Package org.spongepowered.asm.mixin.injection

Annotation Interface ModifyVariable

@Target(METHOD) @Retention(RUNTIME) public @interface ModifyVariable
Specifies that this mixin method should inject a variable modifier callback to itself in the target method(s) identified by method(). The variable modifier can modify a single local variable in the target method frame.

If the variable being modified is a method argument, specifying the argsOnly=true reduces the overhead of calculating the local variable table for the method.

ModifyVariable callbacks should always take one argument of the type to capture and return the same type. For example a ModifyVariable for a local of type String should have the signature:

private String myHandlerMethod(String variable) {
    ...

The callback receives the current value of the local variable, and should return the new value.

The injector has two operating modes, explicit and implicit , and can operate either on the entire LVT or on the method arguments only.

Explicit mode

In explicit mode, the variable to capture can be specified by specifying values for the discriminator arguments ordinal(), index() and name(). The injector uses the discriminators in order to attempt to locate the variable to capture. If no local variable matches any discriminators, the capture fails.

Implicit mode

If no values for the capture discrimiators are specified, the injector operates in implicit mode. If exactly one variable of the capture type exists in the target LVT, then capture will succeed. However, if more than one variable of the required type is encountered in the LVT then an InvalidInjectionException is thrown.

Context-sensitive injection points

Normally the injection points specified by at() are independent of the injector being used. With ModifyVariable there are two special injection points which consume the discriminator from the annotation:

  • LOAD - matches xLOAD insns for the variable matched by the discriminator in the target method.
  • STORE - matches xSTORE insns for the variable matched by the discriminator in the target method.

See the javadoc for each injection point for details.

Notes

Like other injectors, the callback signature can optionally include the target method arguments by simply appending them to the handler method signature.

  • Required Element Summary

    Required Elements
    Modifier and Type
    Required Element
    Description
    At
    at
    An At annotation which describes the InjectionPoint in the target method.

  • Optional Element Summary

    Optional Elements
    Modifier and Type
    Optional Element
    Description
    int
    allow
    Injection points are in general expected to match every candidate instruction in the target method or slice, except in cases where options such as At.ordinal() are specified which naturally limit the number of results.
    boolean
    argsOnly
    Under normal circumstances the injector will consider all local variables including method arguments and all other local variables.
    String
    constraints
    Returns constraints which must be validated for this injector to succeed.
    int
    expect
    Like require() but only enabled if the mixin.debug.countInjections option is set to true and defaults to 1.
    int
    index
    Gets the absolute index of the local variable within the local variable table to capture.
    String[]
    method
    String representation of one or more target selectors which identify the target methods.
    String[]
    name
    Gets the name of the variable to capture.
    int
    order
    By default almost all injectors for a target class apply their injections at the same time.
    int
    ordinal
    Gets the local variable ordinal by type.
    boolean
    print
    When creating a ModifyVariable callback, you may wish to first inspect the local variable table in the target method at the injection point.
    boolean
    remap
    By default, the annotation processor will attempt to locate an obfuscation mapping for all ModifyVariable methods since it is anticipated that in general the target of a ModifyVariable annotation will be an obfuscated method in the target class.
    int
    require
    In general, injectors are intended to "fail soft" in that a failure to locate the injection point in the target method is not considered an error condition.
    Slice
    slice
    A Slice annotation which describes the method bisection used in the at() query for this injector.
    Desc[]
    target
    Literal representation of one or more @Desc annotations which identify the target methods.

  • Element Details

    • method

      String[] method
      String representation of one or more target selectors which identify the target methods.
      Returns:
      target method(s) for this injector
      Default:
      {}

    • target

      Desc[] target
      Literal representation of one or more @Desc annotations which identify the target methods.
      Returns:
      target method(s) for this injector as descriptors
      Default:
      {}

    • slice

      Slice slice
      A Slice annotation which describes the method bisection used in the at() query for this injector.
      Returns:
      slice
      Default:
      @org.spongepowered.asm.mixin.injection.Slice

    • at

      At at
      An At annotation which describes the InjectionPoint in the target method.
      Returns:
      At which identifies the location to inject inside the target method.

