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annotate src/main/java/nl/cwi/monetdb/jdbc/MonetWrapper.java @ 18:8e57d20b5e80

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Corrected implementation of java.sql.Wrapper methods isWrapperFor() and unwrap(). They now properly return expected results instead of always return false or throw an SQLException. Added test program Test_Wrapper.java
author Martin van Dinther <martin.van.dinther@monetdbsolutions.com>
date Thu, 13 Oct 2016 16:31:08 +0200 (2016-10-13)
parents a5a898f6886c
children b9b35ca2eec2
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1 /*
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2 * This Source Code Form is subject to the terms of the Mozilla Public
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3 * License, v. 2.0. If a copy of the MPL was not distributed with this
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4 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
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5 *
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6 * Copyright 1997 - July 2008 CWI, August 2008 - 2016 MonetDB B.V.
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7 */
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8
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9 package nl.cwi.monetdb.jdbc;
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10
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11 import java.sql.SQLException;
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13 /**
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14 * A Wrapper class which provide the ability to retrieve the delegate instance
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15 * when the instance in question is in fact a proxy class.
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16 *
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17 * The wrapper pattern is employed by many JDBC driver implementations to provide
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18 * extensions beyond the traditional JDBC API that are specific to a data source.
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19 * Developers may wish to gain access to these resources that are wrapped (the delegates)
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20 * as proxy class instances representing the the actual resources.
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21 * This class contains a standard mechanism to access these wrapped resources
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22 * represented by their proxy, to permit direct access to the resource delegates.
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23 *
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24 * @author Fabian Groffen, Martin van Dinther
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25 * @version 1.1
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26 */
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27 public class MonetWrapper implements java.sql.Wrapper {
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28 /**
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29 * Returns true if this either implements the interface argument or
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30 * is directly or indirectly a wrapper for an object that does.
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31 * Returns false otherwise. If this implements the interface then
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32 * return true, else if this is a wrapper then return the result of
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33 * recursively calling <code>isWrapperFor</code> on the wrapped object.
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34 * If this does not implement the interface and is not a wrapper, return
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35 * false. This method should be implemented as a low-cost operation
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36 * compared to <code>unwrap</code> so that callers can use this method to avoid
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37 * expensive <code>unwrap</code> calls that may fail.
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38 * If this method returns true then calling <code>unwrap</code> with the same argument should succeed.
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39 *
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40 * @param iface a Class defining an interface.
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41 * @return true if this implements the interface or directly or indirectly wraps an object that does.
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42 * @throws SQLException if an error occurs while determining whether this is a wrapper
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43 * for an object with the given interface.
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44 * @since 1.6
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45 */
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46 @Override
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47 public boolean isWrapperFor(Class<?> iface) throws SQLException {
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48 return iface != null && iface.isAssignableFrom(getClass());
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49 }
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51 /**
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52 * Returns an object that implements the given interface to allow
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53 * access to non-standard methods, or standard methods not exposed by the proxy.
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54 * The result may be either the object found to implement the interface
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55 * or a proxy for that object.
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56 * If the receiver implements the interface then the result is the receiver
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57 * or a proxy for the receiver.
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58 * If the receiver is a wrapper and the wrapped object implements the interface
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59 * then the result is the wrapped object or a proxy for the wrapped object.
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60 * Otherwise return the result of calling <code>unwrap</code> recursively on
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61 * the wrapped object or a proxy for that result.
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62 * If the receiver is not a wrapper and does not implement the interface,
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63 * then an <code>SQLException</code> is thrown.
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64 *
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65 * @param iface A Class defining an interface that the result must implement.
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66 * @return an object that implements the interface. May be a proxy for the actual implementing object.
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67 * @throws SQLException If no object found that implements the interface
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68 * @since 1.6
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69 */
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70 @Override
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71 @SuppressWarnings("unchecked")
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72 public <T> T unwrap(Class<T> iface) throws SQLException {
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73 if (isWrapperFor(iface)) {
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74 return (T) this;
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75 }
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76 throw new SQLException("Cannot unwrap to interface: " + (iface != null ? iface.getName() : ""), "0A000");
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77 }
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78 }