MyCATApache/Mycat-Server · error · IOException

Error opening jar

Error message

Error opening jar: ${fileName}

What it means

JarLoader.loadJar opens the jar with new JarFile(fileName); any IOException while opening (missing file, not a zip/jar, corrupted, permission denied) is rethrown as IOException('Error opening jar: <fileName>'). The original cause is discarded, so the message is the only clue.

Solutions

1. Verify the jar path exists and is a valid, complete jar (unzip -t <file>)
2. Fix read permissions so the Mycat process user can open the file
3. Re-upload/re-download the jar if it is truncated or corrupt
4. Check you passed the jar file path, not a directory

Example fix

// before
loader.loadJar("/apps/udf.jar"); // file missing
// after
File jar = new File("/apps/udf.jar");
if (jar.isFile() && jar.canRead()) loader.loadJar(jar.getAbsolutePath());

File jar = new File(fileName);
if (!jar.isFile() || !jar.canRead())
    throw new IllegalStateException("Jar missing or unreadable: " + fileName);
try (java.util.jar.JarFile jf = new java.util.jar.JarFile(jar)) { /* valid jar */ }

try {
    loader.loadJar(fileName);
} catch (IOException e) {
    if (e.getMessage().startsWith("Error opening jar")) {
        // verify path, integrity (unzip -t), and permissions before retry
    }
}

When it happens

Trigger: Calling loadJar with a path that does not exist, points to a non-jar/corrupted file, or a file the process cannot read; jar locked or truncated during download.

Common situations: Typos in the jar path in config; partial/failed upload leaving a truncated jar; directory passed instead of file; wrong permissions after deployment.

Understand the failure class

Background: "open() failed", "failed to open file", "cannot create file" — what a file open error means and how to fix it — this error's family across 42 libraries.

Related errors

• Mkdirs failed to create
• can't find myid properties file
• error while calling spill() on
• error while calling spill() on
• Failed to create local dir in $newDir.

AI-assisted analysis of MyCATApache/Mycat-Server@65f8d8beb7 (2026-09-11). Data as JSON: /api/errors/86732f46441de779. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/io/mycat/config/classloader/JarLoader.java:58

	            in.close();
	          }
	        }
	      }
	    } finally {
	      jar.close();
	    }
	  }
	  
	  public static Class<?> loadJar(String fileName,String mainJavaclass) throws Exception {

		    File file = new File(fileName);
		    String mainClassName = null;

		    JarFile jarFile;
		    try {
		      jarFile = new JarFile(fileName);
		    } catch(IOException io) {
		      throw new IOException("Error opening jar: " + fileName);
		    }

		    Manifest manifest = jarFile.getManifest();
		    if (manifest != null) {
		      mainClassName = manifest.getMainAttributes().getValue("Main-Class");
		    }
		    jarFile.close();

		    if (mainClassName == null) {
		      mainClassName = mainJavaclass;
		    }
		    mainClassName = mainClassName.replaceAll("/", ".");

		    File tmpDir = new File(System.getProperty("java.io.tmpdir"));
		    tmpDir.mkdirs();
		    if (!tmpDir.isDirectory()) { 
		    	System.out.println("Mkdirs failed to create " + tmpDir);
		    }

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