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/*
* #%L
* SciJava library for generic type reasoning.
* %%
* Copyright (C) 2016 - 2025 SciJava developers.
* %%
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
* #L%
*/
package org.scijava.types;
import com.google.common.reflect.TypeToken;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Proxy;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.Set;
import org.scijava.common3.Classes;
import org.scijava.common3.GenericTyped;
/**
* A "typed null" which knows its generic type, and can generate proxy objects
* implementing that type's interfaces, with customizable behavior per interface
* method via callbacks.
* <p>
* The term "nil" was chosen, despite lots of possibility for confusion with
* related paradigms such as the Scala language, because it is very short while
* loosely meaning the same thing as {@code null}, {@code None}, etc.
* </p>
*
* @author Curtis Rueden
*/
public abstract class Nil<T> implements GenericTyped, Proxyable<T> {
/** Generic type of the object. */
private final TypeToken<?> typeToken;
/** An object which holds any method callbacks. */
private final Object callbacks;
/**
* Creates a new {@code Nil} whose generic type (returned by
* {@link #type()}) is the one specified by the generic parameters used at
* construction.
* <p>
* For example:
* </p>
*
* <pre>
* Nil<List<Map<K, V>>> nil = new Nil<List<Map<K, V>>>() {};
* </pre>
* <p>
* Subsequent calls to {@code nil.type()} will return the proper
* generically typed result—in the above example, a
* {@link ParameterizedType} whose raw type is {@code List}, and whose single
* type parameter is a {@link ParameterizedType} with raw type of {@code Map}
* and {@link TypeVariable} type parameters of {@code K} and {@code V}
* respectively, including their bounds inferred by the compiler in the
* context where the expression was written.
* </p>
*/
public Nil() {
typeToken = new TypeToken<T>(getClass()) {};
callbacks = this;
}
public Nil(final Object callbacks) {
typeToken = new TypeToken<T>(getClass()) {};
this.callbacks = callbacks(callbacks);
}
/**
* Creates a {@code Nil} wrapping the given raw {@link Class}.
* <p>
* This method is intentionally private; use {@link Nil#of(Class)} to create
* such a {@code Nil} from calling code.
* </p>
*/
private Nil(final Class<T> rawType) {
typeToken = TypeToken.of(rawType);
callbacks = this;
}
/**
* Creates a {@code Nil} wrapping the given generic {@link Type}.
* <p>
* This method is intentionally private; use {@link Nil#of(Type)} to create
* such a {@code Nil} from calling code.
* </p>
*/
private Nil(final Type type) {
typeToken = TypeToken.of(type);
callbacks = this;
}
/**
* Creates a {@code Nil} wrapping the given generic {@link Type}, and
* possessing the specified method callbacks.
* <p>
* This method is intentionally private; use {@link Nil#of(Type, Object)} to
* create such a {@code Nil} from calling code.
* </p>
*/
private Nil(final Type type, final Object callbacks) {
typeToken = TypeToken.of(type);
this.callbacks = callbacks(callbacks);
}
// -- Static utility methods --
public static <T> Nil<T> of(final Class<T> rawType) {
return new Nil<>(rawType) {};
}
/**
* Creates a {@code Nil} of the given generic {@link Type}, with no extra
* method callbacks.
*/
public static Nil<?> of(final Type type) {
return new Nil<>(type) {};
}
/**
* Creates a {@code Nil} of the given {@link Type}, with extra method
* callbacks contained in the specified object.
*/
public static Nil<?> of(final Type type, final Object callbacks) {
return new Nil<>(type, callbacks) {};
}
// -- Object methods --
@Override
public String toString() {
return typeToken.toString();
}
// -- GenericTyped methods --
@Override
public Type type() {
return typeToken.getType();
}
// -- GenericProxy methods --
/**
* Create a proxy which implements all the same interfaces as this object's
* generic type.
* <p>
* Optionally,
* CTR FIXME - write up how method delegation works.
* </p>
*/
@Override
public T proxy() {
final var loader = Classes.classLoader();
// extract the generic type's interfaces
final Set<?> ifaceSet = typeToken.getTypes().interfaces().rawTypes();
final var appendGT = !ifaceSet.contains(GenericTyped.class);
final var ifaceCount = ifaceSet.size() + (appendGT ? 1 : 0);
final var interfaces = new Class<?>[ifaceCount];
ifaceSet.toArray(interfaces);
if (appendGT) interfaces[ifaceCount - 1] = GenericTyped.class;
// NB: Technically, this cast is not safe, because T might not be
// an interface, and thus might not be one of the proxy's types.
@SuppressWarnings("unchecked")
final var proxy = (T) Proxy.newProxyInstance(loader, interfaces,
new InvocationHandler() {
@Override
public Object invoke(final Object proxy, final Method method,
final Object[] args) throws Throwable
{
try {
// Look for a Nil subclass method of the same signature.
final var m = callbacks.getClass().getMethod(method.getName(), //
method.getParameterTypes());
return m.invoke(callbacks, args);
}
catch (final NoSuchMethodException exc) {
// NB: Default behavior is to do nothing and return null.
return Classes.nullValue(method.getReturnType());
}
}
}
);
return proxy;
}
// -- Helper methods --
/** Unwraps the given callbacks object. */
private Object callbacks(final Object o) {
return o instanceof Nil ? ((Nil<?>) o).callbacks : o;
}
}