| StackMapAttribute is used by CDLC preverifier and also by javac compiller
starting from J2SE 1.5. Definition is given in appendix "CLDC Byte Code
Typechecker Specification" from CDLC 1.1 specification.
Note that this implementation does not calculate StackMapFrame structures
from the method bytecode. If method code is changed or generated from scratch,
then developer is responsible to prepare a correct StackMapFrame structures.
The format of the stack map in the class file is given below. In the following,
- if the length of the method's byte code1 is 65535 or less, then uoffset
represents the type u2; otherwise uoffset represents the type u4.
- If the maximum number of local variables for the method is 65535 or less,
then ulocalvar represents the type u2; otherwise ulocalvar
represents the type u4.
- If the maximum size of the operand stack is 65535 or less, then ustack
represents the type u2; otherwise ustack represents the type u4.
stack_map { // attribute StackMap
u2 attribute_name_index;
u4 attribute_length
uoffset number_of_entries;
stack_map_frame entries[number_of_entries];
}
Each stack map frame has the following format:
stack_map_frame {
uoffset offset;
ulocalvar number_of_locals;
verification_type_info locals[number_of_locals];
ustack number_of_stack_items;
verification_type_info stack[number_of_stack_items];
}
The verification_type_info structure consists of a one-byte tag
followed by zero or more bytes, giving more information about the tag.
Each verification_type_info structure specifies the verification
type of one or two locations.
union verification_type_info {
Top_variable_info;
Integer_variable_info;
Float_variable_info;
Long_variable_info;
Double_variable_info;
Null_variable_info;
UninitializedThis_variable_info;
Object_variable_info;
Uninitialized_variable_info;
}
Top_variable_info {
u1 tag = ITEM_Top; // 0
}
Integer_variable_info {
u1 tag = ITEM_Integer; // 1
}
Float_variable_info {
u1 tag = ITEM_Float; // 2
}
Long_variable_info {
u1 tag = ITEM_Long; // 4
}
Double_variable_info {
u1 tag = ITEM_Double; // 3
}
Null_variable_info {
u1 tag = ITEM_Null; // 5
}
UninitializedThis_variable_info {
u1 tag = ITEM_UninitializedThis; // 6
}
Object_variable_info {
u1 tag = ITEM_Object; // 7
u2 cpool_index;
}
Uninitialized_variable_info {
u1 tag = ITEM_Uninitialized // 8
uoffset offset;
}
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