Is "A Java Exception has occurred" a virus?

No. It is a standard message from Java itself. It says nothing about whether the JAR is safe, though, so if you don't trust the file, scan it first.

Why does reinstalling Java not fix it?

Reinstalling the same version changes nothing if the JAR needs a newer one, and no Java version fixes a JAR with no Main-Class. Check the real error first.

Does this happen on Mac or Linux?

This dialog is specific to javaw.exe on Windows. On Mac and Linux, run java -jar yourfile.jar in a terminal and match the error in Step 3.

The window closes and nothing else happens. Is that the same problem?

Often, yes. javaw.exe shows the dialog only when the launch fails early. If the app crashes after it has started, nothing appears at all. java -jar in a terminal shows both. ---

A Java Exception Has Occurred — How to Fix It

The Java Virtual Machine Launcher dialog hides the real error. Find it with one command, then fix the Java version, main class or missing files behind it.

A Java Exception Has Occurred — How to Fix It

Step 1: Check the JAR Itself (No Java Needed)

Step 2: Run It From Command Prompt to See the Real Error

Step 3: Match the Error

Step 4: Minecraft Mods and Installers

A Similar Message: "A Fatal Exception Has Occurred"

FAQ

Summary