org.apache.pig.impl.plan.PlanVisitor Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one
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* 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,
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* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.pig.impl.plan;
import java.util.Deque;
import java.util.LinkedList;
/**
* A visitor mechanism for navigating and operating on a plan of
* Operators. This class contains the logic to traverse the plan. It does not
* visit individual nodes. That is left to implementing classes (such as
* LOVisitor).
*
*/
abstract public class PlanVisitor > {
protected P mPlan;
/**
* Guaranteed to always point to the walker currently being used.
*/
protected PlanWalker mCurrentWalker;
private Deque> mWalkers;
/**
* Entry point for visiting the plan.
* @throws VisitorException if an error is encountered while visiting.
*/
public void visit() throws VisitorException {
mCurrentWalker.walk(this);
}
public P getPlan() {
return mPlan;
}
/**
* @param plan OperatorPlan this visitor will visit.
* @param walker PlanWalker this visitor will use to traverse the plan.
*/
protected PlanVisitor(P plan, PlanWalker walker) {
mPlan = plan;
mCurrentWalker = walker;
mWalkers = new LinkedList>();
}
/**
* Push the current walker onto the stack of saved walkers and begin using
* the newly passed walker as the current walker.
* @param walker new walker to set as the current walker.
*/
protected void pushWalker(PlanWalker walker) {
mWalkers.push(mCurrentWalker);
mCurrentWalker = walker;
}
/**
* Pop the next to previous walker off of the stack and set it as the current
* walker. This will drop the reference to the current walker.
* @throws VisitorException if there are no more walkers on the stack. In
* this case the current walker is not reset.
*/
protected void popWalker() throws VisitorException {
if (mWalkers.isEmpty()) {
throw new VisitorException("No more walkers to pop.");
}
mCurrentWalker = mWalkers.pop();
}
}