A numerical identifier for the class file. It should always be CAFEBABE, in hex, stored in the first 4 bytes of the class file. The easiest way to corrupt a class file’s magic number is to upload it to your website with FTP failing to turn on the binary option
Sometimes during the transmission of files, the connection may be aborted, or may be scrambled, causing class loading to fail. Sometimes when copying files over to a web server, they may become garbled or a disk error might occur. For this reason, special care is taken by the JVM and the class loader, to verify that classes are intact. One of the precautions is that every class definition contains at the beginning the magic number, a sequence of four bytes that identify a file as a Java class definition file.