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JDT Core Programmer Guide/ECJ/Bindings

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. This type is used in signatures of members of BinaryTypeBinding to avoid the need to read all class file dependencies. By construction, types in class files are always represented by their fully qualified name, so lookup of these types does not require any context.

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).

Flag vectors

  • tagBits (TypeBinding,MethodBinding,VariableBinding): set of bits as declared in TagBits
    • Is*: fine grained classification of a binding
    • Begin*, End*: pairs of flags that indicate when a given processing step is active / complete (used to avoid re-entrance / recursion).
    • AreFieldsComplete, AreFieldsSorted, AreMethodsComplete, AreMethodsSorted: describes that status of arrays fields and methods
    • Has*: various diagnostics
    • Annotation*: marks when a given annotation has been applied to the element.
  • extendedTagBits (TypeBinding): overflow from tagBits, constants are in ExtendedTagBits.
    New bits should preferrably be allocated here, rather than the crowded TagBits.
  • typeBits (ReferenceBinding): classification of types, constants in TypeIds:
    • classification of resources (below Closeable), see Analysis->Black lists / white lists
    • BitUninitialized
    • BitUninternedType: classify JDT's own types TypeBinding (compiler) and ITypeBinding (DOM) as not suitable for reference comparison (==, !=), unless documented as //$IDENTITY-COMPARISON.
    • Bit*Null*Annotation: detect annotation types, configured for use by ECJ's annotation based null analysis.
    • BitMap, BitCollection, BitList: mark types which have methods with well-known problems (see UnlikelyArgumentCheck).

Synthetic Bindings

While the normal process goes AST -> Bindings -> byte code, some elements are synthesized by the compiler skipping the initial AST stage. These elements are implemented as SyntheticFieldBinding, SyntheticMethodBinding, SyntheticArgumentBinding.

Some of these are managed in SourceTypeBinding.synthetics, an array of maps, where the array index is one of FIELD_EMUL, METHOD_EMUL, CLASS_LITERAL_EMUL.

Each SyntheticMethodBinding classifies itself in its field purpose.

The following constants represent the different purposes of synthetic methods (best effort description, may not completely capture all usage scenarios):

Purpose bytecode name Description
FieldReadAccess access$n Read access to a bytecode-inaccessible field
FieldWriteAccess access$n Write access to a bytecode-inaccessible field
SuperFieldReadAccess access$n Read access to a bytecode-inaccessible field
SuperFieldWriteAccess access$n Write access to a bytecode-inaccessible field
MethodAccess access$n Invocation of a bytecode-inaccessible method
ConstructorAccess <init> Invocation of a bytecode-inaccessible constructor
SuperMethodAccess access$n Invocation of an bytecode-inaccessible constructor in an anonymous instance creation
SuperMethodAccess same as the original method Invocation of a super method inherited from a non-public class into a public class
BridgeMethod same as the original method Method that overrides an inherited method and invokes an overriding method with a more specific signature (parameterization or covariant return)
EnumValues values generated method values()
EnumValueOf values generated method valueOf()
SwitchTable $SWITCH_TABLE$enumName generated lookup function for switch statements
TooManyEnumsConstants " enum constant initialization$n" generated method for enum initialization if byte code cannot fit into <clinit> method, see bug 331334.
LambdaMethod lambda$n Implementation of a lambda expression
ArrayConstructor lambda$n Lambda implementation of a method reference for array allocation
ArrayClone lambda$n Lambda implementation of a method reference for array cloning
FactoryMethod lambda$n Lambda implementation for a regular constructor reference
DeserializeLambda $deserializeLambda$n Generated method for deserializing a serializable lambda

Here "bytecode-inaccessible" is typically an access from a nested class to a private member of an enclosing class. Since in bytecode, nested types appear like toplevel types, the privilege to access private members of enclosings needs to be faked by synthetic delegation methods, which are public, but should not be called from client code. Much of this is obsoleted as of Java 11 due to JEP 181 (Nest-Based Access Control).

Many of these synthetic bindings are created in the manageSyntheticAccessIfNecessary() family of methods in various AST types.

Finally, CodeStream directly generates the bytecode from the synthetic binding, using one of the generateSyntheticBodyFor* methods.

Other classes in the lookup package

the structure of this section should be improved.


  • TagBits - big bag of flags in bitset tagBits, see #Flag Vectors
  • ExtendedTagBits, see #Flag Vectors
  • ExtraCompilerModifiers - will appear in fields modifier but are never found in .class files
  • ProblemReasons - constants used in Problem*Binding
  • TypeConstants - not only type, also well-known method names etc
  • TypeIds - integer constants relating to types

Management of variants of a type binding

  • TypeSystem
    • AnnotatableTypeSystem

Type Inference

Here be draggons, read [JLS §18] first.

  • InferenceContext18 (variant InferenceContext is used only below 1.8 - not really maintained any more)
  • BoundSet
  • InferenceVariable
  • ReductionResult
    • TypeBound
    • ConstraintFormula
      • ConstraintExpressionFormula
      • ConstraintExceptionFormula
      • ConstraintTypeFormula
  • InferenceSubstitution


see JDT Core Programmer Guide/ECJ/Lookups#Scopes

Reading .class files

  • SignatureWrapper: incremental interpretation of a signature in binary format


  • TypeBindingVisitor

Null Annotations

  • ImplicitNullAnnotationVerifier: check if method overriding is valid wrt null annotations
  • ParameterNonNullDefaultProvider: implements the effect of @NonNullByDefault on method parameters
  • ExternalAnnotationSuperimposer: See hierarchy of org.eclipse.jdt.internal.compiler.env.ITypeAnnotationWalker.

Post-resolution computations

  • MethodVerifier
    • MethodVerifier15

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