Last updated: October 18, 2017
The Java incompatible magic value error, such as Exception: java.lang.ClassFormatError: Incompatible magic value..., will usually occur when a Java client class file cannot download from a server, a firewall returns an error page in its place, and that error page replaces the client class file. The error message displays in your computer’s Java Console.
The “magic value” referenced in the Java incompatible magic value error is usually the first 4 bytes in a file (which should always be 0xCA 0xFE 0xBA 0xBE). The error may occur because your firewall blocks the JAR file type. This prevents potentially malicious content from downloading within your network. You may need to contact your network/firewall administrator to request they make an exception for JAR files so Java can download them. To confirm this, you may download the client JAR file directly in your web browser when the Java error displays. Present the error message to your network/firewall administrator so they can provide a solution for you.
Additionally, the cause of the error may be due to uploading files to a web host’s server that include a JAR file. The web host, in turn, ignores or prevents that specific file type from uploading. If the JAR file is not successfully uploaded to the web host server, then the client will not be able to download it since it is not present. Double-check that all the files you wanted to upload to a server actually uploaded.
Below are more steps you may also wish to take that could potentially remedy the error:
1. Check that you are running the latest version of Java.
2. Clear your web browser’s cache of temporary files, and close ALL your open web browser windows. Then load the Java applet in a new web browser window.
3. Clear your Java cache of temporary files from your Java Control Panel. To do so, open your computer’s Control Panel, click Programs, and double-click Java. Under the General tab, in the Temporary Internet Files section, click the Settings button. Click the Delete Files button. Check the boxes for Applications and Applets and for Trace and Log Files, then click the OK button.
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