javax.ws.rs.core.GenericEntity Maven / Gradle / Ivy
package javax.ws.rs.core;
/*
* The contents of this file are subject to the terms
* of the Common Development and Distribution License
* (the "License"). You may not use this file except
* in compliance with the License.
*
* You can obtain a copy of the license at
* http://www.opensource.org/licenses/cddl1.php
* See the License for the specific language governing
* permissions and limitations under the License.
*
* This file incorporates work covered by the following copyright and
* permission notice:
*
* Copyright (C) 2006 Google Inc.
*
* 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
*
* 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.
*/
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import javax.ws.rs.ext.MessageBodyWriter;
/**
* Represents a response 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 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 MessageBodyWriter}:
*
* List<String> list = new ArrayList<String>();
*GenericEntity<List<String>> entity = new GenericEntity<List<String>>(list) {};
*Response response = Response.ok(entity).build();
*
* where list is the instance of List<String>
* 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.
*
*/
public class GenericEntity {
final Class rawType;
final Type type;
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 null
* @throws IllegalArgumentException if entity is null
*/
protected GenericEntity(T entity) {
if (entity == null) {
throw new IllegalArgumentException("The entity must not be null");
}
this.entity = entity;
this.type = getSuperclassTypeParameter(getClass());
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 null
* @param genericType the generic type, must not be 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(T entity, 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(Class c, 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");
}
/**
* Returns the type from super class's type parameter.
*/
private static Type getSuperclassTypeParameter(Class subclass) {
Type superclass = subclass.getGenericSuperclass();
if (!(superclass instanceof ParameterizedType)) {
throw new RuntimeException("Missing type parameter.");
}
ParameterizedType parameterized = (ParameterizedType) superclass;
return parameterized.getActualTypeArguments()[0];
}
/**
* 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;
}
}