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Spring Expression Language (SpEL)

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/*
 * Copyright 2002-2021 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.expression;

import org.springframework.core.convert.TypeDescriptor;
import org.springframework.lang.Nullable;

/**
 * A type converter can convert values between different types encountered during
 * expression evaluation. This is an SPI for the expression parser; see
 * {@link org.springframework.core.convert.ConversionService} for the primary
 * user API to Spring's conversion facilities.
 *
 * @author Andy Clement
 * @author Juergen Hoeller
 * @since 3.0
 */
public interface TypeConverter {

	/**
	 * Return {@code true} if the type converter can convert the specified type
	 * to the desired target type.
	 * @param sourceType a type descriptor that describes the source type
	 * @param targetType a type descriptor that describes the requested result type
	 * @return {@code true} if that conversion can be performed
	 */
	boolean canConvert(@Nullable TypeDescriptor sourceType, TypeDescriptor targetType);

	/**
	 * Convert (or coerce) a value from one type to another, for example from a
	 * {@code boolean} to a {@code String}.
	 * The {@link TypeDescriptor} parameters enable support for typed collections:
	 * A caller may prefer a {@code List}, for example, rather than
	 * simply any {@code List}.
	 * @param value the value to be converted
	 * @param sourceType a type descriptor that supplies extra information about the
	 * source object
	 * @param targetType a type descriptor that supplies extra information about the
	 * requested result type
	 * @return the converted value
	 * @throws EvaluationException if conversion failed or is not possible to begin with
	 */
	@Nullable
	Object convertValue(@Nullable Object value, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType);

}