How can I fix NoClassDefFoundError when using Spring framework's WebSocketClient

Question

I am writing a desktop Java application as a web service client. I want to use WebSocket to implement notification "callback" from the server.

I am using the Spring framework's WebSocketStompClient. Below snippet shows how I initialize it:

import org.springframework.web.socket.messaging.WebSocketStompClient;
import org.springframework.web.socket.sockjs.client.SockJsClient;
import org.springframework.web.socket.sockjs.client.Transport;
import org.springframework.web.socket.sockjs.client.WebSocketTransport;
...
List<Transport> transports = new ArrayList<>();
transports.add(new WebSocketTransport(new StandardWebSocketClient()));
stompClient = new WebSocketStompClient(new SockJsClient(transports));
...

It works perfectly if I run it in IntelliJ IDE, however, if I run by command line "java -cp my.jar MyPackage.MyMainClass", it will fail with the following message:

Error: Unable to initialize main class MyMainClass
Caused by: java.lang.NoClassDefFoundError: org/springframework/web/socket/client/WebSocketClient

The above was produced by Java SE 12.0.2. If I run it using Java SE 1.8, the error message will be:

Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.NoClassDefFoundError: 
org/springframework/web/socket/client/WebSocketClient
    at java.lang.Class.getDeclaredMethods0(Native Method)
    at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
    at java.lang.Class.privateGetMethodRecursive(Unknown Source)
    at java.lang.Class.getMethod0(Unknown Source)
    at java.lang.Class.getMethod(Unknown Source)
    at sun.launcher.LauncherHelper.validateMainClass(Unknown Source)
    at sun.launcher.LauncherHelper.checkAndLoadMain(Unknown Source)

Both java.lang.NoClassDefFoundError and getDeclaredMethods0(Native Method) suggsted that a native module (a DLL) is missing.

Below is my Gradle script:

dependencies {
    implementation project(':logman-common')
    implementation 'org.springframework:spring-web:5.2.2.RELEASE'
    implementation 'org.springframework:spring-websocket:5.2.2.RELEASE'
    implementation 'org.springframework:spring-messaging:5.2.2.RELEASE'
    ... // other unrelated dependencies, such as GUI
}

I don't think it was due to some missing JARs. I tried:

• to add tyrus-standalone-client to the dependency list, or
• to use StandardWebSocketClient instead of SockJsClient,

but the error remained.

I guess some native libraries are missing in my OS (Win10), but that can't explain why I can run it in IntelliJ...

Could you let me know what is missing? or how can I fix this? Thanks!

Answer 1

Try to create a fat jar which contains all the dependencies and classes.

Update build.gradle script to this given below :

apply plugin: 'java'
version = '...'
sourceCompatibility = 1.8
targetCompatibility = 1.8

// to create a fat jar.
jar {
  manifest { 
    attributes "Main-Class": "MyPackage.MyMainClass"
  }  

  from {
    configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) }
  }
} 

dependencies {
    implementation project(':logman-common')
    implementation 'org.springframework:spring-web:5.2.2.RELEASE'
    implementation 'org.springframework:spring-websocket:5.2.2.RELEASE'
    implementation 'org.springframework:spring-messaging:5.2.2.RELEASE'
    ... // other unrelated dependencies, such as GUI
}

It will generate an executable fat jar in the build/libs of your root project which will include all the project files (this includes all of your dependencies, resource files, .class files and so on).

Note: For older versions of Gradle, use this configurations.compile.collect instead of configurations.runtimeClasspath.collect.

You just need to run the jar with this command : java -jar <project-version>.jar

This should solve your problem.