TypeDescriptor.java

/*
 * Copyright 2002-2013 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
 *
 *      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 org.springframework.core.convert;

import java.io.Serializable;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

import org.springframework.core.MethodParameter;
import org.springframework.core.ResolvableType;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ObjectUtils;

/**
 * Context about a type to convert from or to.
 *
 * @author Keith Donald
 * @author Andy Clement
 * @author Juergen Hoeller
 * @author Phillip Webb
 * @author Sam Brannen
 * @since 3.0
 */
public class TypeDescriptor implements Serializable {

	private static final long serialVersionUID = 1L;


	static final Annotation[] EMPTY_ANNOTATION_ARRAY = new Annotation[0];

	private static final Map<Class<?>, TypeDescriptor> commonTypesCache = new HashMap<Class<?>, TypeDescriptor>();

	private static final Class<?>[] CACHED_COMMON_TYPES = { Boolean.class, byte.class,
		Byte.class, char.class, Character.class, short.class, Short.class, int.class,
		Integer.class, long.class, Long.class, float.class, Float.class, double.class,
		Double.class, String.class };
	static {
		for (Class<?> preCachedClass : CACHED_COMMON_TYPES) {
			commonTypesCache.put(preCachedClass, valueOf(preCachedClass));
		}
	}


	private final Class<?> type;

	private final ResolvableType resolvableType;

	private final Annotation[] annotations;


	/**
	 * Create a new type descriptor from a {@link MethodParameter}.
	 * <p>Use this constructor when a source or target conversion point is a
	 * constructor parameter, method parameter, or method return value.
	 * @param methodParameter the method parameter
	 */
	public TypeDescriptor(MethodParameter methodParameter) {
		Assert.notNull(methodParameter, "MethodParameter must not be null");
		if (methodParameter.getNestingLevel() != 1) {
			throw new IllegalArgumentException("MethodParameter argument must have its nestingLevel set to 1");
		}
		this.resolvableType = ResolvableType.forMethodParameter(methodParameter);
		this.type = this.resolvableType.resolve(Object.class);
		this.annotations = (methodParameter.getParameterIndex() == -1 ?
				nullSafeAnnotations(methodParameter.getMethodAnnotations()) :
				nullSafeAnnotations(methodParameter.getParameterAnnotations()));
	}

	/**
	 * Create a new type descriptor from a {@link Field}.
	 * <p>Use this constructor when a source or target conversion point is a field.
	 * @param field the field
	 */
	public TypeDescriptor(Field field) {
		Assert.notNull(field, "Field must not be null");
		this.resolvableType = ResolvableType.forField(field);
		this.type = this.resolvableType.resolve(Object.class);
		this.annotations = nullSafeAnnotations(field.getAnnotations());
	}

	/**
	 * Create a new type descriptor from a {@link Property}.
	 * <p>Use this constructor when a source or target conversion point is a
	 * property on a Java class.
	 * @param property the property
	 */
	public TypeDescriptor(Property property) {
		Assert.notNull(property, "Property must not be null");
		this.resolvableType = ResolvableType.forMethodParameter(property.getMethodParameter());
		this.type = this.resolvableType.resolve(Object.class);
		this.annotations = nullSafeAnnotations(property.getAnnotations());
	}

	private TypeDescriptor(ResolvableType resolvableType, Class<?> type, Annotation[] annotations) {
		this.resolvableType = resolvableType;
		this.type = (type != null ? type : resolvableType.resolve(Object.class));
		this.annotations = nullSafeAnnotations(annotations);
	}


	private Annotation[] nullSafeAnnotations(Annotation[] annotations) {
		return annotations != null ? annotations : EMPTY_ANNOTATION_ARRAY;
	}

	/**
	 * Variation of {@link #getType()} that accounts for a primitive type by returning its object wrapper type.
	 * <p>This is useful for conversion service implementations that wish to normalize to object-based types
	 * and not work with primitive types directly.
	 */
	public Class<?> getObjectType() {
		return ClassUtils.resolvePrimitiveIfNecessary(getType());
	}

