/*
 * Copyright 2002-2018 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.spel.support;

import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.springframework.core.convert.ConversionService;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.expression.BeanResolver;
import org.springframework.expression.ConstructorResolver;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.MethodResolver;
import org.springframework.expression.OperatorOverloader;
import org.springframework.expression.PropertyAccessor;
import org.springframework.expression.TypeComparator;
import org.springframework.expression.TypeConverter;
import org.springframework.expression.TypeLocator;
import org.springframework.expression.TypedValue;
import org.springframework.expression.spel.SpelEvaluationException;
import org.springframework.expression.spel.SpelMessage;
import org.springframework.lang.Nullable;

/**
 * A basic implementation of {@link EvaluationContext} that focuses on a subset
 * of essential SpEL features and customization options, targeting simple
 * condition evaluation and in particular data binding scenarios.
 *
 * <p>In many cases, the full extent of the SpEL language is not required and
 * should be meaningfully restricted. Examples include but are not limited to
 * data binding expressions, property-based filters, and others. To that effect,
 * {@code SimpleEvaluationContext} is tailored to support only a subset of the
 * SpEL language syntax, e.g. excluding references to Java types, constructors,
 * and bean references.
 *
 * <p>When creating a {@code SimpleEvaluationContext} you need to choose the
 * level of support that you need for property access in SpEL expressions:
 * <ul>
 * <li>A custom {@code PropertyAccessor} (typically not reflection-based),
 * potentially combined with a {@link DataBindingPropertyAccessor}</li>
 * <li>Data binding properties for read-only access</li>
 * <li>Data binding properties for read and write</li>
 * </ul>
 *
 * <p>Conveniently, {@link SimpleEvaluationContext#forReadOnlyDataBinding()}
 * enables read access to properties via {@link DataBindingPropertyAccessor};
 * same for {@link SimpleEvaluationContext#forReadWriteDataBinding()} when
 * write access is needed as well. Alternatively, configure custom accessors
 * via {@link SimpleEvaluationContext#forPropertyAccessors}, and potentially
 * activate method resolution and/or a type converter through the builder.
 *
 * <p>Note that {@code SimpleEvaluationContext} is typically not configured
 * with a default root object. Instead it is meant to be created once and
 * used repeatedly through {@code getValue} calls on a pre-compiled
 * {@link org.springframework.expression.Expression} with both an
 * {@code EvaluationContext} and a root object as arguments:
 * {@link org.springframework.expression.Expression#getValue(EvaluationContext, Object)}.
 *
 * <p>For more power and flexibility, in particular for internal configuration
 * scenarios, consider using {@link StandardEvaluationContext} instead.
 *
 * @author Rossen Stoyanchev
 * @author Juergen Hoeller
 * @since 4.3.15
 * @see #forPropertyAccessors
 * @see #forReadOnlyDataBinding()
 * @see #forReadWriteDataBinding()
 * @see StandardEvaluationContext
 * @see StandardTypeConverter
 * @see DataBindingPropertyAccessor
 */
public final class SimpleEvaluationContext implements EvaluationContext {

        private static final TypeLocator typeNotFoundTypeLocator = typeName -> {
                throw new SpelEvaluationException(SpelMessage.TYPE_NOT_FOUND, typeName);
        };


        private final TypedValue rootObject;

        private final List<PropertyAccessor> propertyAccessors;

        private final List<MethodResolver> methodResolvers;

        private final TypeConverter typeConverter;

        private final TypeComparator typeComparator = new StandardTypeComparator();

        private final OperatorOverloader operatorOverloader = new StandardOperatorOverloader();

        private final Map<String, Object> variables = new HashMap<>();


        private SimpleEvaluationContext(List<PropertyAccessor> accessors, List<MethodResolver> resolvers,
                        @Nullable TypeConverter converter, @Nullable TypedValue rootObject) {

                this.propertyAccessors = accessors;
                this.methodResolvers = resolvers;
                this.typeConverter = (converter != null ? converter : new StandardTypeConverter());
                this.rootObject = (rootObject != null ? rootObject : TypedValue.NULL);
        }


