.CLASS File Format – ClassFile structure

Fields

Only the first four fields have fixed offsets. The constant pool’s entries vary in size, so the rest must be found by reading through it.

Nothing may follow the last attribute. HotSpot turns such a file down with the message “Extra bytes at the end of class file”; the explorer shows the bytes as “Extra bytes” and marks them.

magic

The first four bytes are always CA FE BA BE. A Mach-O universal binary starts with the same four bytes; the family page explains how to tell them apart.

minor_version and major_version

Together they give the version as major.minor, such as 45.3 or 61.0. The major version says which features the file may use and which rules the JVM checks; the table of every version is on the family page. From Java SE 9, the major version is the Java release plus 44.

• For major versions 45 to 55 the minor version may be any value. Compilers of JDK 1.0.2 and 1.1 wrote 45.3; later compilers write 0.
• From major version 56 (Java SE 12), the minor version must be 0 or 65535. 65535 means the class uses preview features, and only the JVM of exactly that release with preview features turned on will load it.
• A JVM loads every major version from 45 up to its own, and turns down newer ones.

this_class

An index of a Class entry naming this class or interface, in internal form, such as java/util/EmptyStackException. A class file has to be found under the same name: the class loader looks for java/util/EmptyStackException.class. For a module declaration the name is module-info.

super_class

An index of a Class entry naming the direct superclass. It is 0 only in java/lang/Object, which has none, and in a module-info file. An interface’s super_class is always java/lang/Object. The superclass may not be final nor an interface; the JVM checks this when the class is loaded.

interfaces

interfaces_count is followed by that many two-byte indexes of Class entries. They are the interfaces in the class’s implements clause, or an interface’s extends clause, in the order of the source. Interfaces inherited from superclasses are not listed.

Limits

The sizes of the fields set limits on what a class can hold (JVM Specification §4.11):

• at most 65,535 constant pool slots, interfaces, fields and methods (the counts are two bytes; the constant pool has 65,534 usable slots, because its count is one more than the number of slots);
• a method’s bytecode is 1 to 65,535 bytes, since the branch and exception table offsets are two bytes;
• max_stack and max_locals are at most 65,535;
• a method’s parameters take at most 255 local variable slots, counting this as one and each long and double as two;
• an array type has at most 255 dimensions;
• a Utf8 constant is at most 65,535 bytes, which limits the length of names and string literals.

Names

Names are stored in Utf8 entries, in these forms (JVM Specification §4.2):

• Class and interface names are in internal form: the fully qualified name with / instead of . between packages, as in java/lang/Thread. A nested class has its binary name, such as java/util/Map$Entry. In a Class entry an array type is written as its descriptor instead, such as [Ljava/lang/String;.
• Field, method, local variable and parameter names are unqualified names. They may not contain ., ;, [ or /. Method names also may not contain < or >, except the two special names <init> (a constructor) and <clinit> (the class or interface initialiser).
• Module names keep their dots, such as java.base. Package names in Package entries use /, such as java/lang.

The source-level rules for names and packages are on the Java names and packages and modules pages.

A complete file

This is java/util/EmptyStackException.class from JDK 1.1.4, all 248 bytes of it. Its source is a public class with one constructor that does nothing but call its superclass’s.

00000000  CA FE BA BE 00 03 00 2D 00 0F 07 00 0D 07 00 0E  .......-........
00000010  0A 00 01 00 04 0C 00 06 00 05 01 00 03 28 29 56  .............()V
00000020  01 00 06 3C 69 6E 69 74 3E 01 00 04 43 6F 64 65  ...<init>...Code
00000030  01 00 0D 43 6F 6E 73 74 61 6E 74 56 61 6C 75 65  ...ConstantValue
00000040  01 00 18 45 6D 70 74 79 53 74 61 63 6B 45 78 63  ...EmptyStackExc
00000050  65 70 74 69 6F 6E 2E 6A 61 76 61 01 00 0A 45 78  eption.java...Ex
00000060  63 65 70 74 69 6F 6E 73 01 00 0E 4C 6F 63 61 6C  ceptions...Local
00000070  56 61 72 69 61 62 6C 65 73 01 00 0A 53 6F 75 72  Variables...Sour
00000080  63 65 46 69 6C 65 01 00 1A 6A 61 76 61 2F 6C 61  ceFile...java/la
00000090  6E 67 2F 52 75 6E 74 69 6D 65 45 78 63 65 70 74  ng/RuntimeExcept
000000A0  69 6F 6E 01 00 1D 6A 61 76 61 2F 75 74 69 6C 2F  ion...java/util/
000000B0  45 6D 70 74 79 53 74 61 63 6B 45 78 63 65 70 74  EmptyStackExcept
000000C0  69 6F 6E 00 01 00 02 00 01 00 00 00 00 00 01 00  ion.............
000000D0  01 00 06 00 05 00 01 00 07 00 00 00 11 00 01 00  ................
000000E0  01 00 00 00 05 2A B7 00 03 B1 00 00 00 00 00 01  .....*..........
000000F0  00 0C 00 00 00 02 00 09                          ........

The compiler of this time put a few Utf8 entries in the pool that nothing uses, such as ConstantValue and LocalVariables. That is allowed: the pool may hold anything, used or not.

javap -c -v shows the method like this, and the explorer shows the same text:

  public java.util.EmptyStackException();
    descriptor: ()V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #3                  // Method java/lang/RuntimeException."<init>":()V
         4: return