# How to solve NoClassDefFoundError > 转载自：[How to solve NoClassDefFoundError](https://examples.javacodegeeks.com/java-basics/exceptions/java-lang-noclassdeffounderror-how-to-solve-no-class-def-found-error/) by **Sotirios-Efstathios Maneas** May 27th, 2014 In this tutorial we will discuss How to solve No Class Def Found Error (`NoClassDefFoundError`). This error is thrown when the Java Virtual Machine (JVM) or an instance of the `ClassLoader` class tries to load the definition of a class, but the definition could not be found. It extends the `LinkageError` class, which is used to indicate error cases, where a class has a dependency on some other class and that class has incompatibly changed after the compilation. The definition of a class can be requested during a method call, or while creating a new instance using a new expression. Also, it is important to mention that the definition of the class existed when the application code was compiled, but the definition can no longer be found in the runtime. Finally, the `NoClassDefFoundError` exists since the first version of Java. ## The Structure of NoClassDefFoundError **Constructors** - **NoClassDefFoundError()** Creates an instance of the `NoClassDefFoundError` class. - **NoClassDefFoundError(String s)** Creates an instance of the `NoClassDefFoundError` class, using the specified string as message. ## The NoClassDefFoundError in Java As we have already mentioned, the `NoClassDefFoundError` is thrown when the definition of class in not available during runtime. This error also indicates that the definition of the class was found during the compilation of the application, but it is not available in the application’s classpath during runtime. In general, this is a difficult error to deal with and through this tutorial, we will present a number of different solutions. To begin with, let’s demonstrate an example that throws the aforementioned error. First, we define a `Test` class with a simple constructor: *Test.java*: ```java public class Test { public Test() { System.out.println("A new instance of the Test class was created!"); } } ``` Then, we define a `NoClassDefFoundErrorExample` class that contains a static instance of the `Test` class: *NoClassDefFoundErrorExample.java*: ```java public class NoClassDefFoundErrorExample { private static Test test = new Test(); public static void main(String[] args) { System.out.println("The definition of Test was found!"); } } ``` The next step is to create an executable `.jar` file that shall execute the aforementioned `main` method. In order to achieve that, we first create a manifest file, called `Manifest.txt` and inside the file, we copy the following: ``` Main-Class: NoClassDefFoundErrorExample ``` Using the terminal (Linux or Mac) or the command prompt (Windows), we execute the following commands, in order to first, compile our source Java files and then, create our executable file: ```shell javac Test.java javac NoClassDefFoundErrorExample.java jar cfm NoClassDefFoundErrorExample.jar Manifest.txt NoClassDefFoundErrorExample.class ``` Now, we are ready to execute our code using the following command: ```shell java -jar NoClassDefFoundErrorExample.jar ``` A sample execution is shown below: ``` Exception in thread "main" java.lang.NoClassDefFoundError: TestClass at NoClassDefFoundErrorExample.(NoClassDefFoundErrorExample.java:2) Caused by: java.lang.ClassNotFoundException: TestClass at java.net.URLClassLoader$1.run(URLClassLoader.java:372) at java.net.URLClassLoader$1.run(URLClassLoader.java:361) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:360) at java.lang.ClassLoader.loadClass(ClassLoader.java:424) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ... 1 more ``` The `NoClassDefFoundError` was thrown because the definition of the `Test` class was not included in the application’s classpath. If we execute the following command: ```java jar cfm NoClassDefFoundErrorExample.jar Manifest.txt NoClassDefFoundErrorExample.class Test.class ``` and then, execute our code, we shall get the following output: ``` A new instance of the TestClass was created! The definition of TestClass was found! ``` ## The NoClassDefFoundError during static initialization The `NoClassDefFoundError` can also be thrown during the static initialization of a class. If an exception occurs during the loading and initialization of a class and the definition of another class depends on the former class, then an `ExceptionInInitializerError` is thrown. The following class reproduces the aforementioned problem: *StaticInitializationErrorExample.java*: ```java import java.util.List; import java.util.ArrayList; import java.util.Random; public class StaticInitializationErrorExample { private final static int TOTAL_RECORDS = 100; public static void main(String args[]) { try{ List records = new ArrayList(TOTAL_RECORDS); for(int i=1; i < TOTAL_RECORDS; ++i) records.add(new Record(i)); } catch(Throwable t) { t.printStackTrace(); throw t; } } } class Record { private static Integer ID = getRandomInteger(); public Record(Integer Id){ this.ID = Id; } private static Integer getRandomInteger() { throw new RuntimeException("The method is not implemented..."); //return new Random.nextInt(); } } ``` In this example, we defined a `Record` class with a static field, called `ID`. When, the `Record` class is about to get loaded and initialized, the `getRandomInteger()` method throws a `RuntimeException` and thus, the `main` method that is **static** and requires the definition of the `Record` class throws an `ExceptionInInitializerError`. A sample execution is shown below: ``` java.lang.ExceptionInInitializerError at StaticInitializationErrorExample.main(StaticInitializationErrorExample.java:14) Caused by: java.lang.RuntimeException: The method is not implemented... at Record.getRandomInteger(StaticInitializationErrorExample.java:31) at Record.(StaticInitializationErrorExample.java:24) ... 1 more Exception in thread "main" java.lang.ExceptionInInitializerError at StaticInitializationErrorExample.main(StaticInitializationErrorExample.java:14) Caused by: java.lang.RuntimeException: The method is not implemented... at Record.getRandomInteger(StaticInitializationErrorExample.java:31) at Record.(StaticInitializationErrorExample.java:24) ... 1 more ``` ## How to deal with the NoClassDefFoundError - Verify that all required Java classes are included in the application’s classpath. The most common mistake is **not** to include all the necessary classes, before starting to execute a Java application that has dependencies on some external libraries. - The classpath of the application is correct, but the `Classpath` environment variable is overridden before the application’s execution. - Verify that the aforementioned `ExceptionInInitializerError` does not appear in the stack trace of your application. This was a tutorial on How to solve No Class Def Found Error (`NoClassDefFoundError`) in Java.