Error: Could not find or load main class

When running the Java program by clicking “Run” on top of the static main function, it reports “Could not find or load the main class: xxx”. However, directly invoking java with the source file works without any error. I tried to use the “Java: Clean the Java Language Server Workspace” command, and error persists.

By looking at the log (, here are two errors reported that may be the cause of the issue:

[2019-10-02 16:48:22.718] [exthost] [error] [redhat.java] provider FAILED
[2019-10-02 16:48:22.720] [exthost] [error] Error: The request (id: 28, method: 'textDocument/hover') has been cancelled
	at c:\Users\tingh\.vscode\extensions\redhat.java-0.50.0\dist\extension.js:1:68555
	at c:\Users\tingh\.vscode\extensions\redhat.java-0.50.0\dist\extension.js:1:68849
	at Immediate.module.exports.M.G.Y.size.setImmediate (c:\Users\tingh\.vscode\extensions\redhat.java-0.50.0\dist\extension.js:1:69210)
	at runCallback (timers.js:694:18)
	at tryOnImmediate (timers.js:665:5)
	at processImmediate (timers.js:647:5)
[2019-10-02 16:48:23.302] [exthost] [info] ExtensionService#_doActivateExtension ms-vscode.cpptools {"startup":false,"activationEvent":"onDebug"}
[2019-10-02 16:48:23.302] [exthost] [info] ExtensionService#loadCommonJSModule file:///c:/Users/tingh/.vscode/extensions/ms-vscode.cpptools-0.25.1/dist/main
[2019-10-02 16:48:23.429] [exthost] [info] ExtensionService#_doActivateExtension vscode.debug-server-ready {"startup":false,"activationEvent":"onDebugResolve"}
[2019-10-02 16:48:23.429] [exthost] [info] ExtensionService#loadCommonJSModule file:///c:/Program Files/Microsoft VS Code/resources/app/extensions/debug-server-ready/dist/extension

Environment

• Operating System: Windows 10 x64 Pro 1903, Build 18362.356
• JDK version: Oracle Java 12.0.2
• Visual Studio Code version: 1.38.1
• Java extension version: 0.50.0

Steps To Reproduce

Sample Project: (FileIO.java)

import java.io.File;
import java.io.IOException;
import java.util.Scanner;

public class FileIO {

    public static void main(String[] args)
    throws IOException {
        File file = new File("data.txt");
        Scanner fileInput = new Scanner(file);

        for(int i = 0; i < 5; i++) {
            System.out.println(
                i + ": " + fileInput.nextDouble()
            );
        }

        fileInput.close();
    }
}

Current Result

c:\Research\test_jav>cd C:\Research\test_jav && C:\Users\tingh\.vscode\extensions\vscjava.vscode-java-debug-0.22.0\scripts\launcher.bat "C:\Program 
Files\Java\jdk-12.0.2\bin\java" --enable-preview -Dfile.encoding=UTF-8 -cp C:\Users\tingh\AppData\Roaming\Code\User\workspaceStorage\24ee80637ed5c707879e2f0bc2507a43\redhat.java\jdt_ws\jdt.ls-java-project\bin FileIO
Error: Could not find or load main class FileIO
Caused by: java.lang.ClassNotFoundException: FileIO

Expected Result

c:\Research\test_jav>"C:\Program Files\Java\jdk-12.0.2\bin\java" FileIO.java
0: 11.0
1: 22.0
2: 33.0
3: 44.0
4: 55.0

Additional Informations

[rajkundu]

I was able to reproduce this problem using your files. I coped the source code into test/FileIO.java and created the following file test/data.txt:

11.0
22.0
33.0
44.0
55.0

I got the same error:

Rajs-MacBook-Pro:~/Desktop/test raj$ /Library/Java/JavaVirtualMachines/jdk-12.0.1.jdk/Contents/Home/bin/java --enable-preview -Dfile.encoding=UTF-8 -cp "/Users/raj/Library/Application Support/Code/User/workspaceStorage/afc777b9c5a0a53b50d81c4dcd540ac6/redhat.java/jdt_ws/test_196f9c6f/bin" FileIO 
Error: Could not find or load main class FileIO
Caused by: java.lang.ClassNotFoundException: FileIO

I first noticed this issue today, after upgrading to the latest version of macOS 10.15 Catalina Beta, Build 19A578c. It was with my own project, and the specific error causing the problem (found in the VS Code java logs) was as follows:

!ENTRY org.eclipse.jdt.ls.core 4 0 2019-10-03 00:48:33.995
!MESSAGE Error occured while building workspace. Details: 
 message: Preview features enabled at an invalid source release level 12, preview can be enabled only at source level 13; code: 2098258; resource: /Users/raj/Box/Compsci 201/projects/p2-markov-wordgram/src/SimpleWordGramDriver.java;

VS Code Version Info Version: 1.38.1 Commit: b37e54c98e1a74ba89e03073e5a3761284e3ffb0 Date: 2019-09-11T13:31:32.854Z Electron: 4.2.10 Chrome: 69.0.3497.128 Node.js: 10.11.0 V8: 6.9.427.31-electron.0 OS: Darwin x64 19.0.0

VS Code Extension Info Java Extension Pack Version: 0.8.0 Debugger for Java Version: 0.22.0 Java Test Runner: 0.20.0 Language Support for Java™ by Red Hat: 0.50.0

Java Version Info

Rajs-MacBook-Pro:~ raj$ java --version
java 12.0.1 2019-04-16
Java(TM) SE Runtime Environment (build 12.0.1+12)
Java HotSpot(TM) 64-Bit Server VM (build 12.0.1+12, mixed mode, sharing)

I hope this helps to narrow down the problem. I am very inexperienced and Google couldn’t help me interpret the error messages as well as I’d hoped, so I hope someone else can!

[feyselmubarek]

If you are using vscode for creating java project and want to run java files as a single java file by

javac your_class.java java your_class

simply remove the package declaration at the top of the file and try it. it shows you error in vscode but it doesn’t have any effect since, it is considered to be a single java file, not project