CTarget

The CTarget annotation specifies the position in the method to inject into.
The value field of the annotation specifies the type of the target. Depending on the type, the target field has to be used to provide more information on the target.
By default, the shift field is set to AFTER which means the injection will be performed after the target, but it can be changed to BEFORE which is the default behavior of Mixins.
The ordinal allows choosing the target if multiple targets are available. If the ordinal is not set, all targets will be used.
A target can be made optional by setting the optional field to true. If the target is not found, the injection will be skipped instead of throwing an exception.

Built-in types

The following types are built-in:

Type	Target	Supported shifts	Target information
HEAD	The first instruction in the method	BEFORE
RETURN	All return statements	BEFORE
TAIL	The last return statement	BEFORE
THROW	All throw statements	BEFORE
INVOKE	All method invocations	BEFORE, AFTER	Method Target
FIELD	All field accesses	BEFORE, AFTER	Field Target
GETFIELD	All field reads	BEFORE, AFTER	Field Target
PUTFIELD	All field writes	BEFORE, AFTER	Field Target
NEW	All object instantiations	BEFORE, AFTER	Method Target/Descriptor/Class name
OPCODE	All instructions	BEFORE, AFTER	Opcode name or value
CONSTANT	null/int/long/float/double/String/Type constant	BEFORE, AFTER	Constant value

Additional information about the built-in types:

New type

The NEW type allows matching all object instantiations.
Examples:

//Class name with slashes
@CTarget(value = "NEW", target = "java/lang/String")
@CTarget(value = "NEW", target = "Ljava/lang/String;")
//Method target
@CTarget(value = "NEW", target = "java/lang/String.<init>(Ljava/lang/String;)V")
@CTarget(value = "NEW", target = "Ljava/lang/String;<init>(Ljava/lang/String;)V")
//Descriptor
@CTarget(value = "NEW", target = "(Ljava/lang/String;)V") //Be aware that this does not match the owner!
//Specific constructor
//The return type is the class name
@CTarget(value = "NEW", target = "(Ljava/lang/String;)Ljava/lang/String;")

Opcodes type

The OPCODE type allows matching all instructions with a specific opcode.
Examples:

@CTarget(value = "OPCODE", target = "INVOKESTATIC")
@CTarget(value = "OPCODE", target = "184")

All Opcodes can be found in the Opcodes class in the ASM library.

Constant type

The CONSTANT type allows matching all ldc instructions with a specific constant value.
Syntax for the target field:

Type	Syntax	Example
null	null	null
int	int <constant>	int 1
long	long <constant>	long 2
float	float <constant>	float 3
double	double <constant>	double 4
String	string <text>	string Hello World
Type	type <descriptor>	type Ljava/lang/String;

Custom types

Custom types can be created by implementing the IInjectionTarget interface.
The List<AbstractInsnNode> getTargets(final Map<String, IInjectionTarget> injectionTargets, final MethodNode method, final CTarget target, final CSlice slice) method needs to be implemented and should return a list of instructions that match the target.

The injectionTargets parameter contains all available injection targets.
The method parameter is the method that should be targeted.
The target parameter is the target annotation which contains the target information.
The slice parameter is the slice annotation which contains the slice information (optional).

Example

//Target all INVOKESTATIC instructions
@CTarget(value = "OPCODE", target = "INVOKESTATIC")

//Target the second null constant
@CTarget(value = "CONSTANT", target = "null", ordinal = 1)

//Target the first String.equals() call
@CTarget(value = "INVOKE", target = "java/lang/String.equals(Ljava/lang/Object;)Z", ordinal = 0)

//Target the last return statement
@CTarget(value = "TAIL")

//Target the first field access
@CTarget(value = "FIELD", target = "java/lang/String.length:I", ordinal = 0)