1 /*
   2  * Copyright (c) 1994, 2019, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang;
  27 
  28 /**
  29  * Thrown when an application attempts to use {@code null} in a
  30  * case where an object is required. These include:
  31  * <ul>
  32  * <li>Calling the instance method of a {@code null} object.
  33  * <li>Accessing or modifying the field of a {@code null} object.
  34  * <li>Taking the length of {@code null} as if it were an array.
  35  * <li>Accessing or modifying the slots of {@code null} as if it
  36  *     were an array.
  37  * <li>Throwing {@code null} as if it were a {@code Throwable}
  38  *     value.
  39  * </ul>
  40  * <p>
  41  * Applications should throw instances of this class to indicate
  42  * other illegal uses of the {@code null} object.
  43  *
  44  * {@code NullPointerException} objects may be constructed by the
  45  * virtual machine as if {@linkplain Throwable#Throwable(String,
  46  * Throwable, boolean, boolean) suppression were disabled and/or the
  47  * stack trace was not writable}.
  48  *
  49  * @author  unascribed
  50  * @since   1.0
  51  */
  52 public class NullPointerException extends RuntimeException {
  53     @java.io.Serial
  54     private static final long serialVersionUID = 5162710183389028792L;
  55 
  56     /**
  57      * Constructs a {@code NullPointerException} with no detail message.
  58      */
  59     public NullPointerException() {
  60         super();
  61     }
  62 
  63     /**
  64      * Constructs a {@code NullPointerException} with the specified
  65      * detail message.
  66      *
  67      * @param   s   the detail message.
  68      */
  69     public NullPointerException(String s) {
  70         super(s);
  71     }
  72 
  73     private static final String MUST_COMPUTE_EXTENDED_MESSAGE =
  74         "MUST_COMPUTE_EXTENDED_MESSAGE";
  75     private static final String NO_EXTENDED_MESSAGE =
  76         "NO_EXTENDED_MESSAGE";
  77     private transient String extendedMessage;
  78 
  79     /**
  80      * {@inheritDoc}
  81      */
  82     public synchronized Throwable fillInStackTrace() {
  83         // After the stack trace is changed the extended NPE algorithm
  84         // will compute a wrong message. So compute it beforehand.
  85         if (extendedMessage == null) {
  86             // The first call. Don't compute the message, we'll
  87             // do it lazily.
  88             extendedMessage = MUST_COMPUTE_EXTENDED_MESSAGE;
  89         } else if (extendedMessage == MUST_COMPUTE_EXTENDED_MESSAGE) {
  90             // The second call.
  91             extendedMessage = getExtendedNPEMessage();
  92         }
  93         return super.fillInStackTrace();
  94     }
  95 
  96     /**
  97      * Returns the detail message string of this throwable.
  98      *
  99      * <p> If a non-null message was supplied in a constructor it is
 100      * returned. Otherwise, an implementation specific message or
 101      * {@code null} is returned.
 102      *
 103      * @implNote
 104      * If no explicit message was passed to the constructor, and as
 105      * long as certain internal information is available, a verbose
 106      * description of the null reference is returned.
 107      * The internal information is not available in deserialized
 108      * NullPointerExceptions.
 109      *
 110      * @return the detail message string, which may be {@code null}.
 111      */
 112     public String getMessage() {
 113         String message = super.getMessage();
 114         if (message == null) {
 115             synchronized(this) {
 116                 if (extendedMessage == MUST_COMPUTE_EXTENDED_MESSAGE) {
 117                     // Only the original stack trace was filled in. Message will
 118                     // compute correctly.
 119                     extendedMessage = getExtendedNPEMessage();
 120                 }
 121                 if (extendedMessage != NO_EXTENDED_MESSAGE) {
 122                     return extendedMessage;
 123                 }
 124             }
 125         }
 126         return message;
 127     }
 128 
 129     /**
 130      * Get an extended exception message. This returns a string describing
 131      * the location and cause of the exception. It returns NO_EXTENDED_MESSAGE for
 132      * exceptions where this is not applicable.
 133      */
 134     private String getExtendedNPEMessage() {
 135         String msg = getExtendedNPEMessage0();
 136         if (msg == null) return NO_EXTENDED_MESSAGE;
 137         return msg;
 138     }
 139 
 140     private native String getExtendedNPEMessage0();
 141 }