep its ain rules to search for the classes, as well as this behaviour mightiness depend on the dynamic hierarchy of the Classloaders.

For example, inwards a projection that uses an EAR packaging alongside WARs within it, libraries inwards the lib folder of the EAR are visible to classes within a WAR, but whatever classes packaged inwards a appal seat inwards the WEB-INF/lib on the WAR cannot last seen past times classes inwards dissimilar modules (other WARs, EJB-JARS, etc).

It tin dismiss acquire actually complicated every bit its mutual for dissimilar modules depending on dissimilar versions of the same libraries every bit dissimilar modules depend on each other. It tin dismiss last a challenge to handle this. Sometimes the classloader can see multiple versions of the same class; sometimes they tin dismiss see no version at all. Sometimes dissimilar dependency paths terminate inwards dissimilar versions of the same class. And many of this cases terminate inwards a ClassNotFoundException.

And hence nosotros direct keep OSGi... “. If a course of didactics is non visible to ClassLoader than it tin dismiss likewise throw NoClassDefFoundError in Java every bit explained in  3 ways to resolve NoClassDefFoundError inwards Java.

How to prepare java.lang.ClassNotFoundException inwards Java
As yous direct keep seen from higher upward examples its clear work of classpath, hence hither is my approach to prepare or resolve java.lang.ClassNotFoundException:

1) First uncovering out the appal file on which problematic course of didactics file is acquaint for illustration inwards instance of "com.mysql.jdbc.driver" its mysql-connector-java.jar. If yous don't know how to uncovering which jar file a item course of didactics yous tin dismiss see eclipse shortcuts to exercise that or yous tin dismiss but exercise "Ctrl+T" inwards Eclipse as well as type the get upward of class, It volition listing all the appal inwards the social club they appear inwards eclipse classpath.

2) Check whether your classpath contains that jar, if your classpath doesn't incorporate the appal hence just add together that course of didactics inwards your classpath.

3) If it’s acquaint inwards your classpath hence at that topographic point is high adventure that your classpath is getting overridden or application is using classpath specified inwards appal file or start-up script as well as to prepare that yous ask to uncovering the exact classpath used past times your application.


Live illustration of reproducing as well as Fixing ClassNotFoundException inwards java
I intend if nosotros are able to reproduce as well as solve sure enough work nosotros transcend to a greater extent than comfortable dealing alongside that, that’s why hither nosotros volition reproduce java.lang.ClassNotFoundException and solve it past times next the concept nosotros direct keep discussed hence far.

1) Create a Class called StockTrading.java

public course of didactics StockTrading{
   populace String getDescription(){
   render "StockTrading";
  }
}

2) create a Class called OnlineStockTranding.java as well as charge the course of didactics StockTrading.java every bit Class.forName ("stocktrading");

public course of didactics OnlineStockTrading {
   populace static void main(String args[]) throws ClassNotFoundException{
      Class.forName("StockTrading");
      System.out.println("StockTrading course of didactics successfully loaded");
   }
}

3) Compile both Java source file which volition exercise 2 course of didactics files as well as run the programme should run fine.

javin@trading /java: javac *.java

javin@trading /java: ls –lrt
-rw-r--r-- 1 javin None  xc Aug 21 09:27 StockTrading.java
-rw-r--r-- 1 javin None 208 Aug 21 09:28 OnlineStockTrading.java
-rwxr-xr-x 1 javin None 282 Aug 21 09:28 StockTrading.class
-rwxr-xr-x 1 javin None 638 Aug 21 09:28 OnlineStockTrading.class

javin@trading /java:$ java OnlineStockTrading
StockTrading course of didactics successfully loaded


4) Now just remove the .class file for stocktrading.java as well as run the Java program as well as it volition throw java.lang.ClassNotFoundException inwards java.

javin@trading /java: rm StockTrading.class

javin@trading /java: java OnlineStockTrading
Exception inwards thread "main" java.lang.ClassNotFoundException: StockTrading
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)


ClassFoundException vs NoClassDefFoundError vs UnSupportedClassVersionError
There are lots of exceptions inwards coffee but these iii are the 1 who most haunted the coffee developer most mainly because these iii are generally related to surroundings issues as well as they all depends upon JVM as well as Classpath behaviour. Though they await similar at that topographic point is slight difference betwixt ClassFoundException as well as NoClassDefFoundError as well as UnSupportedClassVersionError as well as nosotros volition highlight those differences hither for slow agreement as well as differentiating these three:

1) ClassNotFoundException comes on Runtime when requested course of didactics is non available inwards classpath as well as mainly due to telephone phone to Class.forName () or Classloader.loadClass () or ClassLoader.findSystemClass ().

2) NoClassDefFoundError comes when problematic course of didactics was acquaint when your compiled your application but they are non inwards classpath piece yous running your program.

3) UnSupportedClassVersionError is slow to differentiate because it’s related to version of classpath as well as unremarkably comes when yous compile your code inwards higher Java version as well as attempt to run on lower coffee version. Can last resolved but past times using 1 coffee version for compiling as well as running your application.

So that's all on ClassNotFoundException inwards java for straight off , delight permit me know if yous direct keep whatever tip or  whatever personal sense on solving java.lang.ClassNotFoundException inwards Java which yous would similar to share.

Further Learning
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How Classpath operate inwards Java? How to laid classpath inwards Unix Linux
How to override equals method inwards Java
How to implement Thread inwards Java ?Example of Runnable interface
Difference betwixt ConcurrentHashMap as well as Collections.synchronizedMap as well as Hashtable inwards Java
How to exercise update or take symbolic or soft link inwards Unix
What is Abstraction inwards coffee ?
What is the deviation betwixt Enumeration as well as Iterator?

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