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package java.security.cert;

import java.util.Collection;
import java.util.Set;

/**
 * An abstract class that performs one or more checks on an
 * {@code X509Certificate}.
 *
 * <p>A concrete implementation of the {@code PKIXCertPathChecker} class
 * can be created to extend the PKIX certification path validation algorithm.
 * For example, an implementation may check for and process a critical private
 * extension of each certificate in a certification path.
 *
 * <p>Instances of {@code PKIXCertPathChecker} are passed as parameters
 * using the {@link PKIXParameters#setCertPathCheckers setCertPathCheckers}
 * or {@link PKIXParameters#addCertPathChecker addCertPathChecker} methods
 * of the {@code PKIXParameters} and {@code PKIXBuilderParameters}
 * class. Each of the {@code PKIXCertPathChecker}s {@link #check check}
 * methods will be called, in turn, for each certificate processed by a PKIX
 * {@code CertPathValidator} or {@code CertPathBuilder}
 * implementation.
 *
 * <p>A {@code PKIXCertPathChecker} may be called multiple times on
 * successive certificates in a certification path. Concrete subclasses
 * are expected to maintain any internal state that may be necessary to
 * check successive certificates. The {@link #init init} method is used
 * to initialize the internal state of the checker so that the certificates
 * of a new certification path may be checked. A stateful implementation
 * <b>must</b> override the {@link #clone clone} method if necessary in
 * order to allow a PKIX {@code CertPathBuilder} to efficiently
 * backtrack and try other paths. In these situations, the
 * {@code CertPathBuilder} is able to restore prior path validation
 * states by restoring the cloned {@code PKIXCertPathChecker}s.
 *
 * <p>The order in which the certificates are presented to the
 * {@code PKIXCertPathChecker} may be either in the forward direction
 * (from target to most-trusted CA) or in the reverse direction (from
 * most-trusted CA to target). A {@code PKIXCertPathChecker} implementation
 * <b>must</b> support reverse checking (the ability to perform its checks when
 * it is presented with certificates in the reverse direction) and <b>may</b>
 * support forward checking (the ability to perform its checks when it is
 * presented with certificates in the forward direction). The
 * {@link #isForwardCheckingSupported isForwardCheckingSupported} method
 * indicates whether forward checking is supported.
 * <p>
 * Additional input parameters required for executing the check may be
 * specified through constructors of concrete implementations of this class.
 * <p>
 * <b>Concurrent Access</b>
 * <p>
 * Unless otherwise specified, the methods defined in this class are not
 * thread-safe. Multiple threads that need to access a single
 * object concurrently should synchronize amongst themselves and
 * provide the necessary locking. Multiple threads each manipulating
 * separate objects need not synchronize.
 *
 * @see PKIXParameters
 * @see PKIXBuilderParameters
 *
 * @since       1.4
 * @author      Yassir Elley
 * @author      Sean Mullan
 */
public abstract class PKIXCertPathChecker
    implements CertPathChecker, Cloneable {

    /**
     * Default constructor.
     */
    protected PKIXCertPathChecker() {}

    /**
     * Initializes the internal state of this {@code PKIXCertPathChecker}.
     * <p>
     * The {@code forward} flag specifies the order that
     * certificates will be passed to the {@link #check check} method
     * (forward or reverse). A {@code PKIXCertPathChecker} <b>must</b>
     * support reverse checking and <b>may</b> support forward checking.
     *
     * @param forward the order that certificates are presented to
     * the {@code check} method. If {@code true}, certificates
     * are presented from target to most-trusted CA (forward); if
     * {@code false}, from most-trusted CA to target (reverse).
     * @throws CertPathValidatorException if this
     * {@code PKIXCertPathChecker} is unable to check certificates in
     * the specified order; it should never be thrown if the forward flag
     * is false since reverse checking must be supported
     */
    @Override
    public abstract void init(boolean forward)
        throws CertPathValidatorException;

    /**
     * Indicates if forward checking is supported. Forward checking refers
     * to the ability of the {@code PKIXCertPathChecker} to perform
     * its checks when certificates are presented to the {@code check}
     * method in the forward direction (from target to most-trusted CA).
     *
     * @return {@code true} if forward checking is supported,
     * {@code false} otherwise
     */
    @Override
    public abstract boolean isForwardCheckingSupported();

    /**
     * Returns an immutable {@code Set} of X.509 certificate extensions
     * that this {@code PKIXCertPathChecker} supports (i.e. recognizes, is
     * able to process), or {@code null} if no extensions are supported.
     * <p>
     * Each element of the set is a {@code String} representing the
     * Object Identifier (OID) of the X.509 extension that is supported.
     * The OID is represented by a set of nonnegative integers separated by
     * periods.
     * <p>
     * All X.509 certificate extensions that a {@code PKIXCertPathChecker}
     * might possibly be able to process should be included in the set.
     *
     * @return an immutable {@code Set} of X.509 extension OIDs (in
     * {@code String} format) supported by this
     * {@code PKIXCertPathChecker}, or {@code null} if no
     * extensions are supported
     */
    public abstract Set<String> getSupportedExtensions();

    /**
     * Performs the check(s) on the specified certificate using its internal
     * state and removes any critical extensions that it processes from the
     * specified collection of OID strings that represent the unresolved
     * critical extensions. The certificates are presented in the order
     * specified by the {@code init} method.
     *
     * @param cert the {@code Certificate} to be checked
     * @param unresolvedCritExts a {@code Collection} of OID strings
     * representing the current set of unresolved critical extensions
     * @throws    CertPathValidatorException if the specified certificate does
     * not pass the check
     */
    public abstract void check(Certificate cert,
            Collection<String> unresolvedCritExts)
            throws CertPathValidatorException;

    /**
     * {@inheritDoc}
     *
     * <p>This implementation calls
     * {@code check(cert, java.util.Collections.<String>emptySet())}.
     */
    @Override
    public void check(Certificate cert) throws CertPathValidatorException {
        check(cert, java.util.Collections.emptySet());
    }

    /**
     * Returns a clone of this object. Calls the {@code Object.clone()}
     * method.
     * All subclasses which maintain state must support and
     * override this method, if necessary.
     *
     * @return a copy of this {@code PKIXCertPathChecker}
     */
    @Override
    public Object clone() {
        try {
            return super.clone();
        } catch (CloneNotSupportedException e) {
            /* Cannot happen */
            throw new InternalError(e.toString(), e);
        }
    }
}
