gson-compile/gson-compile-processor/src/main/java/io/gitlab/jfronny/gson/compile/processor/util/valueprocessor/ValueCreator.java

package io.gitlab.jfronny.gson.compile.processor.util.valueprocessor;

import javax.annotation.processing.ProcessingEnvironment;
import javax.lang.model.element.*;
import javax.lang.model.util.ElementFilter;
import java.util.*;

public class ValueCreator {
    private final ProcessingEnvironment env;

    public ValueCreator(ProcessingEnvironment env) {
        this.env = env;
    }

    public Value from(Element element) throws ElementException {
        return from(element, false);
    }

    /**
     * Creates a [Value] from the given element. This element can be the [TypeElement] of the target class, or a
     * specific constructor or factory method. If [isBuilder] is true, then the element represents the builder class,
     * constructor or factory method.
     */
    public Value from(Element element, boolean isBuilder) throws ElementException {
        if (element instanceof TypeElement tel) {
            return isBuilder ? fromBuilderClass(tel) : fromClass(tel);
        } else if (element instanceof ExecutableElement xel) {
            if (xel.getKind() == ElementKind.CONSTRUCTOR) {
                return isBuilder ? fromBuilderConstructor(xel) : fromConstructor(xel);
            } else {
                return isBuilder ? fromBuilderFactory(xel) : fromFactory(xel);
            }
        } else throw new IllegalArgumentException("Expected TypeElement or ExecutableElement but got: " + element);
    }

    /**
     * Creates a [Value] from the given constructor element. ex:
     * ```
     * public class Value {
     * >   public Value(int arg1) { ... }
     * }
     * ```
     */
    public Value fromConstructor(ExecutableElement constructor) {
        checkKind(constructor, ElementKind.CONSTRUCTOR);
        return create(new ConstructionSource.Constructor(constructor));
    }

    /**
     * Creates a [Value] from the given builder's constructor element. ex:
     * ```
     * public class Builder {
     * >   public Builder() { ... }
     *     public Value build() { ... }
     * }
     * ```
     */
    public Value fromBuilderConstructor(ExecutableElement constructor) {
        checkKind(constructor, ElementKind.CONSTRUCTOR);
        return create(new ConstructionSource.BuilderConstructor(env.getTypeUtils(), constructor));
    }

    /**
     * Creates a [Value] from the given factory method element. ex:
     * ```
     * public class Value {
     * >   public static Value create(int arg) { ... }
     * }
     * ```
     */
    public Value fromFactory(ExecutableElement factory) {
        checkKind(factory, ElementKind.METHOD);
        return create(new ConstructionSource.Factory(env.getTypeUtils(), factory));
    }

    /**
     * Creates a [Value] from the given builder factory method element. ex:
     * ```
     * public class Value {
     * >   public static Builder builder() { ... }
     *     public static class Builder { ... }
     * }
     * ```
     */
    public Value fromBuilderFactory(ExecutableElement builderFactory) {
        checkKind(builderFactory, ElementKind.METHOD);
        return create(new ConstructionSource.BuilderFactory(env.getTypeUtils(), builderFactory));
    }

    /**
     * Creates a [Value] from the given class. ex:
     * ```
     * > public class Value { ... }
     * ```
     */
    public Value fromClass(TypeElement targetClass) throws ElementException {
        ExecutableElement creator = findConstructorOrFactory(targetClass);
        return creator.getKind() == ElementKind.CONSTRUCTOR ? fromConstructor(creator) : fromFactory(creator);
    }

    /**
     * Creates a [Value] from the given builder class. ex:
     * ```
     * > public class Builder {
     *     public Value build() { ... }
     * }
     * ```
     */
    public Value fromBuilderClass(TypeElement builderClass) throws ElementException {
        ExecutableElement creator = findConstructorOrFactory(builderClass);
        return creator.getKind() == ElementKind.CONSTRUCTOR ? fromBuilderConstructor(creator) : fromBuilderFactory(creator);
    }

    private Value create(ConstructionSource constructionSource) {
        return new Value(env, constructionSource);
    }

    private static ExecutableElement findConstructorOrFactory(TypeElement klazz) throws ElementException {
        ExecutableElement noArgConstructor = null;
        List<ExecutableElement> constructors = ElementFilter.constructorsIn(klazz.getEnclosedElements());
        if (constructors.size() == 1) {
            ExecutableElement constructor = constructors.get(0);
            if (constructor.getParameters().isEmpty()) {
                noArgConstructor = constructor;
                constructors.remove(0);
            }
        }
        for (ExecutableElement method : ElementFilter.methodsIn(klazz.getEnclosedElements())) {
            Set<Modifier> modifiers = method.getModifiers();
            if (modifiers.contains(Modifier.STATIC)
                    && !modifiers.contains(Modifier.PRIVATE)
                    && method.getReturnType().equals(klazz.asType())) {
                constructors.add(method);
            }
        }
        if (constructors.isEmpty() && noArgConstructor != null) {
            constructors.add(noArgConstructor);
        }
        if (constructors.size() == 1) {
            return constructors.get(0);
        } else {
            List<ElementException.Message> messages = new ArrayList<>();
            messages.add(new ElementException.Message("More than one constructor or factory method found.", klazz));
            constructors.stream().map(s -> new ElementException.Message("  " + s, s)).forEach(messages::add);
            throw new ElementException(messages);
        }
    }

    private static void checkKind(Element element, ElementKind kind) {
        if (element.getKind() != kind) {
            throw new IllegalArgumentException("Expected " + kind + " but got: " + element);
        }
    }
}