Java Types

Skeletal implementation of interface javax.lang.model.util.Types, plus concrete realization backed by core Java Reflection API, akin to JDK Enhancement Proposal (JEP) 119

Status

Overview

• source compatible with Java 7 and 8
• algorithms, relations, and properties of Java Language Specification implemented exclusively in terms of javax.lang.model interfaces; therefore supports different concrete implementations of javax.lang.model interfaces
• besides the generic skeletal implementation of javax.lang.model.util.Types, also includes a concrete implementation backed by the Java Reflection API
• implemented in plain Java with no dependencies; easily embeddable in projects that have Java types as part of their domain model (for instance, domain-specific languages on top of the JVM that need some sort of support for Java generic types)
• contract tests for the methods and classes that clients of the library need to provide in order to support other implementations of javax.lang.model interfaces
• tests have virtually full code coverage
• contains a complete implementation of javax.lang.model.util.Types methods pertaining to JLS §4 (including primitive types, reference types, type variables, parameterized types, type erasure, raw types, intersection types, and subtyping) and §5.1.10 (capture conversion)
• also includes method for resolving formal type parameters to actual type arguments (for example, determining the actual type argument to Comparable<T>, given type Integer)
• methods pertaining to non-type elements (notably, ExecutableElement and VariableElement) not currently implemented
• Code makes use of JSR 305 annotations (e.g., @Nullable)

License

Revised BSD (3-Clause) License

Binary Releases

Published releases (compiled for Java 7 and up) are available on Maven Central.

<dependency>
    <groupId>net.florianschoppmann.java</groupId>
    <artifactId>java-types</artifactId>
    <version>1.0.1</version>
</dependency>

Documentation

• API documentation
• Maven-generated project documentation

Usage Examples

The following examples show some use cases of class ReflectionTypes, the Java Reflection API-based implementation of javax.lang.model.util.Types provided by this project. While the bulk of the functionality of ReflectionTypes is provided by an abstract, skeletal-implementation class called AbstractTypes, class ReflectionTypes provides methods typeElement() and typeMirror() that facilitate converting Java classes and generic types to TypeElement and TypeMirror instances.

Test whether one type is a subtype of another

(see JLS §4.10 and its references)

ReflectionTypes types = ReflectionTypes.getInstance();
// listSuperNumberType: List<? super Number>
DeclaredType listSuperNumberType = types.getDeclaredType(
    types.typeElement(List.class),
    types.getWildcardType(null, types.typeMirror(Number.class))
);
// iterableExtendsNumberType: Iterable<? extends Number>
DeclaredType iterableExtendsNumberType = types.getDeclaredType(
    types.typeElement(Iterable.class),
    types.getWildcardType(types.typeMirror(Number.class), null)
);
// iterableType: Iterable<?>
DeclaredType iterableType = types.getDeclaredType(
    types.typeElement(Iterable.class),
    types.getWildcardType(null, null)
);
assert types.isSubtype(listSuperNumberType, iterableType);
assert types.isSubtype(iterableExtendsNumberType, iterableType);
assert !types.isSubtype(listSuperNumberType, iterableExtendsNumberType);

Resolve actual type arguments

(see JLS §4.5, §8.1, and their references)

ReflectionTypes types = ReflectionTypes.getInstance();
// actual type arguments to Comparable, given the (raw) subtype ScheduledFuture
List<? extends TypeMirror> typeArguments = types.resolveActualTypeArguments(
    types.typeElement(Comparable.class),
    types.typeMirror(ScheduledFuture.class)
);
assert typeArguments.equals(
    Collections.singletonList(types.typeMirror(Delayed.class))
);

Capture conversion with mutually recursive type-variable bounds

(taken from Example 8.1.2-1 of JLS 8, see also JLS §5.1.10 and its references)

// The following are top-level definitions
interface ConvertibleTo<T> {
    T convert();
}
class ReprChange<T extends ConvertibleTo<S>,
                 S extends ConvertibleTo<T>> {
    T t;
    void set(S s) { t = s.convert();    }
    S get()       { return t.convert(); }
}
class Amount implements ConvertibleTo<Integer> {
    @Override public Integer convert() { return 42; }
}

// [...]

ReflectionTypes types = ReflectionTypes.getInstance();
// reprChangeType: ReprChange<Amount, ?>
DeclaredType reprChangeType = types.getDeclaredType(
    types.typeElement(ReprChange.class),
    types.typeMirror(Amount.class),
    types.getWildcardType(null, null)
);
TypeMirror convertedType = types.capture(reprChangeType);

TypeVariable capturedS
    = (TypeVariable) ((DeclaredType) convertedType).getTypeArguments().get(1);
// convertibleToAmountType: ConvertibleTo<Amount>
DeclaredType convertibleToAmountType = types.getDeclaredType(
    types.typeElement(ConvertibleTo.class),
    types.typeMirror(Amount.class)
);
assert capturedS.getUpperBound().equals(convertibleToAmountType);