Package org.spongepowered.asm.mixin.injection

Annotation Type ModifyVariable

  • @Documented
@Retention(RUNTIME)
@Target(METHOD)
public @interface ModifyVariable
    The ModifyVariable annotation allows to apply a function on a single local variable at an arbitrary point during execution. More specifically, the mixin implementation will call the variable modifier handler with the original value of the local variable. The return value of the handler is then used to set the value of the local variable. The point in which the function is applied is dictated through at().

    As method arguments are also local variables, ModifyVariable can be used to modify incoming (that is those of the injected method) arguments. In order to modify outgoing arguments (that is arguments that are passed to methods invoked by the injected method), ModifyArg should be used.

    Signature and visibility modifiers

    The ModifyVariable handler (also known as the "variable modifier") MUST declare the same return type (subtypes are not supported) as the type of the variable it modifies (supertypes are not supported). If the targeted method is static, the handler MUST be static and private. For non-static targets the access modifiers are not of relevance, except for constructors where the handler must be static when not injecting immediately before the final return via TAIL.

    Locals capture is not supported when using ModifyVariable. However, arguments may be captured by appending them to the list of arguments of the handler method.

    Effects on the target method

    This injector annotation, once applied will produce code similar to follows:

    ACONST null ASTORE 0 ALOAD 0 INVOKESTATIC TargetClass.modifyVariableHandler(Ljava/lang/Object;)V ASTORE 0
    which corresponds to following source code:
    Object variable = null; variable = modifyVariableHandler(variable);
    The variable modifier method would look a bit like follows:
    @ModifyVariable() private static Object modifyVariableHandler(Object original) { return original; }

    Other notes and potential pitfalls

    A single ModifyVariable can modify only a single variable, but can modify it multiple times (as per it's at()).

    ModifyVariable supports an injection point selector that is strictly unique to ModifyVariable: LOAD and STORE. These injection point selectors target the xLOAD or xSTORE instructions of the selected local variable (mind you that ModifyVariable can only select a single variable at a single time, so it is not possible to select ALL xLOAD or xSTORE instructions using LOAD and STORE). These injection point selectors can be used by setting the appropriate At.value() to either LOAD or STORE.

    Unlike any other injection point selectors, STORE selects the instruction after the xSTORE instruction. Please be aware of this when using an At.value() of STORE. This behaviour exists as otherwise modifications to the variable will be immediately overwritten, even in cases of variable = variable * 4;. Note that LOAD behaves like most other injection point selectors and targets the instruction before the xLOAD instruction(-s).

    • Required Element Summary

      Required Elements 
      Modifier and Type Required Element Description
      At at
      The injection point where the injection should occur.

    • Optional Element Summary

      Optional Elements 
      Modifier and Type Optional Element Description
      int allow
      The maximum amount of injection points that should be allowed.
      int expect
      The expected amount of injection points.
      java.lang.String[] method
      The targeted method selectors.
      java.lang.String[] name
      The names of the local variable that is to be modified by the ModifyVariable-annotated variable modifier.
      int require
      The minimum amount of injection points.
      Slice slice
      The slice to make use for the injection points defined by at().
      Desc[] target
      The targeted methods.

    • Element Detail

      • at

        At at
        The injection point where the injection should occur. If none of the injection points apply no exception is thrown by default (this default can be changed through require()), however transformation does not occur (Micromixin still copies the handler into the target class anyways though).

        The injection points of the ModifyVariable can target any arbitrary instruction, however attempting to target an instruction that lies outside the lifetime of the local variable may have adverse effects and may cause the resulting class file to fail to validate or even cause transformation failures.

        ModifyVariable supports an injection point selector that is strictly unique to ModifyVariable: LOAD and STORE. These injection point selectors target the xLOAD or xSTORE instructions of the selected local variable (mind you that ModifyVariable can only select a single variable at a single time, so it is not possible to select ALL xLOAD or xSTORE instructions using LOAD and STORE). These injection point selectors can be used by setting the appropriate At.value() to either LOAD or STORE.

        Unlike any other injection point selectors, STORE selects the instruction after the xSTORE instruction. Please be aware of this when using an At.value() of STORE. This behaviour exists as otherwise modifications to the variable will be immediately overwritten, even in cases of variable = variable * 4;. Note that LOAD behaves like most other injection point selectors and targets the instruction before the xLOAD instruction(-s).

        Returns:
        The injection point targeted by this ModifyVariable handler.

      • allow

        int allow
        The maximum amount of injection points that should be allowed. If the value of this element is below 1 or if the value is below the minimum amount of allowable injection points then the limit is not being enforced. However, expect() has no influence on allow().

        Furthermore this limit is only valid per target class. That is, if multiple target classes are defined as per Mixin.value() or Mixin.targets() then this limit is only applicable for all the injection points in the targeted class. This limitation is caused due to the fact that the targeted classes are not known until they are loaded in by the classloader, at which point all the injection logic occurs.