	/**
	 * The type of the backing class, method parameter, field, or property described by this TypeDescriptor.
	 * <p>Returns primitive types as-is.
	 * <p>See {@link #getObjectType()} for a variation of this operation that resolves primitive types
	 * to their corresponding Object types if necessary.
	 * @return the type, or {@code null}
	 * @see #getObjectType()
	 */
	public Class<?> getType() {
		return this.type;
	}

	/**
	 * Narrows this {@link TypeDescriptor} by setting its type to the class of the provided value.
	 * <p>If the value is {@code null}, no narrowing is performed and this TypeDescriptor is returned unchanged.
	 * <p>Designed to be called by binding frameworks when they read property, field, or method return values.
	 * Allows such frameworks to narrow a TypeDescriptor built from a declared property, field, or method return value type.
	 * For example, a field declared as {@code java.lang.Object} would be narrowed to {@code java.util.HashMap}
	 * if it was set to a {@code java.util.HashMap} value. The narrowed TypeDescriptor can then be used to convert
	 * the HashMap to some other type. Annotation and nested type context is preserved by the narrowed copy.
	 * @param value the value to use for narrowing this type descriptor
	 * @return this TypeDescriptor narrowed (returns a copy with its type updated to the class of the provided value)
	 */
	public TypeDescriptor narrow(Object value) {
		if (value == null) {
			return this;
		}
		ResolvableType narrowed = ResolvableType.forType(value.getClass(), this.resolvableType);
		return new TypeDescriptor(narrowed, null, this.annotations);
	}

	/**
	 * Cast this {@link TypeDescriptor} to a superclass or implemented interface
	 * preserving annotations and nested type context.
	 * @param superType the super type to cast to (can be {@code null}
	 * @return a new TypeDescriptor for the up-cast type
	 * @throws IllegalArgumentException if this type is not assignable to the super-type
	 * @since 3.2
	 */
	public TypeDescriptor upcast(Class<?> superType) {
		if (superType == null) {
			return null;
		}
		Assert.isAssignable(superType, getType());
		return new TypeDescriptor(this.resolvableType.as(superType), superType, this.annotations);
	}

	/**
	 * Returns the name of this type: the fully qualified class name.
	 */
	public String getName() {
		return ClassUtils.getQualifiedName(getType());
	}

	/**
	 * Is this type a primitive type?
	 */
	public boolean isPrimitive() {
		return getType().isPrimitive();
	}

	/**
	 * The annotations associated with this type descriptor, if any.
	 * @return the annotations, or an empty array if none
	 */
	public Annotation[] getAnnotations() {
		return this.annotations;
	}

	/**
	 * Determine if this type descriptor has the specified annotation.
	 * @param annotationType the annotation type
	 * @return <tt>true</tt> if the annotation is present
	 */
	public boolean hasAnnotation(Class<? extends Annotation> annotationType) {
		return getAnnotation(annotationType) != null;
	}

	/**
	 * Obtain the annotation associated with this type descriptor of the specified type.
	 * @param annotationType the annotation type
	 * @return the annotation, or {@code null} if no such annotation exists on this type descriptor
	 */
	@SuppressWarnings("unchecked")
	public <T extends Annotation> T getAnnotation(Class<T> annotationType) {
		for (Annotation annotation : getAnnotations()) {
			if (annotation.annotationType().equals(annotationType)) {
				return (T) annotation;
			}
		}
		for (Annotation metaAnn : getAnnotations()) {
			T ann = metaAnn.annotationType().getAnnotation(annotationType);
			if (ann != null) {
				return ann;
			}
		}
		return null;
	}

