public Class Label

The offset of this label in the bytecode of its method, in bytes. This value is set if and only if the FLAG_RESOLVED flag is set.

A sentinel element used to indicate the end of a list of labels.

See Also

nextListElement

A flag indicating that a label is only used for debug attributes. Such a label is not the start of a basic block, the target of a jump instruction, or an exception handler. It can be safely ignored in control flow graph analysis algorithms (for optimization purposes).

A flag indicating that a label is the target of a jump instruction, or the start of an exception handler.

A flag indicating that this label has at least one associated line number.

A flag indicating that a label corresponds to a reachable basic block.

A flag indicating that the bytecode offset of a label is known.

A flag indicating that the basic block corresponding to a label ends with a subroutine call. By construction in MethodWriter#visitJumpInsn, labels with this flag set have at least two outgoing edges:

• the first one corresponds to the instruction that follows the jsr instruction in the bytecode, i.e. where execution continues when it returns from the jsr call. This is a virtual control flow edge, since execution never goes directly from the jsr to the next instruction. Instead, it goes to the subroutine and eventually returns to the instruction following the jsr. This virtual edge is used to compute the real outgoing edges of the basic blocks ending with a ret instruction, in addSubroutineRetSuccessors.
• the second one corresponds to the target of the jsr instruction,

A flag indicating that the basic block corresponding to a label is the end of a subroutine.

A flag indicating that the basic block corresponding to a label is the start of a subroutine.

The type and status of this label or its corresponding basic block. Must be zero or more of FLAG_DEBUG_ONLY, FLAG_JUMP_TARGET, FLAG_RESOLVED, FLAG_REACHABLE, FLAG_SUBROUTINE_CALLER, FLAG_SUBROUTINE_START, FLAG_SUBROUTINE_END.

The bit mask to extract the 'handle' of a forward reference to this label. The extracted handle is the bytecode offset where the forward reference value is stored (using either 2 or 4 bytes, as indicated by the FORWARD_REFERENCE_TYPE_MASK).

See Also

forwardReferences

The bit mask to extract the type of a forward reference to this label. The extracted type is either FORWARD_REFERENCE_TYPE_SHORT or FORWARD_REFERENCE_TYPE_WIDE.

See Also

forwardReferences

The type of forward references stored with two bytes in the bytecode. This is the case, for instance, of a forward reference from an ifnull instruction.

The type of forward references stored in four bytes in the bytecode. This is the case, for instance, of a forward reference from a lookupswitch instruction.

The number of elements to add to the forwardReferences array when it needs to be resized to store a new forward reference.

The input and output stack map frames of the basic block corresponding to this label. This field is only used when the MethodWriter#COMPUTE_ALL_FRAMES or MethodWriter#COMPUTE_INSERTED_FRAMES option is used.

The number of elements in the input stack of the basic block corresponding to this label. This field is computed in MethodWriter#computeMaxStackAndLocal.

The number of elements to add to the otherLineNumbers array when it needs to be resized to store a new source line number.

The successor of this label, in the order they are visited in MethodVisitor#visitLabel. This linked list does not include labels used for debug info only. If the MethodWriter#COMPUTE_ALL_FRAMES or MethodWriter#COMPUTE_INSERTED_FRAMES option is used then it does not contain either successive labels that denote the same bytecode offset (in this case only the first label appears in this list).

The next element in the list of labels to which this label belongs, or null if it does not belong to any list. All lists of labels must end with the EMPTY_LIST sentinel, in order to ensure that this field is null if and only if this label does not belong to a list of labels. Note that there can be several lists of labels at the same time, but that a label can belong to at most one list at a time (unless some lists share a common tail, but this is not used in practice).

List of labels are used in MethodWriter#computeAllFrames and MethodWriter#computeMaxStackAndLocal to compute stack map frames and the maximum stack size, respectively, as well as in markSubroutine and addSubroutineRetSuccessors to compute the basic blocks belonging to subroutines and their outgoing edges. Outside of these methods, this field should be null (this property is a precondition and a postcondition of these methods).

The outgoing edges of the basic block corresponding to this label, in the control flow graph of its method. These edges are stored in a linked list of Edge objects, linked to each other by their Edge#nextEdge field.

The maximum height reached by the output stack, relatively to the top of the input stack, in the basic block corresponding to this label. This maximum is always positive or null.

The number of elements in the output stack, at the end of the basic block corresponding to this label. This field is only computed for basic blocks that end with a RET instruction.

The id of the subroutine to which this basic block belongs, or 0. If the basic block belongs to several subroutines, this is the id of the "oldest" subroutine that contains it (with the convention that a subroutine calling another one is "older" than the callee). This field is computed in MethodWriter#computeMaxStackAndLocal, if the method contains JSR instructions.

Makes the given visitor visit this label and its source line numbers, if applicable.

Parameters

methodVisitor:MethodVisitor

a method visitor.

visitLineNumbers:boolean

whether to visit of the label's source line numbers, if any.

Adds a source line number corresponding to this label.

Parameters

lineNumber:int

a source line number (which should be strictly positive).

Finds the basic blocks that end a subroutine starting with the basic block corresponding to this label and, for each one of them, adds an outgoing edge to the basic block following the given subroutine call. In other words, completes the control flow graph by adding the edges corresponding to the return from this subroutine, when called from the given caller basic block.

Parameters

subroutineCaller:Label

a basic block that ends with a jsr to the basic block corresponding to this label. This label is supposed to correspond to the start of a subroutine.

Returns the "canonical" Label instance corresponding to this label's bytecode offset, if known, otherwise the label itself. The canonical instance is the first label (in the order of their visit by MethodVisitor#visitLabel) corresponding to this bytecode offset. It cannot be known for labels which have not been visited yet.

This method should only be used when the MethodWriter#COMPUTE_ALL_FRAMES option is used.

Returns:Label

the label itself if frame is null, otherwise the Label's frame owner. This corresponds to the "canonical" label instance described above thanks to the way the label frame is set in MethodWriter#visitLabel.

Finds the basic blocks that belong to the subroutine starting with the basic block corresponding to this label, and marks these blocks as belonging to this subroutine. This method follows the control flow graph to find all the blocks that are reachable from the current basic block WITHOUT following any jsr target.

Parameters

subroutineId:short

the id of the subroutine starting with the basic block corresponding to this label.

Puts a reference to this label in the bytecode of a method. If the bytecode offset of the label is known, the relative bytecode offset between the label and the instruction referencing it is computed and written directly. Otherwise, a null relative offset is written and a new forward reference is declared for this label.

Parameters

code:ByteVector

the bytecode of the method. This is where the reference is appended.

sourceInsnBytecodeOffset:int

the bytecode offset of the instruction that contains the reference to be appended.

wideReference:boolean

whether the reference must be stored in 4 bytes (instead of 2 bytes).

Sets the bytecode offset of this label to the given value and resolves the forward references to this label, if any. This method must be called when this label is added to the bytecode of the method, i.e. when its bytecode offset becomes known. This method fills in the blanks that where left in the bytecode by each forward reference previously added to this label.

Parameters

code:byte[]

the bytecode of the method.

bytecodeOffset:int

the bytecode offset of this label.

Returns:boolean

true if a blank that was left for this label was too small to store the offset. In such a case the corresponding jump instruction is replaced with an equivalent ASM specific instruction using an unsigned two bytes offset. These ASM specific instructions are later replaced with standard bytecode instructions with wider offsets (4 bytes instead of 2), in ClassReader.