Interface Label

public sealed interface Label

A marker for a position within the instructions of a method body. The position is a cursor position in the list of instructions, similar to that of a ListIterator.

Reading Labels

Labels read from class files represent positions in the code array of a Code attribute. It is associated with a bci (bytecode index), also known as pc (program counter), the index into the code array; the actual cursor position is immediately before the given index, so a label at the beginning of the instructions has bci 0, and a label at the end of the instructions has bci codeLength() + 1. The bci can be inspected through CodeAttribute::labelToBci.

In generic CodeModels, a label may not have a bci value; the position of a label can be found by searching for the corresponding LabelTarget within that model.

Writing Labels

Many models in java.lang.classfile refer to labels. To write a label, a label must be obtained, it must be bound to a CodeBuilder.

To obtain a label:

• Use a label read from other models.
• Use pre-defined labels from a CodeBuilder, such as CodeBuilder::startLabel, CodeBuilder::endLabel, or BlockCodeBuilder::breakLabel. They are already bound.
• Create labels with CodeBuilder::newLabel or CodeBuilder::newBoundLabel.

A label must be bound exactly once in the CodeBuilder where it is used; otherwise, writing fails. To bind an unbound label:

• Send a read LabelTarget to a CodeBuilder.
• Use CodeBuilder::labelBinding.

Note that a label read from another model is not automatically bound in a CodeBuilder; they are separate entities and the label is bound to different positions in them.