ClassNotFoundException v/s NoClassDefFoundError

In this section , Let's talk about ClassNotFoundException And NoClassDefFoundError The difference between .

Usually, both of these cases are caused by the lack of necessary .class file , But there are differences between them .

ClassNotFoundException	NoClassDefFoundError
This usually happens when the program is executing , Use class load Statement load missing .class file ：Class.forName(“class.name”);ClassLoader.loadClass(“class.name”);ClassLoader.findSystemClass(“class.name”); reason ： Because the runtime did not update JAR File and execute the program , Missing required file in classpath .	The two errors are similar , But the difference is that it is necessary .class Files are available at compile time , Lost at runtime . Probable cause ： It is deleted after compilation. There may be a version mismatch
The fully qualified class name is java.lang.ClassNotFoundException	The fully qualified class name is java.lang.NoClassDefoundError
It belongs to the exception category , namely java.lang.Exception The direct subclass of .	It belongs to the error category , namely java.lang.Error, java.lang.LinkageError Subclasses of .
It's a Checked Exception, So whenever you encounter something like 1 Click the class loading statement described above , Need to deal with it .	all Error All belong to unchecked Exception Category , therefore NoClassDeffoundError Also belong to unchecked Exception.
When checking for exceptions , Programmers can use try catch Block or declaration throws Clause to provide processing code . therefore , It can be solved .	Program execution Java Runtime System The resulting error is therefore , It is difficult to reproduce .
Example 1	Example 2

• Class.forName(“class.name”);
• ClassLoader.loadClass(“class.name”);
• ClassLoader.findSystemClass(“class.name”);

reason ： Due to runtime Not updated JAR file And execute the program , Missing required file in classpath . The two errors are similar , But the difference is that it is necessary .class Files are available at compile time , Lost at runtime . Probable cause ：

• It is deleted after compilation
• Possible Version mismatch

The fully qualified class name is java.lang.ClassNotFoundException The fully qualified class name is java.lang.NoClassDefoundError It belongs to the exception category , namely java.lang.Exception The direct subclass of . It belongs to the error category , namely java.lang.Error, java.lang.LinkageError Subclasses of . It's a Checked Exception, So whenever you encounter something like 1 Click the class loading statement described above , Need to deal with it . all Error All belong to unchecked Exception Category , therefore NoClassDeffoundError Also belong to unchecked Exception. When checking for exceptions , Programmers can use try catch Block or declaration throws Clause to provide processing code . therefore , It can be solved . Program execution Java Runtime System Error caused therefore , It is difficult to reproduce . Example 1 Example 2

Example 1:ClassNotFoundException Demo example on ：

public class JdbcConnectionExample { public static void main(String[] args) { // declare variables // Step 1: Loading or registering Oracle JDBC driver class try { Class.forName("oracle.jdbc.driver.OracleDriver"); } catch(ClassNotFoundException cnfex) { System.out.println("Problem in loading Oracle JDBC driver"); cnfex.printStackTrace(); } // Step 2: Opening database connection } }

Output results ：

java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver Problem in loading Oracle JDBC driver at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Unknown Source) at in.bench.resources.top.exception.in.java.JdbcConnectionExample .main(JdbcConnectionExample.java:11)

In the example above , We are trying to use Class.forName() Static method loading Oracle Driver file for database , But it is not available at runtime . Possible causes of such anomalies , perform JDBC Program without using the required JAR File update classpath , Solution ： To correct this anomaly , Just put the necessary ojdbc14.jar Included in the classpath , Then perform the same procedure .

Example 2:NoClassDefoundError A demo example of ：

public class SimilarException { // using below declared TestFile class static TestFile tf = new TestFile(); public static void main(String[] args) { // invoke method tf.display(); } } class TestFile { public void display() { System.out.println("Display message"); } }

Running results ：

java.lang.NoClassDefFoundError: in/bench/resources/top/exception/in/java/TestFile at in.bench.resources.top.exception.in.java.SimilarException.<clinit>( SimilarException.java:6) Caused by: java.lang.ClassNotFoundException: in.bench.resources.top.exception.in.java.TestFile at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) ... 1 more Exception in thread "main"

In the example above , After compiling, we deliberately delete testfile.class To display this exception type . We are trying to execute a program , however classpath Missing required class file in TestFile.class. Possible causes of this exception type , A required file exists during compilation , But the file was lost while executing the same program The above program shows “has-a” Relationship , Compile successfully , And during the execution of the program ,JVM Can't find what you need .class file .