主类存在于JAR中却无法识别：JDK1.8下JAR运行报错求助

Let me walk you through the most likely fixes for this frustrating error—this is a super common issue with JAR packaging and manifest files, so we’ll get it sorted out!

1. Double-check if Start is in the default package #

If your Start class is part of a package (e.g., com.myapp.Start), your Main-Class entry in the manifest must include the full package path, not just Start. To confirm:

• Run this command in your C:\correctDirectory folder:

  "C:\Program Files\Java\jdk1.8.0\bin\javap.exe" Start
  

• Look at the output: if it says public class Start, you’re in the default package (good). If it shows public class com.myapp.Start, update your manifest to Main-Class: com.myapp.Start.

2. Verify your manifest file’s format is perfect #

Manifest files are picky about formatting—even tiny mistakes break things:

• Make sure there’s exactly one space after the colon in Main-Class: Start (no missing space, no extra spaces).
• Ensure the file ends with a newline character (you mentioned this, but double-check—sometimes text editors hide this).
• Extract the actual manifest from your JAR to confirm:

  "C:\Program Files\Java\jdk1.8.0\bin\jar.exe" xf ERSR.jar META-INF/MANIFEST.MF
  

  Open MANIFEST.MF in a plain text editor (Notepad works) to check for hidden characters, extra lines, or typos.

3. Confirm Start.class is in the correct location in the JAR #

When you run jar tf ERSR.jar, look closely at the output:

• Start.class should be listed directly at the root of the JAR (e.g., just Start.class, not some/folder/Start.class).
• If you extracted class files from subfolders, you might have accidentally included the folder structure in the JAR. For example, if Start.class was in bin/Start.class, make sure you’re running the jar command from the bin folder, or copy the class files directly to C:\correctDirectory before packaging.

4. Test running the class directly (outside the JAR) #

To rule out classpath issues, run the Start class directly:

cd C:\correctDirectory
"C:\Program Files\Java\jdk1.8.0\bin\java.exe" Start

• If this works, the problem is definitely with your JAR packaging or manifest.
• If this also fails, you might be missing dependencies, or Start.class isn’t actually in your current directory.

5. Simplify your java command #

Your current run command has nested quotes which might cause unexpected behavior. Try simplifying it:

cd C:\correctDirectory
"C:\Program Files\Java\jdk1.8.0\bin\java.exe" -jar ERSR.jar

Changing directories first avoids any path-related quirks with the JAR file location.

内容的提问来源于stack exchange，提问作者Ken Reid