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 mustComputeExtendedNPEMessage = "1"; 74 private static final String noExtendedNPEMessage = "2"; 75 private volatile transient String extendedMessage; 76 77 /** 78 * {@inheritDoc} 79 */ 80 public synchronized Throwable fillInStackTrace() { 81 // After the stack trace is changed the extended NPE algorithm 82 // will compute a wrong message. So compute it beforehand. 83 if (extendedMessage == null) { 84 // The first call. Don't compute the message, we'll 85 // do it lazily. 86 extendedMessage = mustComputeExtendedNPEMessage; 87 } else if (extendedMessage == mustComputeExtendedNPEMessage) { 88 // The second call. 89 extendedMessage = getExtendedNPEMessage(); 90 } 91 return super.fillInStackTrace(); 92 } 93 94 /** 95 * Returns the detail message string of this throwable. 96 * 97 * <p> If a non-null message was supplied in a constructor it is 98 * returned. Otherwise, an implementation specific message or 99 * {@code null} is returned. 100 * 101 * @implNote 102 * If no explicit message was passed to the constructor, and as 103 * long as certain internal information is available, a verbose 104 * description of the null reference is returned. 105 * The internal information is not available in deserialized 106 * NullPointerExceptions. 107 * 108 * @return the detail message string, which may be {@code null}. 109 */ 110 public String getMessage() { 111 String message = super.getMessage(); 112 if (message == null) { 113 synchronized(this) { 114 if (extendedMessage == mustComputeExtendedNPEMessage) { 115 // Only the original stack trace was filled in. Message will 116 // compute correctly. 117 extendedMessage = getExtendedNPEMessage(); 118 } 119 if (extendedMessage != noExtendedNPEMessage) { 120 return extendedMessage; 121 } 122 } 123 } 124 return message; 125 } 126 127 /** 128 * Get an extended exception message. This returns a string describing 129 * the location and cause of the exception. It returns noExtendedNPEMessage for 130 * exceptions where this is not applicable. 131 */ 132 private String getExtendedNPEMessage() { 133 String msg = getExtendedNPEMessage0(); 134 if (msg == null) return noExtendedNPEMessage; 135 return msg; 136 } 137 138 private native String getExtendedNPEMessage0(); 139 }