        /**
         * Return the specified root object, if any.
         */
        @Override
        public TypedValue getRootObject() {
                return this.rootObject;
        }

        /**
         * Return the specified {@link PropertyAccessor} delegates, if any.
         * @see #forPropertyAccessors
         */
        @Override
        public List<PropertyAccessor> getPropertyAccessors() {
                return this.propertyAccessors;
        }

        /**
         * Return an empty list, always, since this context does not support the
         * use of type references.
         */
        @Override
        public List<ConstructorResolver> getConstructorResolvers() {
                return Collections.emptyList();
        }

        /**
         * Return the specified {@link MethodResolver} delegates, if any.
         * @see Builder#withMethodResolvers
         */
        @Override
        public List<MethodResolver> getMethodResolvers() {
                return this.methodResolvers;
        }

        /**
         * {@code SimpleEvaluationContext} does not support the use of bean references.
         * @return always {@code null}
         */
        @Override
        @Nullable
        public BeanResolver getBeanResolver() {
                return null;
        }

        /**
         * {@code SimpleEvaluationContext} does not support use of type references.
         * @return {@code TypeLocator} implementation that raises a
         * {@link SpelEvaluationException} with {@link SpelMessage#TYPE_NOT_FOUND}.
         */
        @Override
        public TypeLocator getTypeLocator() {
                return typeNotFoundTypeLocator;
        }

        /**
         * The configured {@link TypeConverter}.
         * <p>By default this is {@link StandardTypeConverter}.
         * @see Builder#withTypeConverter
         * @see Builder#withConversionService
         */
        @Override
        public TypeConverter getTypeConverter() {
                return this.typeConverter;
        }

        /**
         * Return an instance of {@link StandardTypeComparator}.
         */
        @Override
        public TypeComparator getTypeComparator() {
                return this.typeComparator;
        }

        /**
         * Return an instance of {@link StandardOperatorOverloader}.
         */
        @Override
        public OperatorOverloader getOperatorOverloader() {
                return this.operatorOverloader;
        }

        @Override
        public void setVariable(String name, @Nullable Object value) {
                this.variables.put(name, value);
        }

        @Override
        @Nullable
        public Object lookupVariable(String name) {
                return this.variables.get(name);
        }


        /**
         * Create a {@code SimpleEvaluationContext} for the specified {@link PropertyAccessor}
         * delegates: typically a custom {@code PropertyAccessor} specific to a use case
         * (e.g. attribute resolution in a custom data structure), potentially combined with
         * a {@link DataBindingPropertyAccessor} if property dereferences are needed as well.
         * @param accessors the accessor delegates to use
         * @see DataBindingPropertyAccessor#forReadOnlyAccess()
         * @see DataBindingPropertyAccessor#forReadWriteAccess()
         */
        public static Builder forPropertyAccessors(PropertyAccessor... accessors) {
                for (PropertyAccessor accessor : accessors) {
                        if (accessor.getClass() == ReflectivePropertyAccessor.class) {
                                throw new IllegalArgumentException("SimpleEvaluationContext is not designed for use with a plain " +
                                                "ReflectivePropertyAccessor. Consider using DataBindingPropertyAccessor or a custom subclass.");
                        }
                }
                return new Builder(accessors);
        }

        /**
         * Create a {@code SimpleEvaluationContext} for read-only access to
         * public properties via {@link DataBindingPropertyAccessor}.
         * @see DataBindingPropertyAccessor#forReadOnlyAccess()
         * @see #forPropertyAccessors
         */
        public static Builder forReadOnlyDataBinding() {
                return new Builder(DataBindingPropertyAccessor.forReadOnlyAccess());
        }

        /**
         * Create a {@code SimpleEvaluationContext} for read-write access to
         * public properties via {@link DataBindingPropertyAccessor}.
         * @see DataBindingPropertyAccessor#forReadWriteAccess()
         * @see #forPropertyAccessors
         */
        public static Builder forReadWriteDataBinding() {
                return new Builder(DataBindingPropertyAccessor.forReadWriteAccess());
        }