	/**
	 * Returns true if an object of this type descriptor can be assigned to the location described by the given type descriptor.
	 * <p>For example, valueOf(String.class).isAssignableTo(valueOf(CharSequence.class)) returns true because a String value can be assigned to a CharSequence variable.
	 * On the other hand, valueOf(Number.class).isAssignableTo(valueOf(Integer.class)) returns false because, while all Integers are Numbers, not all Numbers are Integers.
	 * <p>
	 * For arrays, collections, and maps, element and key/value types are checked if declared.
	 * For example, a List&lt;String&gt; field value is assignable to a Collection&lt;CharSequence&gt; field, but List&lt;Number&gt; is not assignable to List&lt;Integer&gt;.
	 * @return true if this type is assignable to the type represented by the provided type descriptor
	 * @see #getObjectType()
	 */
	public boolean isAssignableTo(TypeDescriptor typeDescriptor) {
		boolean typesAssignable = typeDescriptor.getObjectType().isAssignableFrom(getObjectType());
		if (!typesAssignable) {
			return false;
		}
		if (isArray() && typeDescriptor.isArray()) {
			return getElementTypeDescriptor().isAssignableTo(typeDescriptor.getElementTypeDescriptor());
		}
		else if (isCollection() && typeDescriptor.isCollection()) {
			return isNestedAssignable(getElementTypeDescriptor(), typeDescriptor.getElementTypeDescriptor());
		}
		else if (isMap() && typeDescriptor.isMap()) {
			return isNestedAssignable(getMapKeyTypeDescriptor(), typeDescriptor.getMapKeyTypeDescriptor()) &&
				isNestedAssignable(getMapValueTypeDescriptor(), typeDescriptor.getMapValueTypeDescriptor());
		}
		else {
			return true;
		}
	}

	private boolean isNestedAssignable(TypeDescriptor nestedTypeDescriptor, TypeDescriptor otherNestedTypeDescriptor) {
		if (nestedTypeDescriptor == null || otherNestedTypeDescriptor == null) {
			return true;
		}
		return nestedTypeDescriptor.isAssignableTo(otherNestedTypeDescriptor);
	}

	/**
	 * Is this type a {@link Collection} type?
	 */
	public boolean isCollection() {
		return Collection.class.isAssignableFrom(getType());
	}

	/**
	 * Is this type an array type?
	 */
	public boolean isArray() {
		return getType().isArray();
	}

	/**
	 * If this type is an array, returns the array's component type.
	 * If this type is a {@link Collection} and it is parameterized, returns the Collection's element type.
	 * If the Collection is not parameterized, returns null indicating the element type is not declared.
	 * @return the array component type or Collection element type, or {@code null} if this type is a Collection but its element type is not parameterized
	 * @throws IllegalStateException if this type is not a java.util.Collection or Array type
	 */
	public TypeDescriptor getElementTypeDescriptor() {
		assertCollectionOrArray();
		if (this.resolvableType.isArray()) {
			return getRelatedIfResolvable(this, this.resolvableType.getComponentType());
		}
		return getRelatedIfResolvable(this, this.resolvableType.asCollection().getGeneric());

	}

	/**
	 * If this type is a {@link Collection} or an Array, creates a element TypeDescriptor from the provided collection or array element.
	 * <p>Narrows the {@link #getElementTypeDescriptor() elementType} property to the class of the provided collection or array element.
	 * For example, if this describes a java.util.List&lt;java.lang.Number&lt; and the element argument is a java.lang.Integer, the returned TypeDescriptor will be java.lang.Integer.
	 * If this describes a java.util.List&lt;?&gt; and the element argument is a java.lang.Integer, the returned TypeDescriptor will be java.lang.Integer as well.
	 * <p>Annotation and nested type context will be preserved in the narrowed TypeDescriptor that is returned.
	 * @param element the collection or array element
	 * @return a element type descriptor, narrowed to the type of the provided element
	 * @throws IllegalStateException if this type is not a java.util.Collection or Array type
	 * @see #narrow(Object)
	 */
	public TypeDescriptor elementTypeDescriptor(Object element) {
		return narrow(element, getElementTypeDescriptor());
	}

	/**
	 * Is this type a {@link Map} type?
	 */
	public boolean isMap() {
		return Map.class.isAssignableFrom(getType());
	}

	/**
	 * If this type is a {@link Map} and its key type is parameterized, returns the map's key type.
	 * If the Map's key type is not parameterized, returns null indicating the key type is not declared.
	 * @return the Map key type, or {@code null} if this type is a Map but its key type is not parameterized
	 * @throws IllegalStateException if this type is not a java.util.Map
	 */
	public TypeDescriptor getMapKeyTypeDescriptor() {
		assertMap();
		return getRelatedIfResolvable(this, this.resolvableType.asMap().getGeneric(0));
	}

