[DOC-20] Document class file content for JBMC / java_bytecode readme

jbmc/src/java_bytecode/README.md

@@ -25,7 +25,88 @@ To be documented.
 
 \subsection java-class-section How a java program / class is represented in a .class
 
-To be documented.
+Every Java class is compiled into a .class file. Inner classes, anonymous
+classes or classes for tableswitches are also compiled into their own .class
+files.
+
+There exists an [official
+specification](https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html)
+
+Each class files contains information about its version, the constant pool,
+information about the contained class, its super-class, its implemented
+interfaces, its fields, methods and finally additional attributes, such as
+information about lambda functions used in the methods of the class or inner
+classes the it contains.
+
+The content of a .class file can be inspected via the `javap` tool which is part
+of the JDK installation. Useful options are `-c` for code, `-p` to display
+protected / private methods `-l` for line numbers and local variable
+information, as well as `-verbose` which prints a lot of additional information.
+
+In general, all variable length entries in a class file are represented by first
+having an integer `n` that specifies the number of entries and then an array of
+`n` such entries in the file. All variable length attribute also contain
+information about their length in bytes.
+
+The integer format used in class files are unsigned integers of size 8/16/32
+bit, which are named as `u1`/`u2`/`u4`.
+
+\subsection java-class-access-flags Access Flags
+
+The JVM specification defines different access flags, e.g., `final`, `static`,
+`protected`, `private` etc. where different ones are applicable to the class
+itself, its fields or methods. All access flags are represented as bits, the set
+of bits that are defined for one entity is represented as disjunction of those
+values. Each of these values is defined as a constant with a name prefixed with
+`ACC_` in JBMC, e.g., as `#define ACC_PUBLIC 0x0001` or `#define ACC_ENUM
+0x4000`.
+
+\subsection java-class-constant-pool Constant Pool
+
+The constant pool contains all strings and referred values that are used in the
+.class. This includes the names of the class itself and its super-class, as well
+as the names and signatures of all fields and methods. All strings in the
+constant pool are in UTF-16 format.
+
+\subsection java-class-fields Fields
+
+Each member variable of a class has a field entry with a corresponding field
+information structure. This contains the name of the field, its raw JVM type
+(called the descriptor) and an optional signature.
+
+A signature is an extension to the Java raw types and contains information about
+the generic type of an object if applicable.
+
+The name of the field, the descriptor and the signature are all represented as
+indices into the constant pool of the class file.
+
+\subsection java-class-methods Methods
+
+Methods are represented in a similar way as fields. Each method has an
+associated name, descriptor and optional signature entry in the constant pool
+table.
+
+An implemented method also has several attributes. One is the `Code` attribute
+that stores the actual bytecode instructions. There is also an optional
+`LocalVariableTable` which contains the names of local variables and
+parameters. In addition to this there is also an optional
+`LocalVariableTypeTable` that contains information about generic local variables
+and parameters. Finally the exception table is defined as entries in the
+`Exceptions` attribute.
+
+Note: most information about generic types is optional and exists mainly as
+debugger information. This is because the Java compiler ensures that typing is
+correct and creates code accordingly. The same holds true for information on
+local variables. It is therefore advisable to compile Java projects with the
+`-g` option that adds debugging information in all cases.
+
+\subsection java-class-attributes Attributes
+
+The last section contains additional attributes, e.g., `SourceFile` which
+specified from which source file the .class was compiled, `BootstrapMethods`
+which is used for lambda functions or `InnerClasses` which refers to inner
+classes of the class in question.
+
 
 \section java-bytecode-runtime-exceptions Adding runtime exceptions (java_bytecode_instrument)