java/jakarta/el/ELResolver.java

/*
 * 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.el;

/**
 * @author Jacob Hookom [jacob/hookom.net]
 */
public abstract class ELResolver {

    /**
     * Obtain the value of the given property on the given object using the given context.
     *
     * @param context  The EL context for this evaluation
     * @param base     The base object on which the property is to be found
     * @param property The property whose value is to be returned
     *
     * @return the value of the provided property
     *
     * @throws NullPointerException      If the supplied context is <code>null</code>
     * @throws PropertyNotFoundException If the base/property combination provided to the resolver is one that the
     *                                       resolver can handle but no match was found or a match was found but was not
     *                                       readable
     * @throws ELException               Wraps any exception throw whilst resolving the property
     */
    public abstract Object getValue(ELContext context, Object base, Object property);

    /**
     * Invokes a method on the given object.
     *
     * @param context    The EL context for this evaluation
     * @param base       The base object on which the method is to be found
     * @param method     The method to invoke
     * @param paramTypes The types of the parameters of the method to invoke
     * @param params     The parameters with which to invoke the method
     *
     * @return This default implementation always returns <code>null</code>
     *
     * @since EL 2.2
     */
    public Object invoke(ELContext context, Object base, Object method, Class<?>[] paramTypes, Object[] params) {
        return null;
    }

    /**
     * Obtain the most generally acceptable type that may be used to set the given property on the given object using
     * the given context.
     *
     * @param context  The EL context for this evaluation
     * @param base     The base object on which the property is to be found
     * @param property The property whose type is to be returned
     *
     * @return the most general type that maybe used to set the provided property or {@code null} if the resolver is
     *             read-only.
     *
     * @throws NullPointerException      If the supplied context is <code>null</code>
     * @throws PropertyNotFoundException If the base/property combination provided to the resolver is one that the
     *                                       resolver can handle but no match was found or a match was found but was not
     *                                       readable
     * @throws ELException               Wraps any exception throw whilst resolving the property
     */
    public abstract Class<?> getType(ELContext context, Object base, Object property);

    /**
     * Set the value of the given property on the given object using the given context.
     *
     * @param context  The EL context for this evaluation
     * @param base     The base object on which the property is to be found
     * @param property The property whose value is to be set
     * @param value    The value to set the property to
     *
     * @throws NullPointerException         If the supplied context is <code>null</code>
     * @throws PropertyNotFoundException    If the base/property combination provided to the resolver is one that the
     *                                          resolver can handle but no match was found
     * @throws PropertyNotWritableException If the base/property combination provided to the resolver is one that the
     *                                          resolver can handle but the property was not writable
     * @throws ELException                  Wraps any exception throw whilst resolving the property
     */
    public abstract void setValue(ELContext context, Object base, Object property, Object value);

    /**
     * Determine if the given property on the given object is read-only using the given context.
     *
     * @param context  The EL context for this evaluation
     * @param base     The base object on which the property is to be found
     * @param property The property to be checked for read only status
     *
     * @return <code>true</code> if the identified property is read only, otherwise <code>false</code>
     *
     * @throws NullPointerException      If the supplied context is <code>null</code>
     * @throws PropertyNotFoundException If the base/property combination provided to the resolver is one that the
     *                                       resolver can handle but no match was found
     * @throws ELException               Wraps any exception throw whilst resolving the property
     */
    public abstract boolean isReadOnly(ELContext context, Object base, Object property);

    /**
     * Obtain the most common type that is acceptable for the given base object.
     *
     * @param context The context in which the examination takes place
     * @param base    The object to examine
     *
     * @return {code null} if the most common type cannot be determined, otherwise the most common type
     */
    public abstract Class<?> getCommonPropertyType(ELContext context, Object base);

    /**
     * Converts the given object to the given type.
     *
     * @param <T>     The type to which the object should be converted
     * @param context The EL context for this evaluation
     * @param obj     The object to convert
     * @param type    The type to which the object should be converted
     *
     * @return This default implementation always returns <code>null</code>
     *
     * @since EL 3.0
     */
    public <T> T convertToType(ELContext context, Object obj, Class<T> type) {
        context.setPropertyResolved(false);
        return null;
    }

    /**
     * This class is used as a key for {@link ELContext#getContext(Class)}. The key references a context object that if
     * present and set to {@code Boolean#TRUE}, indicates that the identifier being resolved is a single, stand-alone
     * identifier. This allows {@link ELResolver} instances - and in particular
     * {@code jakarta.servlet.jsp.el.ImportELResolver} - to optimise the resolution of the identifier and avoid
     * unnecessary and expensive class loader lookups.
     * <p>
     * The EL implementation is required to set this key with the value {@code Boolean#TRUE} when resolving a single,
     * stand-alone identifier.
     *
     * @since EL 6.1
     */
    public static class StandaloneIdentifierMarker {

        private StandaloneIdentifierMarker() {
            // Non-public default constructor as there is no need to create instances of this class.
        }
    }
}