	/**
	 * If this type is a {@link Map}, creates a mapKey {@link TypeDescriptor} from the provided map key.
	 * <p>Narrows the {@link #getMapKeyTypeDescriptor() mapKeyType} property to the class of the provided map key.
	 * For example, if this describes a java.util.Map&lt;java.lang.Number, java.lang.String&lt; and the key argument is a java.lang.Integer, the returned TypeDescriptor will be java.lang.Integer.
	 * <p>If this describes a java.util.Map&lt;?, ?&gt; and the key argument is a java.lang.Integer, the returned TypeDescriptor will be java.lang.Integer as well.
	 * <p>Annotation and nested type context will be preserved in the narrowed TypeDescriptor that is returned.
	 * @param mapKey the map key
	 * @return the map key type descriptor
	 * @throws IllegalStateException if this type is not a java.util.Map
	 * @see #narrow(Object)
	 */
	public TypeDescriptor getMapKeyTypeDescriptor(Object mapKey) {
		return narrow(mapKey, getMapKeyTypeDescriptor());
	}

	/**
	 * If this type is a {@link Map} and its value type is parameterized, returns the map's value type.
	 * <p>If the Map's value type is not parameterized, returns null indicating the value type is not declared.
	 * @return the Map value type, or {@code null} if this type is a Map but its value type is not parameterized
	 * @throws IllegalStateException if this type is not a java.util.Map
	 */
	public TypeDescriptor getMapValueTypeDescriptor() {
		assertMap();
		return getRelatedIfResolvable(this, this.resolvableType.asMap().getGeneric(1));
	}

	/**
	 * If this type is a {@link Map}, creates a mapValue {@link TypeDescriptor} from the provided map value.
	 * <p>Narrows the {@link #getMapValueTypeDescriptor() mapValueType} property to the class of the provided map value.
	 * For example, if this describes a java.util.Map&lt;java.lang.String, java.lang.Number&lt; and the value argument is a java.lang.Integer, the returned TypeDescriptor will be java.lang.Integer.
	 * If this describes a java.util.Map&lt;?, ?&gt; and the value argument is a java.lang.Integer, the returned TypeDescriptor will be java.lang.Integer as well.
	 * <p>Annotation and nested type context will be preserved in the narrowed TypeDescriptor that is returned.
	 * @param mapValue the map value
	 * @return the map value type descriptor
	 * @throws IllegalStateException if this type is not a java.util.Map
	 */
	public TypeDescriptor getMapValueTypeDescriptor(Object mapValue) {
		return narrow(mapValue, getMapValueTypeDescriptor());
	}

	/**
	 * Returns the value of {@link TypeDescriptor#getType() getType()} for the {@link #getElementTypeDescriptor() elementTypeDescriptor}.
	 * @deprecated in Spring 3.1 in favor of {@link #getElementTypeDescriptor()}
	 * @throws IllegalStateException if this type is not a java.util.Collection or Array type
	 */
	@Deprecated
	public Class<?> getElementType() {
		return getType(getElementTypeDescriptor());
	}

	/**
	 * Returns the value of {@link TypeDescriptor#getType() getType()} for the {@link #getMapKeyTypeDescriptor() getMapKeyTypeDescriptor}.
	 * @deprecated in Spring 3.1 in favor of {@link #getMapKeyTypeDescriptor()}
	 * @throws IllegalStateException if this type is not a java.util.Map
	 */
	@Deprecated
	public Class<?> getMapKeyType() {
		return getType(getMapKeyTypeDescriptor());
	}

