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/*
* 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
*
* 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.expression.spel.ast;
import org.springframework.expression.TypedValue;
import org.springframework.expression.spel.SpelEvaluationException;
import org.springframework.expression.spel.SpelMessage;
/**
* Represents a reference to a value. With a reference it is possible to get or set the
* value. Passing around value references rather than the values themselves can avoid
* incorrect duplication of operand evaluation. For example in 'list[index++]++' without a
* value reference for 'list[index++]' it would be necessary to evaluate list[index++]
* twice (once to get the value, once to determine where the value goes) and that would
* double increment index.
*
* @author Andy Clement
* @since 3.2
*/
public interface ValueRef {
/**
* Returns the value this ValueRef points to, it should not require expression
* component re-evaluation.
* @return the value
*/
TypedValue getValue();
/**
* Sets the value this ValueRef points to, it should not require expression component
* re-evaluation.
* @param newValue the new value
*/
void setValue(Object newValue);
/**
* Indicates whether calling setValue(Object) is supported.
* @return true if setValue() is supported for this value reference.
*/
boolean isWritable();
/**
* A ValueRef for the null value.
*/
class NullValueRef implements ValueRef {
static final NullValueRef INSTANCE = new NullValueRef();
@Override
public TypedValue getValue() {
return TypedValue.NULL;
}
@Override
public void setValue(Object newValue) {
// The exception position '0' isn't right but the overhead of creating
// instances of this per node (where the node is solely for error reporting)
// would be unfortunate.
throw new SpelEvaluationException(0, SpelMessage.NOT_ASSIGNABLE, "null");
}
@Override
public boolean isWritable() {
return false;
}
}
/**
* A ValueRef holder for a single value, which cannot be set.
*/
class TypedValueHolderValueRef implements ValueRef {
private final TypedValue typedValue;
private final SpelNodeImpl node; // used only for error reporting
public TypedValueHolderValueRef(TypedValue typedValue, SpelNodeImpl node) {
this.typedValue = typedValue;
this.node = node;
}
@Override
public TypedValue getValue() {
return this.typedValue;
}
@Override
public void setValue(Object newValue) {
throw new SpelEvaluationException(this.node.pos, SpelMessage.NOT_ASSIGNABLE, this.node.toStringAST());
}
@Override
public boolean isWritable() {
return false;
}
}
}
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