The documentation for the .class format (in the Java VM doc) is inconsistent. Page 51 says, correctly, that the attribute_length field for all attributes is 4 bytes long; but later pages consistently indicate 2 bytes long. Incorrect uses of "u2" instead of "u4" include:

Page 54, "Source File Attribute",
Page 55, "Constant Value Attribute",
Page 57, "Code Attribute",
Page 58, "Line Number Table Attribute",
Page 59, "Local Variable Table Attribute",