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• GenericEntity.java

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jakarta.ws.rs.core.GenericEntity Maven / Gradle / Ivy

/*
 * Copyright (c) 2011, 2019 Oracle and/or its affiliates. All rights reserved.
 *
 * Copyright (c) 2006 Google Inc.
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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
 *
 *      http://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 jakarta.ws.rs.core;

import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.Objects;

/**
 * Represents a message entity of a generic type {@code T}.
 * 

 * Normally type erasure removes generic type information such that a {@link Response} instance that contains, e.g., an
 * entity of type {@code List} appears to contain a raw {@code List} at runtime. When the generic type is
 * required to select a suitable {@link jakarta.ws.rs.ext.MessageBodyWriter}, this class may be used to wrap the entity
 * and capture its generic type.
 * 
 * 

 * There are two ways to create an instance:
 * 
 * 

 * 
Create a (typically anonymous) subclass of this class which enables retrieval of the type information at runtime
 * despite type erasure. For example, the following code shows how to create a {@link Response} containing an entity of
 * type {@code List} whose generic type will be available at runtime for selection of a suitable
 * {@link jakarta.ws.rs.ext.MessageBodyWriter}:
 *
 * 
 * List<String> list = new ArrayList<String>();
 * GenericEntity<List<String>> entity = new GenericEntity<List<String>>(list) {
 * };
 * Response response = Response.ok(entity).build();
 * 
 *
 * 

 * where {@code list} is the instance of {@code List} that will form the response body and entity is an instance
 * of an anonymous subclass of {@code GenericEntity}.
 * 
 * 
 * 
Create an instance directly by supplying the generic type information with the entity. For example the following
 * code shows how to create a response containing the result of a method invoked via reflection:
 *
 * 
 * Method method = ...;
 * GenericEntity<Object> entity = new GenericEntity<Object>(
 *    method.invoke(...), method.getGenericReturnType());
 * Response response = Response.ok(entity).build();
 * 
 * 

 * The above obtains the generic type from the return type of the method, the raw type is the class of entity.
 * 
 * 
 * 
 *
 * @param  response entity instance type
 * @author Paul Sandoz
 * @author Marc Hadley
 * @see GenericType
 * @since 1.0
 */
public class GenericEntity {

    private final Class rawType;
    private final Type type;
    private final T entity;

    /**
     * Constructs a new generic entity. Derives represented class from type parameter. Note that this constructor is
     * protected, users should create a (usually anonymous) subclass as shown above.
     *
     * @param entity the entity instance, must not be {@code null}.
     * @throws IllegalArgumentException if entity is {@code null}.
     */
    protected GenericEntity(final T entity) {
        if (entity == null) {
            throw new IllegalArgumentException("The entity must not be null");
        }
        this.entity = entity;
        this.type = GenericType.getTypeArgument(getClass(), GenericEntity.class);
        this.rawType = entity.getClass();
    }

    /**
     * Create a new instance of GenericEntity, supplying the generic type information. The entity must be assignable to a
     * variable of the supplied generic type, e.g. if {@code entity} is an instance of {@code ArrayList} then
     * {@code genericType} could be the same or a superclass of {@code ArrayList} with the same generic type like
     * {@code List}.
     *
     * @param entity the entity instance, must not be {@code null}.
     * @param genericType the generic type, must not be {@code null}.
     * @throws IllegalArgumentException if the entity is not assignable to a variable of the supplied generic type or if
     * entity or genericType is null.
     */
    public GenericEntity(final T entity, final Type genericType) {
        if (entity == null || genericType == null) {
            throw new IllegalArgumentException("Arguments must not be null.");
        }
        this.entity = entity;
        this.rawType = entity.getClass();
        checkTypeCompatibility(this.rawType, genericType);
        this.type = genericType;
    }

    private void checkTypeCompatibility(final Class c, final Type t) {
        if (t instanceof Class) {
            Class ct = (Class) t;
            if (ct.isAssignableFrom(c)) {
                return;
            }
        } else if (t instanceof ParameterizedType) {
            ParameterizedType pt = (ParameterizedType) t;
            Type rt = pt.getRawType();
            checkTypeCompatibility(c, rt);
            return;
        } else if (c.isArray() && (t instanceof GenericArrayType)) {
            GenericArrayType at = (GenericArrayType) t;
            Type rt = at.getGenericComponentType();
            checkTypeCompatibility(c.getComponentType(), rt);
            return;
        }
        throw new IllegalArgumentException("The type is incompatible with the class of the entity.");
    }

    /**
     * Gets the raw type of the enclosed entity. Note that this is the raw type of the instance, not the raw type of the
     * type parameter. I.e. in the example in the introduction, the raw type is {@code ArrayList} not {@code List}.
     *
     * @return the raw type.
     */
    public final Class getRawType() {
        return rawType;
    }

    /**
     * Gets underlying {@code Type} instance. Note that this is derived from the type parameter, not the enclosed instance.
     * I.e. in the example in the introduction, the type is {@code List} not {@code ArrayList}.
     *
     * @return the type
     */
    public final Type getType() {
        return type;
    }

    /**
     * Get the enclosed entity.
     *
     * @return the enclosed entity.
     */
    public final T getEntity() {
        return entity;
    }

    @Override
    public boolean equals(final Object obj) {
        boolean result = this == obj;
        if (!result && obj instanceof GenericEntity) {
            // Compare inner type for equality
            GenericEntity that = (GenericEntity) obj;
            return this.type.equals(that.type) && this.entity.equals(that.entity);
        }
        return result;
    }

    @Override
    public int hashCode() {
        return Objects.hash(entity, type);
    }

    @Override
    public String toString() {
        return "GenericEntity{" + entity.toString() + ", " + type.toString() + "}";
    }
}