	/**
	 * Returns the value of {@link TypeDescriptor#getType() getType()} for the {@link #getMapValueTypeDescriptor() getMapValueTypeDescriptor}.
	 * @deprecated in Spring 3.1 in favor of {@link #getMapValueTypeDescriptor()}
	 * @throws IllegalStateException if this type is not a java.util.Map
	 */
	@Deprecated
	public Class<?> getMapValueType() {
		return getType(getMapValueTypeDescriptor());
	}

	private Class<?> getType(TypeDescriptor typeDescriptor) {
		return (typeDescriptor == null ? null : typeDescriptor.getType());
	}

	private void assertCollectionOrArray() {
		if (!isCollection() && !isArray()) {
			throw new IllegalStateException("Not a java.util.Collection or Array");
		}
	}

	private void assertMap() {
		if (!isMap()) {
			throw new IllegalStateException("Not a java.util.Map");
		}
	}

	private TypeDescriptor narrow(Object value, TypeDescriptor typeDescriptor) {
		if (typeDescriptor != null) {
			return typeDescriptor.narrow(value);
		}
		return (value != null ? new TypeDescriptor(this.resolvableType, value.getClass(), this.annotations) : null);
	}

	@Override
	public boolean equals(Object obj) {
		if (this == obj) {
			return true;
		}
		if (!(obj instanceof TypeDescriptor)) {
			return false;
		}
		TypeDescriptor other = (TypeDescriptor) obj;
		if (!ObjectUtils.nullSafeEquals(this.type, other.type)) {
			return false;
		}
		if (getAnnotations().length != other.getAnnotations().length) {
			return false;
		}
		for (Annotation ann : this.getAnnotations()) {
			if (other.getAnnotation(ann.annotationType()) == null) {
				return false;
			}
		}
		if (isCollection() || isArray()) {
			return ObjectUtils.nullSafeEquals(this.getElementTypeDescriptor(), other.getElementTypeDescriptor());
		}
		else if (isMap()) {
			return ObjectUtils.nullSafeEquals(this.getMapKeyTypeDescriptor(), other.getMapKeyTypeDescriptor()) &&
					ObjectUtils.nullSafeEquals(this.getMapValueTypeDescriptor(), other.getMapValueTypeDescriptor());
		}
		else {
			return true;
		}
	}

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

	@Override
	public String toString() {
		StringBuilder builder = new StringBuilder();
		for (Annotation ann : getAnnotations()) {
			builder.append("@").append(ann.annotationType().getName()).append(' ');
		}
		builder.append(this.resolvableType.toString());
		return builder.toString();
	}

	/**
	 * Create a new type descriptor from the given type.
	 * <p>Use this to instruct the conversion system to convert an object to a
	 * specific target type, when no type location such as a method parameter or
	 * field is available to provide additional conversion context.
	 * <p>Generally prefer use of {@link #forObject(Object)} for constructing type
	 * descriptors from source objects, as it handles the {@code null} object case.
	 * @param type the class
	 * @return the type descriptor
	 */
	public static TypeDescriptor valueOf(Class<?> type) {
		Assert.notNull(type, "Type must not be null");
		TypeDescriptor desc = commonTypesCache.get(type);
		return (desc != null ? desc : new TypeDescriptor(ResolvableType.forClass(type), null, null));
	}

	/**
	 * Create a new type descriptor from a {@link java.util.Collection} type.
	 * <p>Useful for converting to typed Collections.
	 * <p>For example, a {@code List<String>} could be converted to a
	 * {@code List<EmailAddress>} by converting to a targetType built with this method.
	 * The method call to construct such a {@code TypeDescriptor} would look something
	 * like: {@code collection(List.class, TypeDescriptor.valueOf(EmailAddress.class));}
	 * @param collectionType the collection type, which must implement {@link Collection}.
	 * @param elementTypeDescriptor a descriptor for the collection's element type,
	 * used to convert collection elements
	 * @return the collection type descriptor
	 */
	public static TypeDescriptor collection(Class<?> collectionType, TypeDescriptor elementTypeDescriptor) {
		Assert.notNull(collectionType, "CollectionType must not be null");
		if (!Collection.class.isAssignableFrom(collectionType)) {
			throw new IllegalArgumentException("collectionType must be a java.util.Collection");
		}
		ResolvableType element = (elementTypeDescriptor == null ? null
				: elementTypeDescriptor.resolvableType);
		return new TypeDescriptor(ResolvableType.forClassWithGenerics(collectionType,
				element), null, null);
	}

