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Apache Commons Bytecode Engineering Library
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/*
 * 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 org.apache.bcel.verifier;

import java.util.ArrayList;
import java.util.List;

/**
 * A PassVerifier actually verifies a class file; it is instantiated
 * by a Verifier.
 * The verification should conform with a certain pass as described
 * in The Java Virtual Machine Specification, 2nd edition.
 * This book describes four passes. Pass one means loading the
 * class and verifying a few static constraints. Pass two actually
 * verifies some other constraints that could enforce loading in
 * referenced class files. Pass three is the first pass that actually
 * checks constraints in the code array of a method in the class file;
 * it has two parts with the first verifying static constraints and
 * the second part verifying structural constraints (where a data flow
 * analysis is used for). The fourth pass, finally, performs checks
 * that can only be done at run-time.
 * JustIce does not have a run-time pass, but certain constraints that
 * are usually delayed until run-time for performance reasons are also
 * checked during the second part of pass three.
 * PassVerifier instances perform caching.
 * That means, if you really want a new verification run of a certain
 * pass you must use a new instance of a given PassVerifier.
 *
 * @see Verifier
 * @see #verify()
 */
public abstract class PassVerifier {

    /** The (warning) messages. */
    private final List messages = new ArrayList<>();
    /** The VerificationResult cache. */
    private VerificationResult verificationResult = null;


    /**
     * This method runs a verification pass conforming to the
     * Java Virtual Machine Specification, 2nd edition, on a
     * class file.
     * PassVerifier instances perform caching;
     * i.e. if the verify() method once determined a VerificationResult,
     * then this result may be returned after every invocation of this
     * method instead of running the verification pass anew; likewise with
     * the result of getMessages().
     *
     * @see #getMessages()
     * @see #addMessage(String)
     */
    public VerificationResult verify() {
        if (verificationResult == null) {
            verificationResult = do_verify();
        }
        return verificationResult;
    }


    /** Does the real verification work, uncached. */
    public abstract VerificationResult do_verify();


    /**
     * This method adds a (warning) message to the message pool of this
     * PassVerifier. This method is normally only internally used by
     * BCEL's class file verifier "JustIce" and should not be used from
     * the outside.
     *
     * @see #getMessages()
     */
    public void addMessage( final String message ) {
        messages.add(message);
    }


    /**
     * Returns the (warning) messages that this PassVerifier accumulated
     * during its do_verify()ing work.
     *
     * @see #addMessage(String)
     * @see #do_verify()
     */
    public String[] getMessages() {
        verify(); // create messages if not already done (cached!)
        return messages.toArray(new String[messages.size()]);
    }
}