        This limit is shared across all methods (as defined by method() or target()) targeted by the handler within a class.

        Returns:
        The maximum amount targeted of injection points within the target class.
        Default:
        -1

      • expect

        int expect
        The expected amount of injection points. This behaves similar to require(), however while require() will cause a class file transformation failure, expect() is a weaker form of it. Under the spongeian implementation, this attribute behaves like require() if and only if the appropriate debug flags are activated. The micromixin transformer will meanwhile "just" unconditionally write a warning to the logger.

        This attribute should be used to identify potentially outdated injectors.

        Returns:
        The expected amount of injection points
        Default:
        -1

      • method

        java.lang.String[] method
        The targeted method selectors. The amounts of methods that may match and are selected is not bound to any hard value and as such it should be limited by setting attributes such as require() or expect() as otherwise the injector might accidentally not match anything with no way of knowing what exactly went wrong.

        The following are all valid formats of explicit target selectors:

        • targetMethod
        • targetMethod(Lcom/example/Argument;)V
        • (Lcom/example/Argument;)V
        • targetMethod(I)Lcom/example/ReturnValue;
        • targetMethod()Z
        • Lcom/example/Target;targetMethod(Lcom/example/Argument;)V
        • Lcom/example/Target;(Lcom/example/Argument;)V
        • Lcom/example/Target;targetMethod(Lcom/example/Argument;) V
        • Lcom/example/Target;target Method(Lcom/example/Argument;)V
        • Lcom/example/Target;targetMethod(Lcom/exam ple/Argument;)V

        The parts of the explicit target selector (owner, name, descriptor) must always have the same order, but the individual parts must not necessarily be present.

        While permissible, it is strongly discouraged to make use of whitespace in explicit target selectors. When they are used, the spongeian mixin implementation (and also micromixin) will discard all whitespace characters (tabs included). This is documented behaviour (in both micromixin and sponge's mixin) and is unlikely to change in the future. This discouragement exists as this feature may cause target selectors to be illegible.

        It is generally recommended to not be lazy when it comes to explicit selectors, the more information is provided the better. Information that is not supplied is comparable to a wildcard - the first matching method will be targeted, even if nonsense. It is especially not recommended to discard the method name, even if that is theoretically valid.

        The spongeian implementation also supports schemes other than the explicit selectors. However the Micromixin implementation only supports explicit selectors as documented above. Where as the spongeian implementation supports quantifiers in explicit selectors, Micromixin does not support them (yet). As such, quantifiers are not included in the documentation.

        It is rather advisable to use target() over method(), especially for beginners, since latter provides behaviour that can more easily be anticipated.

        Returns:
        The target selectors that define the target method of the handler.
        Default:
        {}

      • name

        java.lang.String[] name
        The names of the local variable that is to be modified by the ModifyVariable-annotated variable modifier.

        This property only takes effect if no ordinal or index is defined (ordinals and indices are not supported in micromixin-transformer as of 2024-05-14, nor are they present in micromixin-annotations). Further, this property requires the presence of the LVT (local variable table) and is as such unlikely to be present in obfuscated environments where either the local variable table is outright missing or contains bogus values (in minecraft, the local variable table contains bogus names, where as in galimulator it is stripped). Using decompiler-provided names will not work in these circumstances.

        When providing multiple names, only one will be matched. Neither micromixin-transformer nor the spongeian mixin implementation make any guarantees which name will be picked if multiple names were to exist.

        Micromixin-transformer may not properly handle cases where local variables are defined multiple times under the same name (this is commonly the case for the counter variable of for loops), please exercise caution in this circumstance.

        Both micromixin-transformer and the spongeian implementation support implicit local variable selection, meaning that this property is solely a filter to narrow down the amounts of instructions that is spit out by the injection point selector. The type of the local variable and that of the handler method is of course another filter.

        Returns:
        The possible names of the selected variable.
        Default:
        {}

      • require

        int require
        The minimum amount of injection points. If less injection points are found (as per at()). an exception is thrown during transformation. The default amount of required injection points can be set by mixin configuration file, but by default that is no minimum amount of required injection points.
        Returns:
        The minimum amount of injection points
        Default:
        -1

      • slice

        Slice slice
        The slice to make use for the injection points defined by at().

        The id of the slice is ignored as per the spongeian documentation, but this behaviour may not be reflected by micromixin-transformer. It is as such not recommended to depend on this behaviour. See At.slice() and Slice.id() for further information.

        Returns:
        The slice used to filter the possible instructions matched by injection points.
        Default:
        @org.spongepowered.asm.mixin.injection.Slice

      • target

        Desc[] target
        The targeted methods. Only one method is picked from the list of provided methods. As such the list should generally only be used to mark method aliases among others.
        Returns:
        The target selectors that define the target method of the handler.
        Default:
        {}