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 int numStackTracesFilledIn = 0; 74 75 /** 76 * Fills in the execution stack trace. This method records within this 77 * {@code NullPointerException} object information about the current state of 78 * the stack frames for the current thread. 79 * 80 * <p>If the stack trace of this {@code NullPointerException} {@linkplain 81 * Throwable#Throwable(String, Throwable, boolean, boolean) is not 82 * writable}, calling this method has no effect. 83 * 84 * @implNote 85 * This delegates to {@code super.fillInStackTrace()}. In addition, 86 * it records some internal information used to compute 87 * the verbose {@code getMessage()} return value. 88 * 89 * @return a reference to this {@code Throwable} instance. 90 * @see java.lang.Throwable#printStackTrace() 91 */ 92 public synchronized Throwable fillInStackTrace() { 93 Throwable result = super.fillInStackTrace(); 94 if (result == this) numStackTracesFilledIn++; 95 return result; 96 } 97 98 /** 99 * Returns the detail message string of this throwable. 100 * 101 * <p> If a non-null message was supplied in a constructor it is 102 * returned. Otherwise, an implementation specific message or 103 * {@code null} is returned. 104 * 105 * @implNote 106 * If no explicit message was passed to the constructor, and as 107 * long as certain internal information is available, a verbose 108 * description of the null reference is returned. 109 * The internal information is not available in deserialized 110 * NullPointerExceptions. 111 * 112 * @return the detail message string, which may be {@code null}. 113 */ 114 public String getMessage() { 115 String message = super.getMessage(); 116 // If the stack trace was changed the extended NPE algorithm 117 // will compute a wrong message. 118 if (message == null && numStackTracesFilledIn == 0) { 119 return getExtendedNPEMessage(); 120 } 121 return message; 122 } 123 124 /** 125 * Get an extended exception message. This returns a string describing 126 * the location and cause of the exception. It returns null for 127 * exceptions where this is not applicable. 128 */ 129 private native String getExtendedNPEMessage(); 130 }