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Difference between revisions of "JDT Core Programmer Guide/ECJ/Bindings"

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Revision as of 18:26, 2 July 2020

Bindings implement what some text books call "Symbols": unique values used for linking name references to their target.

Main Kinds of Bindings

See the type hierarchy of org.eclipse.jdt.internal.compiler.lookup.Binding. This class also declares constants which define possible answers from Binding.kind().

  • One reason why kinds are or-able bits: a NameReference can use an int as bitset to encode if it can legally resolve to a variable, or a type, or both, where variable can be either a field or a local.

As a special type, UnresolvedReferenceBinding is a placeholder for a not-yet resolved ReferenceBinding. Resolving an UnresolvedReferenceBinding will update its holder, too.

Another group of bindings is synthesized by the compiler from thin air, see #Synthetic Bindings.

General rules for Bindings

No null

In constrast to AST, where null is typically a legal field value, bindings typically use constants of the NO_* family to denote "nothing here". null would typically indicate "not initialized".


Since bindings are unique by construction, it would normally be OK to compare bindings using == or !=. Specifically with the introduction of TYPE_USE annotations, even the same type can be represented by different bindings to account for attached annotations. For that reason, specific comparison methods have been added, which should be used throughout:

  • TypeBinding.equalsEquals()
  • TypeBinding.notEquals()

These methods ignore difference only in annotations. Those rare cases where annotation difference should indeed be considered have to be marked in source with //$IDENTITY-COMPARISON$, to suppress a warning implemented specifically for the compiler's own sake.

Internally, each TypeBinding has an id. Low values correspond to well-known types (see TypeIds), while higher values are allocated dynamically. Ids are interesting as the family of all type bindings derived from the same original share the same id. Ids also simplify checks for well-known types.


Contrary to the general rule, a number of reasons exist, why the same source entity can be represented by several distinct bindings:

  • When the same package exists in several modules, we create one PackageBinding per module, plus a SplitPackageBinding to combine the slices into one. The current strategy concerning SplitPackageBinding was developed via bug 547181 which has a lot of explanations.
  • From a generic type (SourceTypeBinding or BinaryTypeBinding) a number of parameterizations can be created using ParameterizedTypeBinding.
    • For each contained MethodBinding a ParameterizedMethodBinding is created to carry the instantiation of type variables.
    • For each contained FieldBinding a ParameterizedFieldBinding is created to carry the instantiation of type variables.
  • From a generic method (MethodBinding) a number of parameterizations can be created using ParameterizedGenericMethodBinding. While ParameterizedMethodBinding captures type variables of the declaring class, ParameterizedGenericMethodBinding captures type variables declared by the method itself.
  • From each TypeBinding a number of "clones" (of the same type) can be created with different sets of TYPE_USE annotations. Indeed, method TypeBinding.clone() is used for this process, and TypeBinding.prototype() will answer the original from which an annotated type was cloned.
  • From each WildcardBinding a number of CaptureBinding can be created, but that's not of ECJ's invention, but specified in JLS.

When comparing bindings, it is sometimes necessary, to explicitly strip a wrapper binding using one of these methods:

  • TypeBinding.original() strips annotations, and erases parameterized, raw and array types
  • TypeBinding.erasure() full erasure (no type arguments), but may or may not retain annotations
  • TypeBinding.unannotated() strips annotations only
  • TypeBinding.actualType() answers the erasure from ParameterizedTypeBinding and WildcardBinding, null otherwise
  • ParameterizedTypeBinding.genericType(): like actualType() but performes lazy resolving of UnresolvedReferenceBinding
  • MethodBinding.genericMethod(): strips instantiation of a method's own type variables
  • MethodBinding.original(): strips any parameterization
  • MethodBinding.shallowOriginal(): strips instantiation of a method's own type variables, but leaves instantiations of class parameters in place (unclear if different from genericMethod()).

Parameterization and nesting

A plain method inside a ParameterizedType will be represented by a ParameterizedMethodBinding. But what about a nested type inside a generic outer type?

  • If the nested type is static, any type parameters from its outer class are irrelevant for the nested type, as it cannot access them without an outer instance.
  • If the nested type is non-static, reference to this type are represented by a ParameterizedTypeBinding even if the nested type itself declares no type parameters, because here the nested type can refer to type variables of the enclosing type & instance (this was wrong prior to bug 460491).

Synthetic Bindings


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