	/**
	 * Create a new type descriptor from a {@link java.util.Map} type.
	 * <p>Useful for converting to typed Maps.
	 * <p>For example, a Map&lt;String, String&gt; could be converted to a Map&lt;Id, EmailAddress&gt; by converting to a targetType built with this method:
	 * The method call to construct such a TypeDescriptor would look something like: map(Map.class, TypeDescriptor.valueOf(Id.class), TypeDescriptor.valueOf(EmailAddress.class));
	 * @param mapType the map type, which must implement {@link Map}
	 * @param keyTypeDescriptor a descriptor for the map's key type, used to convert map keys
	 * @param valueTypeDescriptor the map's value type, used to convert map values
	 * @return the map type descriptor
	 */
	public static TypeDescriptor map(Class<?> mapType, TypeDescriptor keyTypeDescriptor, TypeDescriptor valueTypeDescriptor) {
		if (!Map.class.isAssignableFrom(mapType)) {
			throw new IllegalArgumentException("mapType must be a java.util.Map");
		}
		ResolvableType key = (keyTypeDescriptor == null ? null : keyTypeDescriptor.resolvableType);
		ResolvableType value = (valueTypeDescriptor == null ? null : valueTypeDescriptor.resolvableType);
		return new TypeDescriptor(ResolvableType.forClassWithGenerics(mapType, key, value), null, null);
	}

	/**
	 * Create a new type descriptor as an array of the specified type.
	 * <p>For example to create a {@code Map<String,String>[]} use
	 * {@code TypeDescriptor.array(TypeDescriptor.map(Map.class, TypeDescriptor.value(String.class), TypeDescriptor.value(String.class)))}.
	 * @param elementTypeDescriptor the {@link TypeDescriptor} of the array element or {@code null}
	 * @return an array {@link TypeDescriptor} or {@code null} if {@code elementTypeDescriptor} is {@code null}
	 * @since 3.2.1
	 */
	public static TypeDescriptor array(TypeDescriptor elementTypeDescriptor) {
		if(elementTypeDescriptor == null) {
			return null;
		}
		return new TypeDescriptor(
				ResolvableType.forArrayComponent(elementTypeDescriptor.resolvableType),
				null, elementTypeDescriptor.getAnnotations());
	}

	/**
	 * Creates a type descriptor for a nested type declared within the method parameter.
	 * <p>For example, if the methodParameter is a {@code List<String>} and the
	 * nesting level is 1, the nested type descriptor will be String.class.
	 * <p>If the methodParameter is a {@code List<List<String>>} and the nesting
	 * level is 2, the nested type descriptor will also be a String.class.
	 * <p>If the methodParameter is a {@code Map<Integer, String>} and the nesting
	 * level is 1, the nested type descriptor will be String, derived from the map value.
	 * <p>If the methodParameter is a {@code List<Map<Integer, String>>} and the
	 * nesting level is 2, the nested type descriptor will be String, derived from the map value.
	 * <p>Returns {@code null} if a nested type cannot be obtained because it was not declared.
	 * For example, if the method parameter is a {@code List<?>}, the nested type
	 * descriptor returned will be {@code null}.
	 * @param methodParameter the method parameter with a nestingLevel of 1
	 * @param nestingLevel the nesting level of the collection/array element or
	 * map key/value declaration within the method parameter
	 * @return the nested type descriptor at the specified nesting level, or null
	 * if it could not be obtained
	 * @throws IllegalArgumentException if the nesting level of the input
	 * {@link MethodParameter} argument is not 1
	 * @throws IllegalArgumentException if the types up to the specified nesting
	 * level are not of collection, array, or map types
	 */
	public static TypeDescriptor nested(MethodParameter methodParameter, int nestingLevel) {
		if (methodParameter.getNestingLevel() != 1) {
			throw new IllegalArgumentException("methodParameter nesting level must be 1: use the nestingLevel parameter to specify the desired nestingLevel for nested type traversal");
		}
		return nested(new TypeDescriptor(methodParameter), nestingLevel);
	}