        /**
         * Builder for {@code SimpleEvaluationContext}.
         */
        public static class Builder {

                private final List<PropertyAccessor> accessors;

                private List<MethodResolver> resolvers = Collections.emptyList();

                @Nullable
                private TypeConverter typeConverter;

                @Nullable
                private TypedValue rootObject;

                public Builder(PropertyAccessor... accessors) {
                        this.accessors = Arrays.asList(accessors);
                }

                /**
                 * Register the specified {@link MethodResolver} delegates for
                 * a combination of property access and method resolution.
                 * @param resolvers the resolver delegates to use
                 * @see #withInstanceMethods()
                 * @see SimpleEvaluationContext#forPropertyAccessors
                 */
                public Builder withMethodResolvers(MethodResolver... resolvers) {
                        for (MethodResolver resolver : resolvers) {
                                if (resolver.getClass() == ReflectiveMethodResolver.class) {
                                        throw new IllegalArgumentException("SimpleEvaluationContext is not designed for use with a plain " +
                                                        "ReflectiveMethodResolver. Consider using DataBindingMethodResolver or a custom subclass.");
                                }
                        }
                        this.resolvers = Arrays.asList(resolvers);
                        return this;
                }

                /**
                 * Register a {@link DataBindingMethodResolver} for instance method invocation purposes
                 * (i.e. not supporting static methods) in addition to the specified property accessors,
                 * typically in combination with a {@link DataBindingPropertyAccessor}.
                 * @see #withMethodResolvers
                 * @see SimpleEvaluationContext#forReadOnlyDataBinding()
                 * @see SimpleEvaluationContext#forReadWriteDataBinding()
                 */
                public Builder withInstanceMethods() {
                        this.resolvers = Collections.singletonList(DataBindingMethodResolver.forInstanceMethodInvocation());
                        return this;
                }


                /**
                 * Register a custom {@link ConversionService}.
                 * <p>By default a {@link StandardTypeConverter} backed by a
                 * {@link org.springframework.core.convert.support.DefaultConversionService} is used.
                 * @see #withTypeConverter
                 * @see StandardTypeConverter#StandardTypeConverter(ConversionService)
                 */
                public Builder withConversionService(ConversionService conversionService) {
                        this.typeConverter = new StandardTypeConverter(conversionService);
                        return this;
                }
                /**
                 * Register a custom {@link TypeConverter}.
                 * <p>By default a {@link StandardTypeConverter} backed by a
                 * {@link org.springframework.core.convert.support.DefaultConversionService} is used.
                 * @see #withConversionService
                 * @see StandardTypeConverter#StandardTypeConverter()
                 */
                public Builder withTypeConverter(TypeConverter converter) {
                        this.typeConverter = converter;
                        return this;
                }

                /**
                 * Specify a default root object to resolve against.
                 * <p>Default is none, expecting an object argument at evaluation time.
                 * @see org.springframework.expression.Expression#getValue(EvaluationContext)
                 * @see org.springframework.expression.Expression#getValue(EvaluationContext, Object)
                 */
                public Builder withRootObject(Object rootObject) {
                        this.rootObject = new TypedValue(rootObject);
                        return this;
                }

                /**
                 * Specify a typed root object to resolve against.
                 * <p>Default is none, expecting an object argument at evaluation time.
                 * @see org.springframework.expression.Expression#getValue(EvaluationContext)
                 * @see org.springframework.expression.Expression#getValue(EvaluationContext, Object)
                 */
                public Builder withTypedRootObject(Object rootObject, TypeDescriptor typeDescriptor) {
                        this.rootObject = new TypedValue(rootObject, typeDescriptor);
                        return this;
                }

                public SimpleEvaluationContext build() {
                        return new SimpleEvaluationContext(this.accessors, this.resolvers, this.typeConverter, this.rootObject);
                }
        }

}