    • print

      boolean print
      When creating a ModifyVariable callback, you may wish to first inspect the local variable table in the target method at the injection point. Set this value to true to print the local variable table and perform no injection.
      Returns:
      true to print the LVT to the console
      Default:
      false

    • ordinal

      int ordinal
      Gets the local variable ordinal by type. For example, if there are 3 String arguments in the local variable table, ordinal 0 specifies the first, 1 specifies the second, etc. Use ordinal when the index within the LVT is known. Takes precedence over index().
      Returns:
      variable ordinal
      Default:
      -1

    • index

      int index
      Gets the absolute index of the local variable within the local variable table to capture. The local variable at the specified index must be of the same type as the capture. Takes precedence over name().
      Returns:
      argument index to modify or -1 for automatic
      Default:
      -1

    • name

      String[] name
      Gets the name of the variable to capture. Only used if the variable cannot be located via ordinal() or index().
      Returns:
      possible names to capture, only useful when the LVT in the target method is known to be complete.
      Default:
      {}

    • argsOnly

      boolean argsOnly
      Under normal circumstances the injector will consider all local variables including method arguments and all other local variables. This involves reading (and possibly generating) the local variable table for the method which can have mixed results. Set this value to true to only consider method arguments.
      Returns:
      true if this injector should only consider method arguments and not all locals.
      Default:
      false

    • remap

      boolean remap
      By default, the annotation processor will attempt to locate an obfuscation mapping for all ModifyVariable methods since it is anticipated that in general the target of a ModifyVariable annotation will be an obfuscated method in the target class. However since it is possible to also apply mixins to non-obfuscated targets (or non- obfuscated methods in obfuscated targets, such as methods added by Forge) it may be necessary to suppress the compiler error which would otherwise be generated. Setting this value to false will cause the annotation processor to skip this annotation when attempting to build the obfuscation table for the mixin.
      Returns:
      True to instruct the annotation processor to search for obfuscation mappings for this annotation
      Default:
      true

    • require

      int require
      In general, injectors are intended to "fail soft" in that a failure to locate the injection point in the target method is not considered an error condition. Another transformer may have changed the method structure or any number of reasons may cause an injection to fail. This also makes it possible to define several injections to achieve the same task given expected mutation of the target class and the injectors which fail are simply ignored.

      However, this behaviour is not always desirable. For example, if your application depends on a particular injection succeeding you may wish to detect the injection failure as an error condition. This argument is thus provided to allow you to stipulate a minimum number of successful injections for this callback handler. If the number of injections specified is not achieved then an InjectionError is thrown at application time. Use this option with care.

      Returns:
      Minimum required number of injected callbacks, default specified by the containing config
      Default:
      -1

    • expect

      int expect
      Like require() but only enabled if the mixin.debug.countInjections option is set to true and defaults to 1. Use this option during debugging to perform simple checking of your injectors. Causes the injector to throw a InvalidInjectionException if the expected number of injections is not realised.
      Returns:
      Minimum number of expected callbacks, default 1
      Default:
      1

    • allow

      int allow
      Injection points are in general expected to match every candidate instruction in the target method or slice, except in cases where options such as At.ordinal() are specified which naturally limit the number of results.

      This option allows for sanity-checking to be performed on the results of an injection point by specifying a maximum allowed number of matches, similar to that afforded by Group.max(). For example if your injection is expected to match 4 invocations of a target method, but instead matches 5, this can become a detectable tamper condition by setting this value to 4.

      Setting any value 1 or greater is allowed. Values less than 1 or less than require() are ignored. require() supercedes this argument such that if allow is less than require the value of require is always used.

      Note that this option is not a limit on the query behaviour of this injection point. It is only a sanity check used to ensure that the number of matches is not too high

      Returns:
      Maximum allowed number of injections for this
      Default:
      -1

    • constraints

      String constraints
      Returns constraints which must be validated for this injector to succeed. See ConstraintParser.Constraint for details of constraint formats.
      Returns:
      Constraints for this annotation
      Default:
      ""

    • order

      int order
      By default almost all injectors for a target class apply their injections at the same time. In other words, if multiple mixins target the same class then injectors are applied in priority order (since the mixins themselves are merged in priority order, and injectors run in the order they were merged). The exception being redirect injectors, which apply in a later pass.

      The default order for injectors is 1000, and redirect injectors use 10000.

      Specifying a value for order alters this default behaviour and causes the injector to inject either earlier or later than it normally would. For example specifying 900 will cause the injector to apply before others, while 1100 will apply later. Injectors with the same order will still apply in order of their mixin's priority.

      Returns:
      the application order for this injector, uses DEFAULT (1000) if not specified
      Default:
      1000