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 import java.lang.reflect.Field;
  29 
  30 /**
  31  * Thrown when an application attempts to use {@code null} in a
  32  * case where an object is required. These include:
  33  * <ul>
  34  * <li>Calling the instance method of a {@code null} object.
  35  * <li>Accessing or modifying the field of a {@code null} object.
  36  * <li>Taking the length of {@code null} as if it were an array.
  37  * <li>Accessing or modifying the slots of {@code null} as if it
  38  *     were an array.
  39  * <li>Throwing {@code null} as if it were a {@code Throwable}
  40  *     value.
  41  * </ul>
  42  * <p>
  43  * Applications should throw instances of this class to indicate
  44  * other illegal uses of the {@code null} object.
  45  *
  46  * {@code NullPointerException} objects may be constructed by the
  47  * virtual machine as if {@linkplain Throwable#Throwable(String,
  48  * Throwable, boolean, boolean) suppression were disabled and/or the
  49  * stack trace was not writable}.
  50  *
  51  * @author  unascribed
  52  * @since   1.0
  53  */
  54 public
  55 class NullPointerException extends RuntimeException {
  56     private static final long serialVersionUID = 5162710183389028792L;
  57 
  58     /*
  59      * If set a message describing the exception is computed when first calling
  60      * getMessage(). A precise description of the location in the code and
  61      * the entitiy being null will be computed. This message is stored in
  62      * detailMessage and this flag is set to false.
  63      */
  64     private transient boolean lazyComputeMessage;
  65 
  66     /**
  67      * Constructs a {@code NullPointerException} with no detail message.
  68      */
  69     public NullPointerException() {
  70         super();
  71         lazyComputeMessage = true;
  72     }
  73 
  74     /**
  75      * Constructs a {@code NullPointerException} with the specified
  76      * detail message.
  77      *
  78      * @param   s   the detail message.
  79      */
  80     public NullPointerException(String s) {
  81         super(s);
  82     }
  83     
  84     /**
  85      * Returns the detail message string of this throwable.
  86      *
  87      * @return  the detail message string of this {@code Throwable} instance
  88      *          (which may be {@code null}).
  89      */
  90     public String getMessage() {
  91         synchronized (this) {
  92             if (!lazyComputeMessage) {
  93                 return super.getMessage();
  94             }
  95         }
  96 
  97         String extendedMessage = getExtendedNPEMessage(super.getMessage());
  98         
  99         if (extendedMessage != null) {
 100             synchronized (this) {
 101                 if (lazyComputeMessage) {
 102                     setMessage(extendedMessage);
 103                     lazyComputeMessage = false;
 104                 }
 105             }
 106         }
 107 
 108         return extendedMessage;
 109     }
 110 
 111     // Install the extendedMessage in the Throwable.defaultMessage before
 112     // serializing.
 113     private Object writeReplace() {
 114         getMessage();
 115         return this;
 116     }
 117 
 118     // Get an extended exception message. This adds a string describing
 119     // the location and cause of the exception. It returns defaultMessage
 120     // for exceptions where this is not applicable.
 121     private native String getExtendedNPEMessage(String defaultMessage);
 122 }