OT Bytecode Attributes/OTSpecialAccess

Attribute OTSpecialAccess

Intent

This attribute combines information about several situations where accessing one element across classes requires special treatment:

• Access from a role requires generation of a synthetic access method
• Accessing a baseclass itself requires changing its access modifiers.

Location:

Team class containing one or more roles requesting decapsulation.

Content:

Two consecutive lists:

• a polymorphic list of decapsulation info for members (methods, fields, super-methods)
• a list of base classes (some of this may be subject to decapsulation)

Purpose:

The OTRE uses this attribute to make accessible those elements that shall be accessed using decapsulation. This is done by generating access methods for all affected methods, fields and super-methods.

• method-decapsulation (inaccessible base method accessed by role)
  • constructor: => OTRE changes access modifiers
  • method: => OTRE generates accessor method (differentiate static/instance methods)
• field-decapsulation (inaccessible base field accessed by role)

=> OTRE adds setter/getter

• super-method-access (a base method is accessed using a base.super call)

=> OTRE provides accessor method to invoke the target method using invokespecial

• base-class access -- two situations

• base-class decapsulation (role accesses invisible base-class)

=> OTRE removes protection

• report super classes of directly bound base classes if super-method-access is applied

=> This information is used by OT/Equinox to ensure correct load order

Format:

 OTSpecialAccess {
       u2 attribute_name_index;
       u4 attribute_length;
       u2 member_decapsulation_count;
       MemberDecapsulation member_decapsulations[member_decapsulation_count];
       u2 baseclass_count;
       Baseclass baseclasses[baseclass_count];
 }
 union MemberDecapsulation {
       MethodDecapsulation methodDesc;
       FieldDecapsulation fieldDesc;
       SuperMethodAccess superMethodDesc;
 }
 MethodDecapsulation {
       u1 selector=1;
       u2 class_name_index;
       u2 encoded_method_selector_index;
       u2 signature_index;
 }
 FieldDecapsulation {
       u1 selector=2;
       u1 flags;
       u2 class_name_index;
       u2 field_name_index;
       u2 field_signature_index;
 }
 SuperMethodAccess {
       u1 selector=3;
       u2 class_name_index;
       u2 superclass_name_index;
       u2 method_selector_index;
       u2 signature_index;
 }
 Baseclass {
       u2 baseclass_name_index;
       u1 is_decapsulation;
 }

The items of the OTSpecialAccess structure are as follows:

• attribute_name_index

The constant_pool entry at that index must be a CONSTANT_Utf8_info representing the string "OTSpecialAccess".

• attribute_length

The value of the attribute_length item indicates the length of the attribute, excluding the initial six bytes.

• member_decapsulation_count

Number of MemberDecapsulation entries

• member_decapsulations

Array of entries describing decapsulation of one member. Each element is one of MethodDecapsulation, FieldDecapsulation or SuperMethodAccess.

• baseclass_count

Number of entries for baseclasses

• baseclasses

Array of base classes to which the current team binds roles.

The items of the MethodDecapsulation structure are as follows:

• selector

Constant 1 to discriminate among alternatives of MemberDecapsulation

• class_name_index

Constant pool index pointing to the name of the bound base class

• encoded_method_selector_index

Constant pool index pointing to the name of the decapsulated method. For non-constructor methods this name is encoded either as declaringClass!selector (for static methods) or declaringClass?selector (for instance methods). Here declaringClass is the class actually declaring the method

• signature_index

Constant pool index for the string-encoded signature of the method

The items of the FieldDecapsulation structure are as follows:

• selector

Constant 2 to discriminate among alternatives of MemberDecapsulation

• class_name_index

Constant pool index pointing to the name of the bound base class

• field_name_index

Constant pool index pointing to the name of the decapsulated field.

• signature_index

Constant pool index for the string-encoded signature (type) of the field

The items of the SuperMethodAccess structure are as follows:

• selector

Constant 3 to discriminate among alternatives of MemberDecapsulation

• class_name_index

Constant pool index pointing to the name of the declaring class for the method

• superclass_name_index

Constant pool index pointing to the name of the superclass of the declaring class for the method

• method_selector_index

Constant pool index pointing to the selector of the decapsulated method.

• signature_index

Constant pool index for the string-encoded signature of the method

The items of the Baseclass structure are as follows:

• baseclass_name_index

Constant pool index for the name of a bound baseclass

• is_decapsulation

flag (0=false, 1=true) to signal whether decapsulation is applied to the baseclass itself