org.springframework.core.ParameterizedTypeReference.java Source code

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Here is the source code for org.springframework.core.ParameterizedTypeReference.java

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/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.core;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;

/**
 * The purpose of this class is to enable capturing and passing a generic
 * {@link Type}. In order to capture the generic type and retain it at runtime,
 * you need to create a subclass (ideally as anonymous inline class) as follows:
 *
 * <pre class="code">
 * ParameterizedTypeReference&lt;List&lt;String&gt;&gt; typeRef = new ParameterizedTypeReference&lt;List&lt;String&gt;&gt;() {};
 * </pre>
 *
 * <p>The resulting {@code typeRef} instance can then be used to obtain a {@link Type}
 * instance that carries the captured parameterized type information at runtime.
 * For more information on "super type tokens" see the link to Neal Gafter's blog post.
 *
 * @author Arjen Poutsma
 * @author Rossen Stoyanchev
 * @since 3.2
 * @param <T> the referenced type
 * @see <a href="https://gafter.blogspot.nl/2006/12/super-type-tokens.html">Neal Gafter on Super Type Tokens</a>
 */
public abstract class ParameterizedTypeReference<T> {

    private final Type type;

    protected ParameterizedTypeReference() {
        Class<?> parameterizedTypeReferenceSubclass = findParameterizedTypeReferenceSubclass(getClass());
        Type type = parameterizedTypeReferenceSubclass.getGenericSuperclass();
        Assert.isInstanceOf(ParameterizedType.class, type, "Type must be a parameterized type");
        ParameterizedType parameterizedType = (ParameterizedType) type;
        Type[] actualTypeArguments = parameterizedType.getActualTypeArguments();
        Assert.isTrue(actualTypeArguments.length == 1, "Number of type arguments must be 1");
        this.type = actualTypeArguments[0];
    }

    private ParameterizedTypeReference(Type type) {
        this.type = type;
    }

    public Type getType() {
        return this.type;
    }

    @Override
    public boolean equals(@Nullable Object other) {
        return (this == other || (other instanceof ParameterizedTypeReference
                && this.type.equals(((ParameterizedTypeReference<?>) other).type)));
    }

    @Override
    public int hashCode() {
        return this.type.hashCode();
    }

    @Override
    public String toString() {
        return "ParameterizedTypeReference<" + this.type + ">";
    }

    /**
     * Build a {@code ParameterizedTypeReference} wrapping the given type.
     * @param type a generic type (possibly obtained via reflection,
     * e.g. from {@link java.lang.reflect.Method#getGenericReturnType()})
     * @return a corresponding reference which may be passed into
     * {@code ParameterizedTypeReference}-accepting methods
     * @since 4.3.12
     */
    public static <T> ParameterizedTypeReference<T> forType(Type type) {
        return new ParameterizedTypeReference<T>(type) {
        };
    }

    private static Class<?> findParameterizedTypeReferenceSubclass(Class<?> child) {
        Class<?> parent = child.getSuperclass();
        if (Object.class == parent) {
            throw new IllegalStateException("Expected ParameterizedTypeReference superclass");
        } else if (ParameterizedTypeReference.class == parent) {
            return child;
        } else {
            return findParameterizedTypeReferenceSubclass(parent);
        }
    }

}