	/**
	 * Creates a type descriptor for a nested type declared within the field.
	 * <p>For example, if the field is a {@code List<String>} and the nesting
	 * level is 1, the nested type descriptor will be {@code String.class}.
	 * <p>If the field is a {@code List<List<String>>} and the nesting level is
	 * 2, the nested type descriptor will also be a {@code String.class}.
	 * <p>If the field is a {@code Map<Integer, String>} and the nesting level
	 * is 1, the nested type descriptor will be String, derived from the map value.
	 * <p>If the field is a {@code List<Map<Integer, String>>} and the nesting
	 * level is 2, the nested type descriptor will be String, derived from the map value.
	 * <p>Returns {@code null} if a nested type cannot be obtained because it was not declared.
	 * For example, if the field is a {@code List<?>}, the nested type descriptor returned will be {@code null}.
	 * @param field the field
	 * @param nestingLevel the nesting level of the collection/array element or
	 * map key/value declaration within the field
	 * @return the nested type descriptor at the specified nesting level, or null
	 * if it could not be obtained
	 * @throws IllegalArgumentException if the types up to the specified nesting
	 * level are not of collection, array, or map types
	 */
	public static TypeDescriptor nested(Field field, int nestingLevel) {
		return nested(new TypeDescriptor(field), nestingLevel);
	}

	/**
	 * Creates a type descriptor for a nested type declared within the property.
	 * <p>For example, if the property is a {@code List<String>} and the nesting
	 * level is 1, the nested type descriptor will be {@code String.class}.
	 * <p>If the property is a {@code List<List<String>>} and the nesting level
	 * is 2, the nested type descriptor will also be a {@code String.class}.
	 * <p>If the property is a {@code Map<Integer, String>} and the nesting level
	 * is 1, the nested type descriptor will be String, derived from the map value.
	 * <p>If the property is a {@code List<Map<Integer, String>>} and the nesting
	 * level is 2, the nested type descriptor will be String, derived from the map value.
	 * <p>Returns {@code null} if a nested type cannot be obtained because it was not declared.
	 * For example, if the property is a {@code List<?>}, the nested type descriptor
	 * returned will be {@code null}.
	 * @param property the property
	 * @param nestingLevel the nesting level of the collection/array element or
	 * map key/value declaration within the property
	 * @return the nested type descriptor at the specified nesting level, or
	 * {@code null} if it could not be obtained
	 * @throws IllegalArgumentException if the types up to the specified nesting
	 * level are not of collection, array, or map types
	 */
	public static TypeDescriptor nested(Property property, int nestingLevel) {
		return nested(new TypeDescriptor(property), nestingLevel);
	}

	/**
	 * Create a new type descriptor for an object.
	 * <p>Use this factory method to introspect a source object before asking the conversion system to convert it to some another type.
	 * <p>If the provided object is null, returns null, else calls {@link #valueOf(Class)} to build a TypeDescriptor from the object's class.
	 * @param source the source object
	 * @return the type descriptor
	 */
	public static TypeDescriptor forObject(Object source) {
		return (source != null ? valueOf(source.getClass()) : null);
	}

	private static TypeDescriptor nested(TypeDescriptor typeDescriptor, int nestingLevel) {
		ResolvableType nested = typeDescriptor.resolvableType;
		for (int i = 0; i < nestingLevel; i++) {
			if (Object.class.equals(nested.getType())) {
				// could be a collection type but we don't know about its element type,
				// so let's just assume there is an element type of type Object
			} else {
				nested = nested.getNested(2);
			}
		}
		Assert.state(nested != ResolvableType.NONE, "Unable to obtain nested generic from "
					+ typeDescriptor + " at level " + nestingLevel);
		return getRelatedIfResolvable(typeDescriptor, nested);
	}

	private static TypeDescriptor getRelatedIfResolvable(TypeDescriptor source, ResolvableType type) {
		if (type.resolve() == null) {
			return null;
		}
		return new TypeDescriptor(type, null, source.annotations);
	}

}